json.hpp

#ifndef NLOHMANN_JSON_HPP
#define NLOHMANN_JSON_HPP

#include <algorithm>
#include <array>
#include <cassert>
#include <ciso646>
#include <cmath>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <memory>
#include <sstream>
#include <stdexcept>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

// enable ssize_t on MinGW
#ifdef __GNUC__
    #ifdef __MINGW32__
        #include <sys/types.h>
    #endif
#endif

// disable float-equal warnings on GCC/clang
#if defined(__clang__) || defined(__GNUC__) || defined(__GNUG__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wfloat-equal"
#endif

// enable ssize_t for MSVC
#ifdef _MSC_VER
    #include <basetsd.h>
    using ssize_t = SSIZE_T;
#endif

namespace nlohmann
{


namespace
{
template<typename T>
struct has_mapped_type
{
  private:
    template<typename C> static char test(typename C::mapped_type*);
    template<typename C> static char (&test(...))[2];
  public:
    static constexpr bool value = sizeof(test<T>(0)) == 1;
};

}

template <
    template<typename U, typename V, typename... Args> class ObjectType = std::map,
    template<typename U, typename... Args> class ArrayType = std::vector,
    class StringType = std::string,
    class BooleanType = bool,
    class NumberIntegerType = int64_t,
    class NumberUnsignedType = uint64_t,
    class NumberFloatType = double,
    template<typename U> class AllocatorType = std::allocator
    >
class basic_json
{
  private:
    using basic_json_t = basic_json<ObjectType,
          ArrayType,
          StringType,
          BooleanType,
          NumberIntegerType,
          NumberUnsignedType,
          NumberFloatType,
          AllocatorType>;

  public:

    // container types //


    using value_type = basic_json;

    using reference = value_type&;
    using const_reference = const value_type&;

    using difference_type = std::ptrdiff_t;
    using size_type = std::size_t;

    using allocator_type = AllocatorType<basic_json>;

    using pointer = typename std::allocator_traits<allocator_type>::pointer;
    using const_pointer = typename std::allocator_traits<allocator_type>::const_pointer;

    // forward declaration
    template<typename Base> class json_reverse_iterator;

    class iterator;
    class const_iterator;
    using reverse_iterator = json_reverse_iterator<typename basic_json::iterator>;
    using const_reverse_iterator = json_reverse_iterator<typename basic_json::const_iterator>;



    static allocator_type get_allocator()
    {
        return allocator_type();
    }


    // JSON value data types //


    using object_t = ObjectType<StringType,
          basic_json,
          std::less<StringType>,
          AllocatorType<std::pair<const StringType,
          basic_json>>>;

    using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;

    using string_t = StringType;

    using boolean_t = BooleanType;

    using number_integer_t = NumberIntegerType;

    using number_unsigned_t = NumberUnsignedType;

    using number_float_t = NumberFloatType;



    // JSON type enumeration //

    enum class value_t : uint8_t
    {
        null,           
        object,         
        array,          
        string,         
        boolean,        
        number_integer, 
        number_unsigned,
        number_float,   
        discarded       
    };


  private:
    template<typename T, typename... Args>
    static T* create(Args&& ... args)
    {
        AllocatorType<T> alloc;
        auto deleter = [&](T * object)
        {
            alloc.deallocate(object, 1);
        };
        std::unique_ptr<T, decltype(deleter)> object(alloc.allocate(1), deleter);
        alloc.construct(object.get(), std::forward<Args>(args)...);
        return object.release();
    }

    // JSON value storage //

    union json_value
    {
        object_t* object;
        array_t* array;
        string_t* string;
        boolean_t boolean;
        number_integer_t number_integer;
        number_unsigned_t number_unsigned;
        number_float_t number_float;

        json_value() noexcept = default;
        json_value(boolean_t v) noexcept : boolean(v) {}
        json_value(number_integer_t v) noexcept : number_integer(v) {}
        json_value(number_unsigned_t v) noexcept : number_unsigned(v) {}
        json_value(number_float_t v) noexcept : number_float(v) {}
        json_value(value_t t)
        {
            switch (t)
            {
                case value_t::object:
                {
                    object = create<object_t>();
                    break;
                }

                case value_t::array:
                {
                    array = create<array_t>();
                    break;
                }

                case value_t::string:
                {
                    string = create<string_t>("");
                    break;
                }

                case value_t::boolean:
                {
                    boolean = boolean_t(false);
                    break;
                }

                case value_t::number_integer:
                {
                    number_integer = number_integer_t(0);
                    break;
                }

                case value_t::number_unsigned:
                {
                    number_unsigned = number_unsigned_t(0);
                    break;
                }

                case value_t::number_float:
                {
                    number_float = number_float_t(0.0);
                    break;
                }

                default:
                {
                    break;
                }
            }
        }

        json_value(const string_t& value)
        {
            string = create<string_t>(value);
        }

        json_value(const object_t& value)
        {
            object = create<object_t>(value);
        }

        json_value(const array_t& value)
        {
            array = create<array_t>(value);
        }
    };


  public:
    // JSON parser callback //

    enum class parse_event_t : uint8_t
    {
        object_start,
        object_end,
        array_start,
        array_end,
        key,
        value
    };

    using parser_callback_t = std::function<bool(int depth, parse_event_t event, basic_json& parsed)>;


    // constructors //


    basic_json(const value_t value_type)
        : m_type(value_type), m_value(value_type)
    {}

    basic_json() noexcept = default;

    basic_json(std::nullptr_t) noexcept
        : basic_json(value_t::null)
    {}

    basic_json(const object_t& val)
        : m_type(value_t::object), m_value(val)
    {}

    template <class CompatibleObjectType, typename
              std::enable_if<
                  std::is_constructible<typename object_t::key_type, typename CompatibleObjectType::key_type>::value and
                  std::is_constructible<basic_json, typename CompatibleObjectType::mapped_type>::value, int>::type
              = 0>
    basic_json(const CompatibleObjectType& val)
        : m_type(value_t::object)
    {
        using std::begin;
        using std::end;
        m_value.object = create<object_t>(begin(val), end(val));
    }

    basic_json(const array_t& val)
        : m_type(value_t::array), m_value(val)
    {}

    template <class CompatibleArrayType, typename
              std::enable_if<
                  not std::is_same<CompatibleArrayType, typename basic_json_t::iterator>::value and
                  not std::is_same<CompatibleArrayType, typename basic_json_t::const_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename basic_json_t::reverse_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename basic_json_t::const_reverse_iterator>::value and
                  not std::is_same<CompatibleArrayType, typename array_t::iterator>::value and
                  not std::is_same<CompatibleArrayType, typename array_t::const_iterator>::value and
                  std::is_constructible<basic_json, typename CompatibleArrayType::value_type>::value, int>::type
              = 0>
    basic_json(const CompatibleArrayType& val)
        : m_type(value_t::array)
    {
        using std::begin;
        using std::end;
        m_value.array = create<array_t>(begin(val), end(val));
    }

    basic_json(const string_t& val)
        : m_type(value_t::string), m_value(val)
    {}

    basic_json(const typename string_t::value_type* val)
        : basic_json(string_t(val))
    {}

    template <class CompatibleStringType, typename
              std::enable_if<
                  std::is_constructible<string_t, CompatibleStringType>::value, int>::type
              = 0>
    basic_json(const CompatibleStringType& val)
        : basic_json(string_t(val))
    {}

    basic_json(boolean_t val)
        : m_type(value_t::boolean), m_value(val)
    {}

    template<typename T,
             typename std::enable_if<
                 not (std::is_same<T, int>::value)
                 and std::is_same<T, number_integer_t>::value
                 , int>::type
             = 0>
    basic_json(const number_integer_t val)
        : m_type(value_t::number_integer), m_value(val)
    {}

    basic_json(const int val)
        : m_type(value_t::number_integer),
          m_value(static_cast<number_integer_t>(val))
    {}

    template<typename CompatibleNumberIntegerType, typename
             std::enable_if<
                 std::is_constructible<number_integer_t, CompatibleNumberIntegerType>::value and
                 std::numeric_limits<CompatibleNumberIntegerType>::is_integer and
                 std::numeric_limits<CompatibleNumberIntegerType>::is_signed,
                 CompatibleNumberIntegerType>::type
             = 0>
    basic_json(const CompatibleNumberIntegerType val) noexcept
        : m_type(value_t::number_integer),
          m_value(static_cast<number_integer_t>(val))
    {}

    template<typename T,
             typename std::enable_if<
                 not (std::is_same<T, int>::value)
                 and std::is_same<T, number_unsigned_t>::value
                 , int>::type
             = 0>
    basic_json(const number_unsigned_t val)
        : m_type(value_t::number_unsigned), m_value(val)
    {}

    template < typename CompatibleNumberUnsignedType, typename
               std::enable_if <
                   std::is_constructible<number_unsigned_t, CompatibleNumberUnsignedType>::value and
                   std::numeric_limits<CompatibleNumberUnsignedType>::is_integer and
                   !std::numeric_limits<CompatibleNumberUnsignedType>::is_signed,
                   CompatibleNumberUnsignedType >::type
               = 0 >
    basic_json(const CompatibleNumberUnsignedType val) noexcept
        : m_type(value_t::number_unsigned),
          m_value(static_cast<number_unsigned_t>(val))
    {}

    basic_json(const number_float_t val)
        : m_type(value_t::number_float), m_value(val)
    {
        // replace infinity and NAN by null
        if (not std::isfinite(val))
        {
            m_type = value_t::null;
            m_value = json_value();
        }
    }

    template<typename CompatibleNumberFloatType, typename = typename
             std::enable_if<
                 std::is_constructible<number_float_t, CompatibleNumberFloatType>::value and
                 std::is_floating_point<CompatibleNumberFloatType>::value>::type
             >
    basic_json(const CompatibleNumberFloatType val) noexcept
        : basic_json(number_float_t(val))
    {}

    basic_json(std::initializer_list<basic_json> init,
               bool type_deduction = true,
               value_t manual_type = value_t::array)
    {
        // the initializer list could describe an object
        bool is_an_object = true;

        // check if each element is an array with two elements whose first
        // element is a string
        for (const auto& element : init)
        {
            if (not element.is_array() or element.size() != 2
                    or not element[0].is_string())
            {
                // we found an element that makes it impossible to use the
                // initializer list as object
                is_an_object = false;
                break;
            }
        }

        // adjust type if type deduction is not wanted
        if (not type_deduction)
        {
            // if array is wanted, do not create an object though possible
            if (manual_type == value_t::array)
            {
                is_an_object = false;
            }

            // if object is wanted but impossible, throw an exception
            if (manual_type == value_t::object and not is_an_object)
            {
                throw std::domain_error("cannot create object from initializer list");
            }
        }

        if (is_an_object)
        {
            // the initializer list is a list of pairs -> create object
            m_type = value_t::object;
            m_value = value_t::object;

            assert(m_value.object != nullptr);

            for (auto& element : init)
            {
                m_value.object->emplace(std::move(*(element[0].m_value.string)), std::move(element[1]));
            }
        }
        else
        {
            // the initializer list describes an array -> create array
            m_type = value_t::array;
            m_value.array = create<array_t>(std::move(init));
        }
    }

    static basic_json array(std::initializer_list<basic_json> init =
                                std::initializer_list<basic_json>())
    {
        return basic_json(init, false, value_t::array);
    }

    static basic_json object(std::initializer_list<basic_json> init =
                                 std::initializer_list<basic_json>())
    {
        return basic_json(init, false, value_t::object);
    }

    basic_json(size_type cnt, const basic_json& val)
        : m_type(value_t::array)
    {
        m_value.array = create<array_t>(cnt, val);
    }

    template <class InputIT, typename
              std::enable_if<
                  std::is_same<InputIT, typename basic_json_t::iterator>::value or
                  std::is_same<InputIT, typename basic_json_t::const_iterator>::value
                  , int>::type
              = 0>
    basic_json(InputIT first, InputIT last) : m_type(first.m_object->m_type)
    {
        // make sure iterator fits the current value
        if (first.m_object != last.m_object)
        {
            throw std::domain_error("iterators are not compatible");
        }

        // check if iterator range is complete for primitive values
        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not first.m_it.primitive_iterator.is_begin() or not last.m_it.primitive_iterator.is_end())
                {
                    throw std::out_of_range("iterators out of range");
                }
                break;
            }

            default:
            {
                break;
            }
        }

        switch (m_type)
        {
            case value_t::number_integer:
            {
                assert(first.m_object != nullptr);
                m_value.number_integer = first.m_object->m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                assert(first.m_object != nullptr);
                m_value.number_unsigned = first.m_object->m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                assert(first.m_object != nullptr);
                m_value.number_float = first.m_object->m_value.number_float;
                break;
            }

            case value_t::boolean:
            {
                assert(first.m_object != nullptr);
                m_value.boolean = first.m_object->m_value.boolean;
                break;
            }

            case value_t::string:
            {
                assert(first.m_object != nullptr);
                m_value = *first.m_object->m_value.string;
                break;
            }

            case value_t::object:
            {
                m_value.object = create<object_t>(first.m_it.object_iterator, last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                m_value.array = create<array_t>(first.m_it.array_iterator, last.m_it.array_iterator);
                break;
            }

            default:
            {
                assert(first.m_object != nullptr);
                throw std::domain_error("cannot use construct with iterators from " + first.m_object->type_name());
            }
        }
    }

    // other constructors and destructor //

    basic_json(const basic_json& other)
        : m_type(other.m_type)
    {
        switch (m_type)
        {
            case value_t::object:
            {
                assert(other.m_value.object != nullptr);
                m_value = *other.m_value.object;
                break;
            }

            case value_t::array:
            {
                assert(other.m_value.array != nullptr);
                m_value = *other.m_value.array;
                break;
            }

            case value_t::string:
            {
                assert(other.m_value.string != nullptr);
                m_value = *other.m_value.string;
                break;
            }

            case value_t::boolean:
            {
                m_value = other.m_value.boolean;
                break;
            }

            case value_t::number_integer:
            {
                m_value = other.m_value.number_integer;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value = other.m_value.number_unsigned;
                break;
            }

            case value_t::number_float:
            {
                m_value = other.m_value.number_float;
                break;
            }

            default:
            {
                break;
            }
        }
    }

    basic_json(basic_json&& other) noexcept
        : m_type(std::move(other.m_type)),
          m_value(std::move(other.m_value))
    {
        // invalidate payload
        other.m_type = value_t::null;
        other.m_value = {};
    }

    reference& operator=(basic_json other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        using std::swap;
        swap(m_type, other.m_type);
        swap(m_value, other.m_value);
        return *this;
    }

    ~basic_json()
    {
        switch (m_type)
        {
            case value_t::object:
            {
                AllocatorType<object_t> alloc;
                alloc.destroy(m_value.object);
                alloc.deallocate(m_value.object, 1);
                break;
            }

            case value_t::array:
            {
                AllocatorType<array_t> alloc;
                alloc.destroy(m_value.array);
                alloc.deallocate(m_value.array, 1);
                break;
            }

            case value_t::string:
            {
                AllocatorType<string_t> alloc;
                alloc.destroy(m_value.string);
                alloc.deallocate(m_value.string, 1);
                break;
            }

            default:
            {
                // all other types need no specific destructor
                break;
            }
        }
    }


  public:
    // object inspection //


    string_t dump(const int indent = -1) const
    {
        std::stringstream ss;

        if (indent >= 0)
        {
            dump(ss, true, static_cast<unsigned int>(indent));
        }
        else
        {
            dump(ss, false, 0);
        }

        return ss.str();
    }

    value_t type() const noexcept
    {
        return m_type;
    }

    bool is_primitive() const noexcept
    {
        return is_null() or is_string() or is_boolean() or is_number();
    }

    bool is_structured() const noexcept
    {
        return is_array() or is_object();
    }

    bool is_null() const noexcept
    {
        return m_type == value_t::null;
    }

    bool is_boolean() const noexcept
    {
        return m_type == value_t::boolean;
    }

    bool is_number() const noexcept
    {
        return is_number_integer() or is_number_float();
    }

    bool is_number_integer() const noexcept
    {
        return m_type == value_t::number_integer or m_type == value_t::number_unsigned;
    }

    bool is_number_unsigned() const noexcept
    {
        return m_type == value_t::number_unsigned;
    }

    bool is_number_float() const noexcept
    {
        return m_type == value_t::number_float;
    }

    bool is_object() const noexcept
    {
        return m_type == value_t::object;
    }

    bool is_array() const noexcept
    {
        return m_type == value_t::array;
    }

    bool is_string() const noexcept
    {
        return m_type == value_t::string;
    }

    bool is_discarded() const noexcept
    {
        return m_type == value_t::discarded;
    }

    operator value_t() const noexcept
    {
        return m_type;
    }


  private:
    // value access //

    template <class T, typename
              std::enable_if<
                  std::is_convertible<typename object_t::key_type, typename T::key_type>::value and
                  std::is_convertible<basic_json_t, typename T::mapped_type>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        if (is_object())
        {
            assert(m_value.object != nullptr);
            return T(m_value.object->begin(), m_value.object->end());
        }
        else
        {
            throw std::domain_error("type must be object, but is " + type_name());
        }
    }

    object_t get_impl(object_t*) const
    {
        if (is_object())
        {
            assert(m_value.object != nullptr);
            return *(m_value.object);
        }
        else
        {
            throw std::domain_error("type must be object, but is " + type_name());
        }
    }

    template <class T, typename
              std::enable_if<
                  std::is_convertible<basic_json_t, typename T::value_type>::value and
                  not std::is_same<basic_json_t, typename T::value_type>::value and
                  not std::is_arithmetic<T>::value and
                  not std::is_convertible<std::string, T>::value and
                  not has_mapped_type<T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        if (is_array())
        {
            T to_vector;
            assert(m_value.array != nullptr);
            std::transform(m_value.array->begin(), m_value.array->end(),
                           std::inserter(to_vector, to_vector.end()), [](basic_json i)
            {
                return i.get<typename T::value_type>();
            });
            return to_vector;
        }
        else
        {
            throw std::domain_error("type must be array, but is " + type_name());
        }
    }

    template <class T, typename
              std::enable_if<
                  std::is_convertible<basic_json_t, T>::value and
                  not std::is_same<basic_json_t, T>::value
                  , int>::type = 0>
    std::vector<T> get_impl(std::vector<T>*) const
    {
        if (is_array())
        {
            std::vector<T> to_vector;
            assert(m_value.array != nullptr);
            to_vector.reserve(m_value.array->size());
            std::transform(m_value.array->begin(), m_value.array->end(),
                           std::inserter(to_vector, to_vector.end()), [](basic_json i)
            {
                return i.get<T>();
            });
            return to_vector;
        }
        else
        {
            throw std::domain_error("type must be array, but is " + type_name());
        }
    }

    template <class T, typename
              std::enable_if<
                  std::is_same<basic_json, typename T::value_type>::value and
                  not has_mapped_type<T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        if (is_array())
        {
            assert(m_value.array != nullptr);
            return T(m_value.array->begin(), m_value.array->end());
        }
        else
        {
            throw std::domain_error("type must be array, but is " + type_name());
        }
    }

    array_t get_impl(array_t*) const
    {
        if (is_array())
        {
            assert(m_value.array != nullptr);
            return *(m_value.array);
        }
        else
        {
            throw std::domain_error("type must be array, but is " + type_name());
        }
    }

    template <typename T, typename
              std::enable_if<
                  std::is_convertible<string_t, T>::value
                  , int>::type = 0>
    T get_impl(T*) const
    {
        if (is_string())
        {
            assert(m_value.string != nullptr);
            return *m_value.string;
        }
        else
        {
            throw std::domain_error("type must be string, but is " + type_name());
        }
    }

    template<typename T, typename
             std::enable_if<
                 std::is_arithmetic<T>::value
                 , int>::type = 0>
    T get_impl(T*) const
    {
        switch (m_type)
        {
            case value_t::number_integer:
            {
                return static_cast<T>(m_value.number_integer);
            }

            case value_t::number_unsigned:
            {
                return static_cast<T>(m_value.number_unsigned);
            }

            case value_t::number_float:
            {
                return static_cast<T>(m_value.number_float);
            }

            default:
            {
                throw std::domain_error("type must be number, but is " + type_name());
            }
        }
    }

    boolean_t get_impl(boolean_t*) const
    {
        if (is_boolean())
        {
            return m_value.boolean;
        }
        else
        {
            throw std::domain_error("type must be boolean, but is " + type_name());
        }
    }

    object_t* get_impl_ptr(object_t*) noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    const object_t* get_impl_ptr(const object_t*) const noexcept
    {
        return is_object() ? m_value.object : nullptr;
    }

    array_t* get_impl_ptr(array_t*) noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    const array_t* get_impl_ptr(const array_t*) const noexcept
    {
        return is_array() ? m_value.array : nullptr;
    }

    string_t* get_impl_ptr(string_t*) noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    const string_t* get_impl_ptr(const string_t*) const noexcept
    {
        return is_string() ? m_value.string : nullptr;
    }

    boolean_t* get_impl_ptr(boolean_t*) noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    const boolean_t* get_impl_ptr(const boolean_t*) const noexcept
    {
        return is_boolean() ? &m_value.boolean : nullptr;
    }

    number_integer_t* get_impl_ptr(number_integer_t*) noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    const number_integer_t* get_impl_ptr(const number_integer_t*) const noexcept
    {
        return is_number_integer() ? &m_value.number_integer : nullptr;
    }

    number_unsigned_t* get_impl_ptr(number_unsigned_t*) noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    const number_unsigned_t* get_impl_ptr(const number_unsigned_t*) const noexcept
    {
        return is_number_unsigned() ? &m_value.number_unsigned : nullptr;
    }

    number_float_t* get_impl_ptr(number_float_t*) noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    const number_float_t* get_impl_ptr(const number_float_t*) const noexcept
    {
        return is_number_float() ? &m_value.number_float : nullptr;
    }

    template<typename ReferenceType, typename ThisType>
    static ReferenceType get_ref_impl(ThisType& obj)
    {
        // delegate the call to get_ptr<>()
        using PointerType = typename std::add_pointer<ReferenceType>::type;
        auto ptr = obj.template get_ptr<PointerType>();

        if (ptr != nullptr)
        {
            return *ptr;
        }
        else
        {
            throw std::domain_error("incompatible ReferenceType for get_ref, actual type is " +
                                    obj.type_name());
        }
    }

  public:


    template<typename ValueType, typename
             std::enable_if<
                 not std::is_pointer<ValueType>::value
                 , int>::type = 0>
    ValueType get() const
    {
        return get_impl(static_cast<ValueType*>(nullptr));
    }

    template<typename PointerType, typename
             std::enable_if<
                 std::is_pointer<PointerType>::value
                 , int>::type = 0>
    PointerType get() noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    template<typename PointerType, typename
             std::enable_if<
                 std::is_pointer<PointerType>::value
                 , int>::type = 0>
    const PointerType get() const noexcept
    {
        // delegate the call to get_ptr
        return get_ptr<PointerType>();
    }

    template<typename PointerType, typename
             std::enable_if<
                 std::is_pointer<PointerType>::value
                 , int>::type = 0>
    PointerType get_ptr() noexcept
    {
        // delegate the call to get_impl_ptr<>()
        return get_impl_ptr(static_cast<PointerType>(nullptr));
    }

    template<typename PointerType, typename
             std::enable_if<
                 std::is_pointer<PointerType>::value
                 and std::is_const<typename std::remove_pointer<PointerType>::type>::value
                 , int>::type = 0>
    const PointerType get_ptr() const noexcept
    {
        // delegate the call to get_impl_ptr<>() const
        return get_impl_ptr(static_cast<const PointerType>(nullptr));
    }

    template<typename ReferenceType, typename
             std::enable_if<
                 std::is_reference<ReferenceType>::value
                 , int>::type = 0>
    ReferenceType get_ref()
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    template<typename ReferenceType, typename
             std::enable_if<
                 std::is_reference<ReferenceType>::value
                 and std::is_const<typename std::remove_reference<ReferenceType>::type>::value
                 , int>::type = 0>
    ReferenceType get_ref() const
    {
        // delegate call to get_ref_impl
        return get_ref_impl<ReferenceType>(*this);
    }

    template < typename ValueType, typename
               std::enable_if <
                   not std::is_pointer<ValueType>::value
                   and not std::is_same<ValueType, typename string_t::value_type>::value
#ifndef _MSC_VER  // Fix for issue #167 operator<< abiguity under VS2015
                   and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
#endif
                   , int >::type = 0 >
    operator ValueType() const
    {
        // delegate the call to get<>() const
        return get<ValueType>();
    }



    // element access //


    reference at(size_type idx)
    {
        // at only works for arrays
        if (is_array())
        {
            try
            {
                assert(m_value.array != nullptr);
                return m_value.array->at(idx);
            }
            catch (std::out_of_range&)
            {
                // create better exception explanation
                throw std::out_of_range("array index " + std::to_string(idx) + " is out of range");
            }
        }
        else
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }
    }

    const_reference at(size_type idx) const
    {
        // at only works for arrays
        if (is_array())
        {
            try
            {
                assert(m_value.array != nullptr);
                return m_value.array->at(idx);
            }
            catch (std::out_of_range&)
            {
                // create better exception explanation
                throw std::out_of_range("array index " + std::to_string(idx) + " is out of range");
            }
        }
        else
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }
    }

    reference at(const typename object_t::key_type& key)
    {
        // at only works for objects
        if (is_object())
        {
            try
            {
                assert(m_value.object != nullptr);
                return m_value.object->at(key);
            }
            catch (std::out_of_range&)
            {
                // create better exception explanation
                throw std::out_of_range("key '" + key + "' not found");
            }
        }
        else
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }
    }

    const_reference at(const typename object_t::key_type& key) const
    {
        // at only works for objects
        if (is_object())
        {
            try
            {
                assert(m_value.object != nullptr);
                return m_value.object->at(key);
            }
            catch (std::out_of_range&)
            {
                // create better exception explanation
                throw std::out_of_range("key '" + key + "' not found");
            }
        }
        else
        {
            throw std::domain_error("cannot use at() with " + type_name());
        }
    }

    reference operator[](size_type idx)
    {
        // implicitly convert null to object
        if (is_null())
        {
            m_type = value_t::array;
            m_value.array = create<array_t>();
        }

        // [] only works for arrays
        if (is_array())
        {
            assert(m_value.array != nullptr);
            for (size_t i = m_value.array->size(); i <= idx; ++i)
            {
                m_value.array->push_back(basic_json());
            }

            return m_value.array->operator[](idx);
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    const_reference operator[](size_type idx) const
    {
        // at only works for arrays
        if (is_array())
        {
            assert(m_value.array != nullptr);
            return m_value.array->operator[](idx);
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    reference operator[](const typename object_t::key_type& key)
    {
        // implicitly convert null to object
        if (is_null())
        {
            m_type = value_t::object;
            m_value.object = create<object_t>();
        }

        // [] only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            return m_value.object->operator[](key);
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    const_reference operator[](const typename object_t::key_type& key) const
    {
        // [] only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    template<typename T, std::size_t n>
    reference operator[](T * (&key)[n])
    {
        return operator[](static_cast<const T>(key));
    }

    template<typename T, std::size_t n>
    const_reference operator[](T * (&key)[n]) const
    {
        return operator[](static_cast<const T>(key));
    }

    template<typename T>
    reference operator[](T* key)
    {
        // implicitly convert null to object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
        }

        // at only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            return m_value.object->operator[](key);
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    template<typename T>
    const_reference operator[](T* key) const
    {
        // at only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            assert(m_value.object->find(key) != m_value.object->end());
            return m_value.object->find(key)->second;
        }
        else
        {
            throw std::domain_error("cannot use operator[] with " + type_name());
        }
    }

    template <class ValueType, typename
              std::enable_if<
                  std::is_convertible<basic_json_t, ValueType>::value
                  , int>::type = 0>
    ValueType value(const typename object_t::key_type& key, ValueType default_value) const
    {
        // at only works for objects
        if (is_object())
        {
            // if key is found, return value and given default value otherwise
            const auto it = find(key);
            if (it != end())
            {
                return *it;
            }
            else
            {
                return default_value;
            }
        }
        else
        {
            throw std::domain_error("cannot use value() with " + type_name());
        }
    }

    string_t value(const typename object_t::key_type& key, const char* default_value) const
    {
        return value(key, string_t(default_value));
    }

    reference front()
    {
        return *begin();
    }

    const_reference front() const
    {
        return *cbegin();
    }

    reference back()
    {
        auto tmp = end();
        --tmp;
        return *tmp;
    }

    const_reference back() const
    {
        auto tmp = cend();
        --tmp;
        return *tmp;
    }

    template <class InteratorType, typename
              std::enable_if<
                  std::is_same<InteratorType, typename basic_json_t::iterator>::value or
                  std::is_same<InteratorType, typename basic_json_t::const_iterator>::value
                  , int>::type
              = 0>
    InteratorType erase(InteratorType pos)
    {
        // make sure iterator fits the current value
        if (this != pos.m_object)
        {
            throw std::domain_error("iterator does not fit current value");
        }

        InteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not pos.m_it.primitive_iterator.is_begin())
                {
                    throw std::out_of_range("iterator out of range");
                }

                if (is_string())
                {
                    delete m_value.string;
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                break;
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                result.m_it.object_iterator = m_value.object->erase(pos.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);
                result.m_it.array_iterator = m_value.array->erase(pos.m_it.array_iterator);
                break;
            }

            default:
            {
                throw std::domain_error("cannot use erase() with " + type_name());
            }
        }

        return result;
    }

    template <class InteratorType, typename
              std::enable_if<
                  std::is_same<InteratorType, typename basic_json_t::iterator>::value or
                  std::is_same<InteratorType, typename basic_json_t::const_iterator>::value
                  , int>::type
              = 0>
    InteratorType erase(InteratorType first, InteratorType last)
    {
        // make sure iterator fits the current value
        if (this != first.m_object or this != last.m_object)
        {
            throw std::domain_error("iterators do not fit current value");
        }

        InteratorType result = end();

        switch (m_type)
        {
            case value_t::boolean:
            case value_t::number_float:
            case value_t::number_integer:
            case value_t::number_unsigned:
            case value_t::string:
            {
                if (not first.m_it.primitive_iterator.is_begin() or not last.m_it.primitive_iterator.is_end())
                {
                    throw std::out_of_range("iterators out of range");
                }

                if (is_string())
                {
                    delete m_value.string;
                    m_value.string = nullptr;
                }

                m_type = value_t::null;
                break;
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                result.m_it.object_iterator = m_value.object->erase(first.m_it.object_iterator,
                                              last.m_it.object_iterator);
                break;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);
                result.m_it.array_iterator = m_value.array->erase(first.m_it.array_iterator,
                                             last.m_it.array_iterator);
                break;
            }

            default:
            {
                throw std::domain_error("cannot use erase() with " + type_name());
            }
        }

        return result;
    }

    size_type erase(const typename object_t::key_type& key)
    {
        // this erase only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            return m_value.object->erase(key);
        }
        else
        {
            throw std::domain_error("cannot use erase() with " + type_name());
        }
    }

    void erase(const size_type idx)
    {
        // this erase only works for arrays
        if (is_array())
        {
            if (idx >= size())
            {
                throw std::out_of_range("index out of range");
            }

            assert(m_value.array != nullptr);
            m_value.array->erase(m_value.array->begin() + static_cast<difference_type>(idx));
        }
        else
        {
            throw std::domain_error("cannot use erase() with " + type_name());
        }
    }

    iterator find(typename object_t::key_type key)
    {
        auto result = end();

        if (is_object())
        {
            assert(m_value.object != nullptr);
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    const_iterator find(typename object_t::key_type key) const
    {
        auto result = cend();

        if (is_object())
        {
            assert(m_value.object != nullptr);
            result.m_it.object_iterator = m_value.object->find(key);
        }

        return result;
    }

    size_type count(typename object_t::key_type key) const
    {
        // return 0 for all nonobject types
        assert(not is_object() or m_value.object != nullptr);
        return is_object() ? m_value.object->count(key) : 0;
    }



    // iterators //


    iterator begin()
    {
        iterator result(this);
        result.set_begin();
        return result;
    }

    const_iterator begin() const
    {
        return cbegin();
    }

    const_iterator cbegin() const
    {
        const_iterator result(this);
        result.set_begin();
        return result;
    }

    iterator end()
    {
        iterator result(this);
        result.set_end();
        return result;
    }

    const_iterator end() const
    {
        return cend();
    }

    const_iterator cend() const
    {
        const_iterator result(this);
        result.set_end();
        return result;
    }

    reverse_iterator rbegin()
    {
        return reverse_iterator(end());
    }

    const_reverse_iterator rbegin() const
    {
        return crbegin();
    }

    reverse_iterator rend()
    {
        return reverse_iterator(begin());
    }

    const_reverse_iterator rend() const
    {
        return crend();
    }

    const_reverse_iterator crbegin() const
    {
        return const_reverse_iterator(cend());
    }

    const_reverse_iterator crend() const
    {
        return const_reverse_iterator(cbegin());
    }

  private:
    // forward declaration
    template<typename IteratorType> class iteration_proxy;

  public:
    static iteration_proxy<iterator> iterator_wrapper(reference cont)
    {
        return iteration_proxy<iterator>(cont);
    }

    static iteration_proxy<const_iterator> iterator_wrapper(const_reference cont)
    {
        return iteration_proxy<const_iterator>(cont);
    }



    // capacity //


    bool empty() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return true;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);
                return m_value.array->empty();
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                return m_value.object->empty();
            }

            default:
            {
                // all other types are nonempty
                return false;
            }
        }
    }

    size_type size() const noexcept
    {
        switch (m_type)
        {
            case value_t::null:
            {
                // null values are empty
                return 0;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);
                return m_value.array->size();
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                return m_value.object->size();
            }

            default:
            {
                // all other types have size 1
                return 1;
            }
        }
    }

    size_type max_size() const noexcept
    {
        switch (m_type)
        {
            case value_t::array:
            {
                assert(m_value.array != nullptr);
                return m_value.array->max_size();
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                return m_value.object->max_size();
            }

            default:
            {
                // all other types have max_size() == size()
                return size();
            }
        }
    }



    // modifiers //


    void clear() noexcept
    {
        switch (m_type)
        {
            case value_t::number_integer:
            {
                m_value.number_integer = 0;
                break;
            }

            case value_t::number_unsigned:
            {
                m_value.number_unsigned = 0;
                break;
            }

            case value_t::number_float:
            {
                m_value.number_float = 0.0;
                break;
            }

            case value_t::boolean:
            {
                m_value.boolean = false;
                break;
            }

            case value_t::string:
            {
                assert(m_value.string != nullptr);
                m_value.string->clear();
                break;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);
                m_value.array->clear();
                break;
            }

            case value_t::object:
            {
                assert(m_value.object != nullptr);
                m_value.object->clear();
                break;
            }

            default:
            {
                break;
            }
        }
    }

    void push_back(basic_json&& val)
    {
        // push_back only works for null objects or arrays
        if (not(is_null() or is_array()))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
        }

        // add element to array (move semantics)
        assert(m_value.array != nullptr);
        m_value.array->push_back(std::move(val));
        // invalidate object
        val.m_type = value_t::null;
    }

    reference operator+=(basic_json&& val)
    {
        push_back(std::move(val));
        return *this;
    }

    void push_back(const basic_json& val)
    {
        // push_back only works for null objects or arrays
        if (not(is_null() or is_array()))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an array
        if (is_null())
        {
            m_type = value_t::array;
            m_value = value_t::array;
        }

        // add element to array
        assert(m_value.array != nullptr);
        m_value.array->push_back(val);
    }

    reference operator+=(const basic_json& val)
    {
        push_back(val);
        return *this;
    }

    void push_back(const typename object_t::value_type& val)
    {
        // push_back only works for null objects or objects
        if (not(is_null() or is_object()))
        {
            throw std::domain_error("cannot use push_back() with " + type_name());
        }

        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
        }

        // add element to array
        assert(m_value.object != nullptr);
        m_value.object->insert(val);
    }

    reference operator+=(const typename object_t::value_type& val)
    {
        push_back(val);
        return operator[](val.first);
    }

    iterator insert(const_iterator pos, const basic_json& val)
    {
        // insert only works for arrays
        if (is_array())
        {
            // check if iterator pos fits to this JSON value
            if (pos.m_object != this)
            {
                throw std::domain_error("iterator does not fit current value");
            }

            // insert to array and return iterator
            iterator result(this);
            assert(m_value.array != nullptr);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, val);
            return result;
        }
        else
        {
            throw std::domain_error("cannot use insert() with " + type_name());
        }
    }

    iterator insert(const_iterator pos, basic_json&& val)
    {
        return insert(pos, val);
    }

    iterator insert(const_iterator pos, size_type cnt, const basic_json& val)
    {
        // insert only works for arrays
        if (is_array())
        {
            // check if iterator pos fits to this JSON value
            if (pos.m_object != this)
            {
                throw std::domain_error("iterator does not fit current value");
            }

            // insert to array and return iterator
            iterator result(this);
            assert(m_value.array != nullptr);
            result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
            return result;
        }
        else
        {
            throw std::domain_error("cannot use insert() with " + type_name());
        }
    }

    iterator insert(const_iterator pos, const_iterator first, const_iterator last)
    {
        // insert only works for arrays
        if (not is_array())
        {
            throw std::domain_error("cannot use insert() with " + type_name());
        }

        // check if iterator pos fits to this JSON value
        if (pos.m_object != this)
        {
            throw std::domain_error("iterator does not fit current value");
        }

        if (first.m_object != last.m_object)
        {
            throw std::domain_error("iterators do not fit");
        }

        if (first.m_object == this or last.m_object == this)
        {
            throw std::domain_error("passed iterators may not belong to container");
        }

        // insert to array and return iterator
        iterator result(this);
        assert(m_value.array != nullptr);
        result.m_it.array_iterator = m_value.array->insert(
                                         pos.m_it.array_iterator,
                                         first.m_it.array_iterator,
                                         last.m_it.array_iterator);
        return result;
    }

    iterator insert(const_iterator pos, std::initializer_list<basic_json> ilist)
    {
        // insert only works for arrays
        if (not is_array())
        {
            throw std::domain_error("cannot use insert() with " + type_name());
        }

        // check if iterator pos fits to this JSON value
        if (pos.m_object != this)
        {
            throw std::domain_error("iterator does not fit current value");
        }

        // insert to array and return iterator
        iterator result(this);
        assert(m_value.array != nullptr);
        result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist);
        return result;
    }

    void swap(reference other) noexcept (
        std::is_nothrow_move_constructible<value_t>::value and
        std::is_nothrow_move_assignable<value_t>::value and
        std::is_nothrow_move_constructible<json_value>::value and
        std::is_nothrow_move_assignable<json_value>::value
    )
    {
        std::swap(m_type, other.m_type);
        std::swap(m_value, other.m_value);
    }

    void swap(array_t& other)
    {
        // swap only works for arrays
        if (is_array())
        {
            assert(m_value.array != nullptr);
            std::swap(*(m_value.array), other);
        }
        else
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }
    }

    void swap(object_t& other)
    {
        // swap only works for objects
        if (is_object())
        {
            assert(m_value.object != nullptr);
            std::swap(*(m_value.object), other);
        }
        else
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }
    }

    void swap(string_t& other)
    {
        // swap only works for strings
        if (is_string())
        {
            assert(m_value.string != nullptr);
            std::swap(*(m_value.string), other);
        }
        else
        {
            throw std::domain_error("cannot use swap() with " + type_name());
        }
    }



    // lexicographical comparison operators //


  private:
    friend bool operator<(const value_t lhs, const value_t rhs)
    {
        static constexpr std::array<uint8_t, 8> order = {{
                0, // null
                3, // object
                4, // array
                5, // string
                1, // boolean
                2, // integer
                2, // unsigned
                2, // float
            }
        };

        // discarded values are not comparable
        if (lhs == value_t::discarded or rhs == value_t::discarded)
        {
            return false;
        }

        return order[static_cast<std::size_t>(lhs)] < order[static_cast<std::size_t>(rhs)];
    }

  public:
    friend bool operator==(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                {
                    assert(lhs.m_value.array != nullptr);
                    assert(rhs.m_value.array != nullptr);
                    return *lhs.m_value.array == *rhs.m_value.array;
                }
                case value_t::object:
                {
                    assert(lhs.m_value.object != nullptr);
                    assert(rhs.m_value.object != nullptr);
                    return *lhs.m_value.object == *rhs.m_value.object;
                }
                case value_t::null:
                {
                    return true;
                }
                case value_t::string:
                {
                    assert(lhs.m_value.string != nullptr);
                    assert(rhs.m_value.string != nullptr);
                    return *lhs.m_value.string == *rhs.m_value.string;
                }
                case value_t::boolean:
                {
                    return lhs.m_value.boolean == rhs.m_value.boolean;
                }
                case value_t::number_integer:
                {
                    return lhs.m_value.number_integer == rhs.m_value.number_integer;
                }
                case value_t::number_unsigned:
                {
                    return lhs.m_value.number_unsigned == rhs.m_value.number_unsigned;
                }
                case value_t::number_float:
                {
                    return lhs.m_value.number_float == rhs.m_value.number_float;
                }
                default:
                {
                    return false;
                }
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_integer) == rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_float == static_cast<number_float_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return static_cast<number_integer_t>(lhs.m_value.number_unsigned) == rhs.m_value.number_integer;
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_integer == static_cast<number_integer_t>(rhs.m_value.number_unsigned);
        }

        return false;
    }

    friend bool operator==(const_reference v, std::nullptr_t) noexcept
    {
        return v.is_null();
    }

    friend bool operator==(std::nullptr_t, const_reference v) noexcept
    {
        return v.is_null();
    }

    friend bool operator!=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs == rhs);
    }

    friend bool operator!=(const_reference v, std::nullptr_t) noexcept
    {
        return not v.is_null();
    }

    friend bool operator!=(std::nullptr_t, const_reference v) noexcept
    {
        return not v.is_null();
    }

    friend bool operator<(const_reference lhs, const_reference rhs) noexcept
    {
        const auto lhs_type = lhs.type();
        const auto rhs_type = rhs.type();

        if (lhs_type == rhs_type)
        {
            switch (lhs_type)
            {
                case value_t::array:
                {
                    assert(lhs.m_value.array != nullptr);
                    assert(rhs.m_value.array != nullptr);
                    return *lhs.m_value.array < *rhs.m_value.array;
                }
                case value_t::object:
                {
                    assert(lhs.m_value.object != nullptr);
                    assert(rhs.m_value.object != nullptr);
                    return *lhs.m_value.object < *rhs.m_value.object;
                }
                case value_t::null:
                {
                    return false;
                }
                case value_t::string:
                {
                    assert(lhs.m_value.string != nullptr);
                    assert(rhs.m_value.string != nullptr);
                    return *lhs.m_value.string < *rhs.m_value.string;
                }
                case value_t::boolean:
                {
                    return lhs.m_value.boolean < rhs.m_value.boolean;
                }
                case value_t::number_integer:
                {
                    return lhs.m_value.number_integer < rhs.m_value.number_integer;
                }
                case value_t::number_unsigned:
                {
                    return lhs.m_value.number_unsigned < rhs.m_value.number_unsigned;
                }
                case value_t::number_float:
                {
                    return lhs.m_value.number_float < rhs.m_value.number_float;
                }
                default:
                {
                    return false;
                }
            }
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_integer) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_integer)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_integer);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_float)
        {
            return static_cast<number_float_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_float;
        }
        else if (lhs_type == value_t::number_float and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_float < static_cast<number_float_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_integer and rhs_type == value_t::number_unsigned)
        {
            return lhs.m_value.number_integer < static_cast<number_integer_t>(rhs.m_value.number_unsigned);
        }
        else if (lhs_type == value_t::number_unsigned and rhs_type == value_t::number_integer)
        {
            return static_cast<number_integer_t>(lhs.m_value.number_unsigned) < rhs.m_value.number_integer;
        }

        // We only reach this line if we cannot compare values. In that case,
        // we compare types. Note we have to call the operator explicitly,
        // because MSVC has problems otherwise.
        return operator<(lhs_type, rhs_type);
    }

    friend bool operator<=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (rhs < lhs);
    }

    friend bool operator>(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs <= rhs);
    }

    friend bool operator>=(const_reference lhs, const_reference rhs) noexcept
    {
        return not (lhs < rhs);
    }



    // serialization //


    friend std::ostream& operator<<(std::ostream& o, const basic_json& j)
    {
        // read width member and use it as indentation parameter if nonzero
        const bool pretty_print = (o.width() > 0);
        const auto indentation = (pretty_print ? o.width() : 0);

        // reset width to 0 for subsequent calls to this stream
        o.width(0);

        // do the actual serialization
        j.dump(o, pretty_print, static_cast<unsigned int>(indentation));
        return o;
    }

    friend std::ostream& operator>>(const basic_json& j, std::ostream& o)
    {
        return o << j;
    }



    // deserialization //


    static basic_json parse(const string_t& s, parser_callback_t cb = nullptr)
    {
        return parser(s, cb).parse();
    }

    static basic_json parse(std::istream& i, parser_callback_t cb = nullptr)
    {
        return parser(i, cb).parse();
    }

    static basic_json parse(std::istream&& i, parser_callback_t cb = nullptr)
    {
        return parser(i, cb).parse();
    }

    friend std::istream& operator<<(basic_json& j, std::istream& i)
    {
        j = parser(i).parse();
        return i;
    }

    friend std::istream& operator>>(std::istream& i, basic_json& j)
    {
        j = parser(i).parse();
        return i;
    }



  private:
    // convenience functions //

    string_t type_name() const
    {
        switch (m_type)
        {
            case value_t::null:
                return "null";
            case value_t::object:
                return "object";
            case value_t::array:
                return "array";
            case value_t::string:
                return "string";
            case value_t::boolean:
                return "boolean";
            case value_t::discarded:
                return "discarded";
            default:
                return "number";
        }
    }

    static std::size_t extra_space(const string_t& s) noexcept
    {
        std::size_t result = 0;

        for (const auto& c : s)
        {
            switch (c)
            {
                case '"':
                case '\\':
                case '\b':
                case '\f':
                case '\n':
                case '\r':
                case '\t':
                {
                    // from c (1 byte) to \x (2 bytes)
                    result += 1;
                    break;
                }

                default:
                {
                    if (c >= 0x00 and c <= 0x1f)
                    {
                        // from c (1 byte) to \uxxxx (6 bytes)
                        result += 5;
                    }
                    break;
                }
            }
        }

        return result;
    }

    static string_t escape_string(const string_t& s) noexcept
    {
        const auto space = extra_space(s);
        if (space == 0)
        {
            return s;
        }

        // create a result string of necessary size
        string_t result(s.size() + space, '\\');
        std::size_t pos = 0;

        for (const auto& c : s)
        {
            switch (c)
            {
                // quotation mark (0x22)
                case '"':
                {
                    result[pos + 1] = '"';
                    pos += 2;
                    break;
                }

                // reverse solidus (0x5c)
                case '\\':
                {
                    // nothing to change
                    pos += 2;
                    break;
                }

                // backspace (0x08)
                case '\b':
                {
                    result[pos + 1] = 'b';
                    pos += 2;
                    break;
                }

                // formfeed (0x0c)
                case '\f':
                {
                    result[pos + 1] = 'f';
                    pos += 2;
                    break;
                }

                // newline (0x0a)
                case '\n':
                {
                    result[pos + 1] = 'n';
                    pos += 2;
                    break;
                }

                // carriage return (0x0d)
                case '\r':
                {
                    result[pos + 1] = 'r';
                    pos += 2;
                    break;
                }

                // horizontal tab (0x09)
                case '\t':
                {
                    result[pos + 1] = 't';
                    pos += 2;
                    break;
                }

                default:
                {
                    if (c >= 0x00 and c <= 0x1f)
                    {
                        // convert a number 0..15 to its hex representation (0..f)
                        auto hexify = [](const char v) -> char
                        {
                            return (v < 10) ? ('0' + v) : ('a' + v - 10);
                        };

                        // print character c as \uxxxx
                        for (const char m :
                    { 'u', '0', '0', hexify(c >> 4), hexify(c & 0x0f)
                        })
                        {
                            result[++pos] = m;
                        }

                        ++pos;
                    }
                    else
                    {
                        // all other characters are added as-is
                        result[pos++] = c;
                    }
                    break;
                }
            }
        }

        return result;
    }

    void dump(std::ostream& o,
              const bool pretty_print,
              const unsigned int indent_step,
              const unsigned int current_indent = 0) const
    {
        // variable to hold indentation for recursive calls
        unsigned int new_indent = current_indent;

        switch (m_type)
        {
            case value_t::object:
            {
                assert(m_value.object != nullptr);

                if (m_value.object->empty())
                {
                    o << "{}";
                    return;
                }

                o << "{";

                // increase indentation
                if (pretty_print)
                {
                    new_indent += indent_step;
                    o << "\n";
                }

                for (auto i = m_value.object->cbegin(); i != m_value.object->cend(); ++i)
                {
                    if (i != m_value.object->cbegin())
                    {
                        o << (pretty_print ? ",\n" : ",");
                    }
                    o << string_t(new_indent, ' ') << "\""
                      << escape_string(i->first) << "\":"
                      << (pretty_print ? " " : "");
                    i->second.dump(o, pretty_print, indent_step, new_indent);
                }

                // decrease indentation
                if (pretty_print)
                {
                    new_indent -= indent_step;
                    o << "\n";
                }

                o << string_t(new_indent, ' ') + "}";
                return;
            }

            case value_t::array:
            {
                assert(m_value.array != nullptr);

                if (m_value.array->empty())
                {
                    o << "[]";
                    return;
                }

                o << "[";

                // increase indentation
                if (pretty_print)
                {
                    new_indent += indent_step;
                    o << "\n";
                }

                for (auto i = m_value.array->cbegin(); i != m_value.array->cend(); ++i)
                {
                    if (i != m_value.array->cbegin())
                    {
                        o << (pretty_print ? ",\n" : ",");
                    }
                    o << string_t(new_indent, ' ');
                    i->dump(o, pretty_print, indent_step, new_indent);
                }

                // decrease indentation
                if (pretty_print)
                {
                    new_indent -= indent_step;
                    o << "\n";
                }

                o << string_t(new_indent, ' ') << "]";
                return;
            }

            case value_t::string:
            {
                assert(m_value.string != nullptr);
                o << string_t("\"") << escape_string(*m_value.string) << "\"";
                return;
            }

            case value_t::boolean:
            {
                o << (m_value.boolean ? "true" : "false");
                return;
            }

            case value_t::number_integer:
            {
                o << m_value.number_integer;
                return;
            }

            case value_t::number_unsigned:
            {
                o << m_value.number_unsigned;
                return;
            }

            case value_t::number_float:
            {
                // If the number is an integer then output as a fixed with with
                // precision 1 to output "0.0", "1.0" etc as expected for some
                // round trip tests otherwise  15 digits of precision allows
                // round-trip IEEE 754 string->double->string; to be safe, we
                // read this value from
                // std::numeric_limits<number_float_t>::digits10
                if (std::fmod(m_value.number_float, 1) == 0)
                {
                    o << std::fixed << std::setprecision(1);
                }
                else
                {
                    // std::defaultfloat not supported in gcc version < 5
                    o.unsetf(std::ios_base::floatfield);
                    o << std::setprecision(std::numeric_limits<double>::digits10);
                }
                o << m_value.number_float;
                return;
            }

            case value_t::discarded:
            {
                o << "<discarded>";
                return;
            }

            case value_t::null:
            {
                o << "null";
                return;
            }
        }
    }

  private:
    // member variables //

    value_t m_type = value_t::null;

    json_value m_value = {};


  private:
    // iterators //

    class primitive_iterator_t
    {
      public:
        void set_begin()
        {
            m_it = begin_value;
        }

        void set_end()
        {
            m_it = end_value;
        }

        bool is_begin() const
        {
            return (m_it == begin_value);
        }

        bool is_end() const
        {
            return (m_it == end_value);
        }

        operator difference_type& ()
        {
            return m_it;
        }

        operator difference_type () const
        {
            return m_it;
        }

      private:
        static constexpr difference_type begin_value = 0;
        static constexpr difference_type end_value = begin_value + 1;

        difference_type m_it = std::numeric_limits<std::ptrdiff_t>::denorm_min();
    };

    struct internal_iterator
    {
        typename object_t::iterator object_iterator;
        typename array_t::iterator array_iterator;
        primitive_iterator_t primitive_iterator;

        internal_iterator()
            : object_iterator(), array_iterator(), primitive_iterator()
        {}
    };

    template<typename IteratorType>
    class iteration_proxy
    {
      private:
        class iteration_proxy_internal
        {
          private:
            IteratorType anchor;
            size_t array_index = 0;

          public:
            iteration_proxy_internal(IteratorType it)
                : anchor(it)
            {}

            iteration_proxy_internal& operator*()
            {
                return *this;
            }

            iteration_proxy_internal& operator++()
            {
                ++anchor;
                ++array_index;

                return *this;
            }

            bool operator!= (const iteration_proxy_internal& o) const
            {
                return anchor != o.anchor;
            }

            typename basic_json::string_t key() const
            {
                assert(anchor.m_object != nullptr);

                switch (anchor.m_object->type())
                {
                    // use integer array index as key
                    case value_t::array:
                    {
                        return std::to_string(array_index);
                    }

                    // use key from the object
                    case value_t::object:
                    {
                        return anchor.key();
                    }

                    // use an empty key for all primitive types
                    default:
                    {
                        return "";
                    }
                }
            }

            typename IteratorType::reference value() const
            {
                return anchor.value();
            }
        };

        typename IteratorType::reference container;

      public:
        iteration_proxy(typename IteratorType::reference cont)
            : container(cont)
        {}

        iteration_proxy_internal begin()
        {
            return iteration_proxy_internal(container.begin());
        }

        iteration_proxy_internal end()
        {
            return iteration_proxy_internal(container.end());
        }
    };

  public:
    class const_iterator : public std::iterator<std::random_access_iterator_tag, const basic_json>
    {
        friend class basic_json;

      public:
        using value_type = typename basic_json::value_type;
        using difference_type = typename basic_json::difference_type;
        using pointer = typename basic_json::const_pointer;
        using reference = typename basic_json::const_reference;
        using iterator_category = std::bidirectional_iterator_tag;

        const_iterator() = default;

        const_iterator(pointer object) : m_object(object)
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    m_it.object_iterator = typename object_t::iterator();
                    break;
                }

                case basic_json::value_t::array:
                {
                    m_it.array_iterator = typename array_t::iterator();
                    break;
                }

                default:
                {
                    m_it.primitive_iterator = primitive_iterator_t();
                    break;
                }
            }
        }

        const_iterator(const iterator& other) : m_object(other.m_object)
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    m_it.object_iterator = other.m_it.object_iterator;
                    break;
                }

                case basic_json::value_t::array:
                {
                    m_it.array_iterator = other.m_it.array_iterator;
                    break;
                }

                default:
                {
                    m_it.primitive_iterator = other.m_it.primitive_iterator;
                    break;
                }
            }
        }

        const_iterator(const const_iterator& other) noexcept
            : m_object(other.m_object), m_it(other.m_it)
        {}

        const_iterator& operator=(const_iterator other) noexcept(
            std::is_nothrow_move_constructible<pointer>::value and
            std::is_nothrow_move_assignable<pointer>::value and
            std::is_nothrow_move_constructible<internal_iterator>::value and
            std::is_nothrow_move_assignable<internal_iterator>::value
        )
        {
            std::swap(m_object, other.m_object);
            std::swap(m_it, other.m_it);
            return *this;
        }

      private:
        void set_begin()
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    assert(m_object->m_value.object != nullptr);
                    m_it.object_iterator = m_object->m_value.object->begin();
                    break;
                }

                case basic_json::value_t::array:
                {
                    assert(m_object->m_value.array != nullptr);
                    m_it.array_iterator = m_object->m_value.array->begin();
                    break;
                }

                case basic_json::value_t::null:
                {
                    // set to end so begin()==end() is true: null is empty
                    m_it.primitive_iterator.set_end();
                    break;
                }

                default:
                {
                    m_it.primitive_iterator.set_begin();
                    break;
                }
            }
        }

        void set_end()
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    assert(m_object->m_value.object != nullptr);
                    m_it.object_iterator = m_object->m_value.object->end();
                    break;
                }

                case basic_json::value_t::array:
                {
                    assert(m_object->m_value.array != nullptr);
                    m_it.array_iterator = m_object->m_value.array->end();
                    break;
                }

                default:
                {
                    m_it.primitive_iterator.set_end();
                    break;
                }
            }
        }

      public:
        reference operator*() const
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    assert(m_object->m_value.object);
                    assert(m_it.object_iterator != m_object->m_value.object->end());
                    return m_it.object_iterator->second;
                }

                case basic_json::value_t::array:
                {
                    assert(m_object->m_value.array);
                    assert(m_it.array_iterator != m_object->m_value.array->end());
                    return *m_it.array_iterator;
                }

                case basic_json::value_t::null:
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.primitive_iterator.is_begin())
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        pointer operator->() const
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    assert(m_object->m_value.object);
                    assert(m_it.object_iterator != m_object->m_value.object->end());
                    return &(m_it.object_iterator->second);
                }

                case basic_json::value_t::array:
                {
                    assert(m_object->m_value.array);
                    assert(m_it.array_iterator != m_object->m_value.array->end());
                    return &*m_it.array_iterator;
                }

                default:
                {
                    if (m_it.primitive_iterator.is_begin())
                    {
                        return m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        const_iterator operator++(int)
        {
            auto result = *this;
            ++(*this);
            return result;
        }

        const_iterator& operator++()
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    ++m_it.object_iterator;
                    break;
                }

                case basic_json::value_t::array:
                {
                    ++m_it.array_iterator;
                    break;
                }

                default:
                {
                    ++m_it.primitive_iterator;
                    break;
                }
            }

            return *this;
        }

        const_iterator operator--(int)
        {
            auto result = *this;
            --(*this);
            return result;
        }

        const_iterator& operator--()
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    --m_it.object_iterator;
                    break;
                }

                case basic_json::value_t::array:
                {
                    --m_it.array_iterator;
                    break;
                }

                default:
                {
                    --m_it.primitive_iterator;
                    break;
                }
            }

            return *this;
        }

        bool operator==(const const_iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    return (m_it.object_iterator == other.m_it.object_iterator);
                }

                case basic_json::value_t::array:
                {
                    return (m_it.array_iterator == other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.primitive_iterator == other.m_it.primitive_iterator);
                }
            }
        }

        bool operator!=(const const_iterator& other) const
        {
            return not operator==(other);
        }

        bool operator<(const const_iterator& other) const
        {
            // if objects are not the same, the comparison is undefined
            if (m_object != other.m_object)
            {
                throw std::domain_error("cannot compare iterators of different containers");
            }

            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    throw std::domain_error("cannot compare order of object iterators");
                }

                case basic_json::value_t::array:
                {
                    return (m_it.array_iterator < other.m_it.array_iterator);
                }

                default:
                {
                    return (m_it.primitive_iterator < other.m_it.primitive_iterator);
                }
            }
        }

        bool operator<=(const const_iterator& other) const
        {
            return not other.operator < (*this);
        }

        bool operator>(const const_iterator& other) const
        {
            return not operator<=(other);
        }

        bool operator>=(const const_iterator& other) const
        {
            return not operator<(other);
        }

        const_iterator& operator+=(difference_type i)
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    throw std::domain_error("cannot use offsets with object iterators");
                }

                case basic_json::value_t::array:
                {
                    m_it.array_iterator += i;
                    break;
                }

                default:
                {
                    m_it.primitive_iterator += i;
                    break;
                }
            }

            return *this;
        }

        const_iterator& operator-=(difference_type i)
        {
            return operator+=(-i);
        }

        const_iterator operator+(difference_type i)
        {
            auto result = *this;
            result += i;
            return result;
        }

        const_iterator operator-(difference_type i)
        {
            auto result = *this;
            result -= i;
            return result;
        }

        difference_type operator-(const const_iterator& other) const
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    throw std::domain_error("cannot use offsets with object iterators");
                }

                case basic_json::value_t::array:
                {
                    return m_it.array_iterator - other.m_it.array_iterator;
                }

                default:
                {
                    return m_it.primitive_iterator - other.m_it.primitive_iterator;
                }
            }
        }

        reference operator[](difference_type n) const
        {
            assert(m_object != nullptr);

            switch (m_object->m_type)
            {
                case basic_json::value_t::object:
                {
                    throw std::domain_error("cannot use operator[] for object iterators");
                }

                case basic_json::value_t::array:
                {
                    return *(m_it.array_iterator + n);
                }

                case basic_json::value_t::null:
                {
                    throw std::out_of_range("cannot get value");
                }

                default:
                {
                    if (m_it.primitive_iterator == -n)
                    {
                        return *m_object;
                    }
                    else
                    {
                        throw std::out_of_range("cannot get value");
                    }
                }
            }
        }

        typename object_t::key_type key() const
        {
            assert(m_object != nullptr);

            if (m_object->is_object())
            {
                return m_it.object_iterator->first;
            }
            else
            {
                throw std::domain_error("cannot use key() for non-object iterators");
            }
        }

        reference value() const
        {
            return operator*();
        }

      private:
        pointer m_object = nullptr;
        internal_iterator m_it = internal_iterator();
    };

    class iterator : public const_iterator
    {
      public:
        using base_iterator = const_iterator;
        using pointer = typename basic_json::pointer;
        using reference = typename basic_json::reference;

        iterator() = default;

        iterator(pointer object) noexcept
            : base_iterator(object)
        {}

        iterator(const iterator& other) noexcept
            : base_iterator(other)
        {}

        iterator& operator=(iterator other) noexcept(
            std::is_nothrow_move_constructible<pointer>::value and
            std::is_nothrow_move_assignable<pointer>::value and
            std::is_nothrow_move_constructible<internal_iterator>::value and
            std::is_nothrow_move_assignable<internal_iterator>::value
        )
        {
            base_iterator::operator=(other);
            return *this;
        }

        reference operator*()
        {
            return const_cast<reference>(base_iterator::operator*());
        }

        pointer operator->()
        {
            return const_cast<pointer>(base_iterator::operator->());
        }

        iterator operator++(int)
        {
            iterator result = *this;
            base_iterator::operator++();
            return result;
        }

        iterator& operator++()
        {
            base_iterator::operator++();
            return *this;
        }

        iterator operator--(int)
        {
            iterator result = *this;
            base_iterator::operator--();
            return result;
        }

        iterator& operator--()
        {
            base_iterator::operator--();
            return *this;
        }

        iterator& operator+=(difference_type i)
        {
            base_iterator::operator+=(i);
            return *this;
        }

        iterator& operator-=(difference_type i)
        {
            base_iterator::operator-=(i);
            return *this;
        }

        iterator operator+(difference_type i)
        {
            auto result = *this;
            result += i;
            return result;
        }

        iterator operator-(difference_type i)
        {
            auto result = *this;
            result -= i;
            return result;
        }

        difference_type operator-(const iterator& other) const
        {
            return base_iterator::operator-(other);
        }

        reference operator[](difference_type n) const
        {
            return const_cast<reference>(base_iterator::operator[](n));
        }

        reference value() const
        {
            return const_cast<reference>(base_iterator::value());
        }
    };

    template<typename Base>
    class json_reverse_iterator : public std::reverse_iterator<Base>
    {
      public:
        using base_iterator = std::reverse_iterator<Base>;
        using reference = typename Base::reference;

        json_reverse_iterator(const typename base_iterator::iterator_type& it)
            : base_iterator(it)
        {}

        json_reverse_iterator(const base_iterator& it)
            : base_iterator(it)
        {}

        json_reverse_iterator operator++(int)
        {
            return base_iterator::operator++(1);
        }

        json_reverse_iterator& operator++()
        {
            base_iterator::operator++();
            return *this;
        }

        json_reverse_iterator operator--(int)
        {
            return base_iterator::operator--(1);
        }

        json_reverse_iterator& operator--()
        {
            base_iterator::operator--();
            return *this;
        }

        json_reverse_iterator& operator+=(difference_type i)
        {
            base_iterator::operator+=(i);
            return *this;
        }

        json_reverse_iterator operator+(difference_type i) const
        {
            auto result = *this;
            result += i;
            return result;
        }

        json_reverse_iterator operator-(difference_type i) const
        {
            auto result = *this;
            result -= i;
            return result;
        }

        difference_type operator-(const json_reverse_iterator& other) const
        {
            return this->base() - other.base();
        }

        reference operator[](difference_type n) const
        {
            return *(this->operator+(n));
        }

        typename object_t::key_type key() const
        {
            auto it = --this->base();
            return it.key();
        }

        reference value() const
        {
            auto it = --this->base();
            return it.operator * ();
        }
    };


  private:
    // lexer and parser //

    class lexer
    {
      public:
        enum class token_type
        {
            uninitialized,    
            literal_true,     
            literal_false,    
            literal_null,     
            value_string,     
            value_number,     
            begin_array,      
            begin_object,     
            end_array,        
            end_object,       
            name_separator,   
            value_separator,  
            parse_error,      
            end_of_input      
        };

        using lexer_char_t = unsigned char;

        explicit lexer(const string_t& s) noexcept
            : m_stream(nullptr), m_buffer(s)
        {
            m_content = reinterpret_cast<const lexer_char_t*>(s.c_str());
            assert(m_content != nullptr);
            m_start = m_cursor = m_content;
            m_limit = m_content + s.size();
        }

        explicit lexer(std::istream* s) noexcept
            : m_stream(s), m_buffer()
        {
            assert(m_stream != nullptr);
            getline(*m_stream, m_buffer);
            m_content = reinterpret_cast<const lexer_char_t*>(m_buffer.c_str());
            assert(m_content != nullptr);
            m_start = m_cursor = m_content;
            m_limit = m_content + m_buffer.size();
        }

        lexer() = default;

        // switch off unwanted functions
        lexer(const lexer&) = delete;
        lexer operator=(const lexer&) = delete;

        static string_t to_unicode(const std::size_t codepoint1,
                                   const std::size_t codepoint2 = 0)
        {
            string_t result;

            // calculate the codepoint from the given code points
            std::size_t codepoint = codepoint1;

            // check if codepoint1 is a high surrogate
            if (codepoint1 >= 0xD800 and codepoint1 <= 0xDBFF)
            {
                // check if codepoint2 is a low surrogate
                if (codepoint2 >= 0xDC00 and codepoint2 <= 0xDFFF)
                {
                    codepoint =
                        // high surrogate occupies the most significant 22 bits
                        (codepoint1 << 10)
                        // low surrogate occupies the least significant 15 bits
                        + codepoint2
                        // there is still the 0xD800, 0xDC00 and 0x10000 noise
                        // in the result so we have to subtract with:
                        // (0xD800 << 10) + DC00 - 0x10000 = 0x35FDC00
                        - 0x35FDC00;
                }
                else
                {
                    throw std::invalid_argument("missing or wrong low surrogate");
                }
            }

            if (codepoint < 0x80)
            {
                // 1-byte characters: 0xxxxxxx (ASCII)
                result.append(1, static_cast<typename string_t::value_type>(codepoint));
            }
            else if (codepoint <= 0x7ff)
            {
                // 2-byte characters: 110xxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xC0 | ((codepoint >> 6) & 0x1F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else if (codepoint <= 0xffff)
            {
                // 3-byte characters: 1110xxxx 10xxxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xE0 | ((codepoint >> 12) & 0x0F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 6) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else if (codepoint <= 0x10ffff)
            {
                // 4-byte characters: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
                result.append(1, static_cast<typename string_t::value_type>(0xF0 | ((codepoint >> 18) & 0x07)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 12) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | ((codepoint >> 6) & 0x3F)));
                result.append(1, static_cast<typename string_t::value_type>(0x80 | (codepoint & 0x3F)));
            }
            else
            {
                throw std::out_of_range("code points above 0x10FFFF are invalid");
            }

            return result;
        }

        static std::string token_type_name(token_type t)
        {
            switch (t)
            {
                case token_type::uninitialized:
                    return "<uninitialized>";
                case token_type::literal_true:
                    return "true literal";
                case token_type::literal_false:
                    return "false literal";
                case token_type::literal_null:
                    return "null literal";
                case token_type::value_string:
                    return "string literal";
                case token_type::value_number:
                    return "number literal";
                case token_type::begin_array:
                    return "'['";
                case token_type::begin_object:
                    return "'{'";
                case token_type::end_array:
                    return "']'";
                case token_type::end_object:
                    return "'}'";
                case token_type::name_separator:
                    return "':'";
                case token_type::value_separator:
                    return "','";
                case token_type::parse_error:
                    return "<parse error>";
                case token_type::end_of_input:
                    return "end of input";
                default:
                {
                    // catch non-enum values
                    return "unknown token"; // LCOV_EXCL_LINE
                }
            }
        }

        token_type scan() noexcept
        {
            // pointer for backtracking information
            m_marker = nullptr;

            // remember the begin of the token
            m_start = m_cursor;
            assert(m_start != nullptr);


            {
                lexer_char_t yych;
                unsigned int yyaccept = 0;
                static const unsigned char yybm[] =
                {
                    0,   0,   0,   0,   0,   0,   0,   0,
                    0,  32,  32,   0,   0,  32,   0,   0,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    96,  64,   0,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    192, 192, 192, 192, 192, 192, 192, 192,
                    192, 192,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,   0,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                    64,  64,  64,  64,  64,  64,  64,  64,
                };
                if ((m_limit - m_cursor) < 5)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= ':')
                {
                    if (yych <= ' ')
                    {
                        if (yych <= '\n')
                        {
                            if (yych <= 0x00)
                            {
                                goto basic_json_parser_28;
                            }
                            if (yych <= 0x08)
                            {
                                goto basic_json_parser_30;
                            }
                            if (yych >= '\n')
                            {
                                goto basic_json_parser_4;
                            }
                        }
                        else
                        {
                            if (yych == '\r')
                            {
                                goto basic_json_parser_2;
                            }
                            if (yych <= 0x1F)
                            {
                                goto basic_json_parser_30;
                            }
                        }
                    }
                    else
                    {
                        if (yych <= ',')
                        {
                            if (yych == '"')
                            {
                                goto basic_json_parser_27;
                            }
                            if (yych <= '+')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_16;
                        }
                        else
                        {
                            if (yych <= '/')
                            {
                                if (yych <= '-')
                                {
                                    goto basic_json_parser_23;
                                }
                                goto basic_json_parser_30;
                            }
                            else
                            {
                                if (yych <= '0')
                                {
                                    goto basic_json_parser_24;
                                }
                                if (yych <= '9')
                                {
                                    goto basic_json_parser_26;
                                }
                                goto basic_json_parser_18;
                            }
                        }
                    }
                }
                else
                {
                    if (yych <= 'n')
                    {
                        if (yych <= ']')
                        {
                            if (yych == '[')
                            {
                                goto basic_json_parser_8;
                            }
                            if (yych <= '\\')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_10;
                        }
                        else
                        {
                            if (yych == 'f')
                            {
                                goto basic_json_parser_22;
                            }
                            if (yych <= 'm')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_20;
                        }
                    }
                    else
                    {
                        if (yych <= '{')
                        {
                            if (yych == 't')
                            {
                                goto basic_json_parser_21;
                            }
                            if (yych <= 'z')
                            {
                                goto basic_json_parser_30;
                            }
                            goto basic_json_parser_12;
                        }
                        else
                        {
                            if (yych <= '}')
                            {
                                if (yych <= '|')
                                {
                                    goto basic_json_parser_30;
                                }
                                goto basic_json_parser_14;
                            }
                            else
                            {
                                if (yych == 0xEF)
                                {
                                    goto basic_json_parser_6;
                                }
                                goto basic_json_parser_30;
                            }
                        }
                    }
                }
basic_json_parser_2:
                ++m_cursor;
                yych = *m_cursor;
                goto basic_json_parser_5;
basic_json_parser_3:
                {
                    return scan();
                }
basic_json_parser_4:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_5:
                if (yybm[0 + yych] & 32)
                {
                    goto basic_json_parser_4;
                }
                goto basic_json_parser_3;
basic_json_parser_6:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 0xBB)
                {
                    goto basic_json_parser_64;
                }
basic_json_parser_7:
                {
                    return token_type::parse_error;
                }
basic_json_parser_8:
                ++m_cursor;
                {
                    return token_type::begin_array;
                }
basic_json_parser_10:
                ++m_cursor;
                {
                    return token_type::end_array;
                }
basic_json_parser_12:
                ++m_cursor;
                {
                    return token_type::begin_object;
                }
basic_json_parser_14:
                ++m_cursor;
                {
                    return token_type::end_object;
                }
basic_json_parser_16:
                ++m_cursor;
                {
                    return token_type::value_separator;
                }
basic_json_parser_18:
                ++m_cursor;
                {
                    return token_type::name_separator;
                }
basic_json_parser_20:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'u')
                {
                    goto basic_json_parser_60;
                }
                goto basic_json_parser_7;
basic_json_parser_21:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'r')
                {
                    goto basic_json_parser_56;
                }
                goto basic_json_parser_7;
basic_json_parser_22:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych == 'a')
                {
                    goto basic_json_parser_51;
                }
                goto basic_json_parser_7;
basic_json_parser_23:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_7;
                }
                if (yych <= '0')
                {
                    goto basic_json_parser_50;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_41;
                }
                goto basic_json_parser_7;
basic_json_parser_24:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 'D')
                {
                    if (yych == '.')
                    {
                        goto basic_json_parser_43;
                    }
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_44;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_44;
                    }
                }
basic_json_parser_25:
                {
                    return token_type::value_number;
                }
basic_json_parser_26:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                goto basic_json_parser_42;
basic_json_parser_27:
                yyaccept = 0;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 0x0F)
                {
                    goto basic_json_parser_7;
                }
                goto basic_json_parser_32;
basic_json_parser_28:
                ++m_cursor;
                {
                    return token_type::end_of_input;
                }
basic_json_parser_30:
                yych = *++m_cursor;
                goto basic_json_parser_7;
basic_json_parser_31:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_32:
                if (yybm[0 + yych] & 64)
                {
                    goto basic_json_parser_31;
                }
                if (yych <= 0x0F)
                {
                    goto basic_json_parser_33;
                }
                if (yych <= '"')
                {
                    goto basic_json_parser_35;
                }
                goto basic_json_parser_34;
basic_json_parser_33:
                m_cursor = m_marker;
                if (yyaccept == 0)
                {
                    goto basic_json_parser_7;
                }
                else
                {
                    goto basic_json_parser_25;
                }
basic_json_parser_34:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= 'e')
                {
                    if (yych <= '/')
                    {
                        if (yych == '"')
                        {
                            goto basic_json_parser_31;
                        }
                        if (yych <= '.')
                        {
                            goto basic_json_parser_33;
                        }
                        goto basic_json_parser_31;
                    }
                    else
                    {
                        if (yych <= '\\')
                        {
                            if (yych <= '[')
                            {
                                goto basic_json_parser_33;
                            }
                            goto basic_json_parser_31;
                        }
                        else
                        {
                            if (yych == 'b')
                            {
                                goto basic_json_parser_31;
                            }
                            goto basic_json_parser_33;
                        }
                    }
                }
                else
                {
                    if (yych <= 'q')
                    {
                        if (yych <= 'f')
                        {
                            goto basic_json_parser_31;
                        }
                        if (yych == 'n')
                        {
                            goto basic_json_parser_31;
                        }
                        goto basic_json_parser_33;
                    }
                    else
                    {
                        if (yych <= 's')
                        {
                            if (yych <= 'r')
                            {
                                goto basic_json_parser_31;
                            }
                            goto basic_json_parser_33;
                        }
                        else
                        {
                            if (yych <= 't')
                            {
                                goto basic_json_parser_31;
                            }
                            if (yych <= 'u')
                            {
                                goto basic_json_parser_37;
                            }
                            goto basic_json_parser_33;
                        }
                    }
                }
basic_json_parser_35:
                ++m_cursor;
                {
                    return token_type::value_string;
                }
basic_json_parser_37:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_33;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_38;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_33;
                    }
                }
basic_json_parser_38:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_33;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_39;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_33;
                    }
                }
basic_json_parser_39:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= ':')
                    {
                        goto basic_json_parser_33;
                    }
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_40;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych >= 'g')
                    {
                        goto basic_json_parser_33;
                    }
                }
basic_json_parser_40:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '@')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_31;
                    }
                    goto basic_json_parser_33;
                }
                else
                {
                    if (yych <= 'F')
                    {
                        goto basic_json_parser_31;
                    }
                    if (yych <= '`')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych <= 'f')
                    {
                        goto basic_json_parser_31;
                    }
                    goto basic_json_parser_33;
                }
basic_json_parser_41:
                yyaccept = 1;
                m_marker = ++m_cursor;
                if ((m_limit - m_cursor) < 3)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
basic_json_parser_42:
                if (yybm[0 + yych] & 128)
                {
                    goto basic_json_parser_41;
                }
                if (yych <= 'D')
                {
                    if (yych != '.')
                    {
                        goto basic_json_parser_25;
                    }
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_44;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_44;
                    }
                    goto basic_json_parser_25;
                }
basic_json_parser_43:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_33;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_48;
                }
                goto basic_json_parser_33;
basic_json_parser_44:
                yych = *++m_cursor;
                if (yych <= ',')
                {
                    if (yych != '+')
                    {
                        goto basic_json_parser_33;
                    }
                }
                else
                {
                    if (yych <= '-')
                    {
                        goto basic_json_parser_45;
                    }
                    if (yych <= '/')
                    {
                        goto basic_json_parser_33;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_46;
                    }
                    goto basic_json_parser_33;
                }
basic_json_parser_45:
                yych = *++m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_33;
                }
                if (yych >= ':')
                {
                    goto basic_json_parser_33;
                }
basic_json_parser_46:
                ++m_cursor;
                if (m_limit <= m_cursor)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= '/')
                {
                    goto basic_json_parser_25;
                }
                if (yych <= '9')
                {
                    goto basic_json_parser_46;
                }
                goto basic_json_parser_25;
basic_json_parser_48:
                yyaccept = 1;
                m_marker = ++m_cursor;
                if ((m_limit - m_cursor) < 3)
                {
                    yyfill();    // LCOV_EXCL_LINE;
                }
                yych = *m_cursor;
                if (yych <= 'D')
                {
                    if (yych <= '/')
                    {
                        goto basic_json_parser_25;
                    }
                    if (yych <= '9')
                    {
                        goto basic_json_parser_48;
                    }
                    goto basic_json_parser_25;
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_44;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_44;
                    }
                    goto basic_json_parser_25;
                }
basic_json_parser_50:
                yyaccept = 1;
                yych = *(m_marker = ++m_cursor);
                if (yych <= 'D')
                {
                    if (yych == '.')
                    {
                        goto basic_json_parser_43;
                    }
                    goto basic_json_parser_25;
                }
                else
                {
                    if (yych <= 'E')
                    {
                        goto basic_json_parser_44;
                    }
                    if (yych == 'e')
                    {
                        goto basic_json_parser_44;
                    }
                    goto basic_json_parser_25;
                }
basic_json_parser_51:
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_33;
                }
                yych = *++m_cursor;
                if (yych != 's')
                {
                    goto basic_json_parser_33;
                }
                yych = *++m_cursor;
                if (yych != 'e')
                {
                    goto basic_json_parser_33;
                }
                ++m_cursor;
                {
                    return token_type::literal_false;
                }
basic_json_parser_56:
                yych = *++m_cursor;
                if (yych != 'u')
                {
                    goto basic_json_parser_33;
                }
                yych = *++m_cursor;
                if (yych != 'e')
                {
                    goto basic_json_parser_33;
                }
                ++m_cursor;
                {
                    return token_type::literal_true;
                }
basic_json_parser_60:
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_33;
                }
                yych = *++m_cursor;
                if (yych != 'l')
                {
                    goto basic_json_parser_33;
                }
                ++m_cursor;
                {
                    return token_type::literal_null;
                }
basic_json_parser_64:
                yych = *++m_cursor;
                if (yych != 0xBF)
                {
                    goto basic_json_parser_33;
                }
                ++m_cursor;
                {
                    return scan();
                }
            }


        }

        void yyfill() noexcept
        {
            if (m_stream == nullptr or not * m_stream)
            {
                return;
            }

            const ssize_t offset_start = m_start - m_content;
            const ssize_t offset_marker = m_marker - m_start;
            const ssize_t offset_cursor = m_cursor - m_start;

            m_buffer.erase(0, static_cast<size_t>(offset_start));
            std::string line;
            assert(m_stream != nullptr);
            std::getline(*m_stream, line);
            m_buffer += "\n" + line; // add line with newline symbol

            m_content = reinterpret_cast<const lexer_char_t*>(m_buffer.c_str());
            assert(m_content != nullptr);
            m_start  = m_content;
            m_marker = m_start + offset_marker;
            m_cursor = m_start + offset_cursor;
            m_limit  = m_start + m_buffer.size() - 1;
        }

        string_t get_token() const noexcept
        {
            assert(m_start != nullptr);
            return string_t(reinterpret_cast<typename string_t::const_pointer>(m_start),
                            static_cast<size_t>(m_cursor - m_start));
        }

        string_t get_string() const
        {
            string_t result;
            result.reserve(static_cast<size_t>(m_cursor - m_start - 2));

            // iterate the result between the quotes
            for (const lexer_char_t* i = m_start + 1; i < m_cursor - 1; ++i)
            {
                // process escaped characters
                if (*i == '\\')
                {
                    // read next character
                    ++i;

                    switch (*i)
                    {
                        // the default escapes
                        case 't':
                        {
                            result += "\t";
                            break;
                        }
                        case 'b':
                        {
                            result += "\b";
                            break;
                        }
                        case 'f':
                        {
                            result += "\f";
                            break;
                        }
                        case 'n':
                        {
                            result += "\n";
                            break;
                        }
                        case 'r':
                        {
                            result += "\r";
                            break;
                        }
                        case '\\':
                        {
                            result += "\\";
                            break;
                        }
                        case '/':
                        {
                            result += "/";
                            break;
                        }
                        case '"':
                        {
                            result += "\"";
                            break;
                        }

                        // unicode
                        case 'u':
                        {
                            // get code xxxx from uxxxx
                            auto codepoint = std::strtoul(std::string(reinterpret_cast<typename string_t::const_pointer>(i + 1),
                                                          4).c_str(), nullptr, 16);

                            // check if codepoint is a high surrogate
                            if (codepoint >= 0xD800 and codepoint <= 0xDBFF)
                            {
                                // make sure there is a subsequent unicode
                                if ((i + 6 >= m_limit) or * (i + 5) != '\\' or * (i + 6) != 'u')
                                {
                                    throw std::invalid_argument("missing low surrogate");
                                }

                                // get code yyyy from uxxxx\uyyyy
                                auto codepoint2 = std::strtoul(std::string(reinterpret_cast<typename string_t::const_pointer>
                                                               (i + 7), 4).c_str(), nullptr, 16);
                                result += to_unicode(codepoint, codepoint2);
                                // skip the next 10 characters (xxxx\uyyyy)
                                i += 10;
                            }
                            else
                            {
                                // add unicode character(s)
                                result += to_unicode(codepoint);
                                // skip the next four characters (xxxx)
                                i += 4;
                            }
                            break;
                        }
                    }
                }
                else
                {
                    // all other characters are just copied to the end of the
                    // string
                    result.append(1, static_cast<typename string_t::value_type>(*i));
                }
            }

            return result;
        }

        long double str_to_float_t(long double* /* type */, char** endptr) const
        {
            return std::strtold(reinterpret_cast<typename string_t::const_pointer>(m_start), endptr);
        }

        double str_to_float_t(double* /* type */, char** endptr) const
        {
            return std::strtod(reinterpret_cast<typename string_t::const_pointer>(m_start), endptr);
        }

        float str_to_float_t(float* /* type */, char** endptr) const
        {
            return std::strtof(reinterpret_cast<typename string_t::const_pointer>(m_start), endptr);
        }

        template <typename T_A, typename T_B>
        bool attempt_cast(T_A source, T_B& dest) const
        {
            dest = static_cast<T_B>(source);
            return (source == static_cast<T_A>(dest));
        }

        void get_number(basic_json& result) const
        {
            typename string_t::value_type* endptr;
            assert(m_start != nullptr);
            errno = 0;

            // attempt to parse it as an integer - first checking for a
            // negative number
            if (*reinterpret_cast<typename string_t::const_pointer>(m_start) != '-')
            {
                // positive, parse with strtoull and attempt cast to
                // number_unsigned_t
                if (attempt_cast(std::strtoull(reinterpret_cast<typename string_t::const_pointer>(m_start), &endptr,
                                               10), result.m_value.number_unsigned))
                {
                    result.m_type = value_t::number_unsigned;
                }
                else
                {
                    // cast failed due to overflow - store as float
                    result.m_type = value_t::number_float;
                }
            }
            else
            {
                // Negative, parse with strtoll and attempt cast to
                // number_integer_t
                if (attempt_cast(std::strtoll(reinterpret_cast<typename string_t::const_pointer>(m_start), &endptr,
                                              10), result.m_value.number_unsigned))
                {
                    result.m_type = value_t::number_integer;
                }
                else
                {
                    // cast failed due to overflow - store as float
                    result.m_type = value_t::number_float;
                }
            }

            // check the end of the number was reached and no range error
            // occurred
            if (reinterpret_cast<lexer_char_t*>(endptr) != m_cursor || errno == ERANGE)
            {
                result.m_type = value_t::number_float;
            }

            if (result.m_type  == value_t::number_float)
            {
                // either the number won't fit in an integer (range error from
                // strtoull/strtoll or overflow on cast) or there was something
                // else after the number, which could be an exponent

                // parse with strtod
                result.m_value.number_float = str_to_float_t(static_cast<number_float_t*>(nullptr), &endptr);

                // anything after the number is an error
                if (reinterpret_cast<lexer_char_t*>(endptr) != m_cursor)
                {
                    throw std::invalid_argument(std::string("parse error - ") + get_token() + " is not a number");
                }
            }
        }

      private:
        std::istream* m_stream = nullptr;
        string_t m_buffer;
        const lexer_char_t* m_content = nullptr;
        const lexer_char_t* m_start = nullptr;
        const lexer_char_t* m_marker = nullptr;
        const lexer_char_t* m_cursor = nullptr;
        const lexer_char_t* m_limit = nullptr;
    };

    class parser
    {
      public:
        parser(const string_t& s, parser_callback_t cb = nullptr)
            : callback(cb), m_lexer(s)
        {
            // read first token
            get_token();
        }

        parser(std::istream& _is, parser_callback_t cb = nullptr)
            : callback(cb), m_lexer(&_is)
        {
            // read first token
            get_token();
        }

        basic_json parse()
        {
            basic_json result = parse_internal(true);

            expect(lexer::token_type::end_of_input);

            // return parser result and replace it with null in case the
            // top-level value was discarded by the callback function
            return result.is_discarded() ? basic_json() : result;
        }

      private:
        basic_json parse_internal(bool keep)
        {
            auto result = basic_json(value_t::discarded);

            switch (last_token)
            {
                case lexer::token_type::begin_object:
                {
                    if (keep and (not callback or (keep = callback(depth++, parse_event_t::object_start, result))))
                    {
                        // explicitly set result to object to cope with {}
                        result.m_type = value_t::object;
                        result.m_value = json_value(value_t::object);
                    }

                    // read next token
                    get_token();

                    // closing } -> we are done
                    if (last_token == lexer::token_type::end_object)
                    {
                        get_token();
                        if (keep and callback and not callback(--depth, parse_event_t::object_end, result))
                        {
                            result = basic_json(value_t::discarded);
                        }
                        return result;
                    }

                    // no comma is expected here
                    unexpect(lexer::token_type::value_separator);

                    // otherwise: parse key-value pairs
                    do
                    {
                        // ugly, but could be fixed with loop reorganization
                        if (last_token == lexer::token_type::value_separator)
                        {
                            get_token();
                        }

                        // store key
                        expect(lexer::token_type::value_string);
                        const auto key = m_lexer.get_string();

                        bool keep_tag = false;
                        if (keep)
                        {
                            if (callback)
                            {
                                basic_json k(key);
                                keep_tag = callback(depth, parse_event_t::key, k);
                            }
                            else
                            {
                                keep_tag = true;
                            }
                        }

                        // parse separator (:)
                        get_token();
                        expect(lexer::token_type::name_separator);

                        // parse and add value
                        get_token();
                        auto value = parse_internal(keep);
                        if (keep and keep_tag and not value.is_discarded())
                        {
                            result[key] = std::move(value);
                        }
                    }
                    while (last_token == lexer::token_type::value_separator);

                    // closing }
                    expect(lexer::token_type::end_object);
                    get_token();
                    if (keep and callback and not callback(--depth, parse_event_t::object_end, result))
                    {
                        result = basic_json(value_t::discarded);
                    }

                    return result;
                }

                case lexer::token_type::begin_array:
                {
                    if (keep and (not callback or (keep = callback(depth++, parse_event_t::array_start, result))))
                    {
                        // explicitly set result to object to cope with []
                        result.m_type = value_t::array;
                        result.m_value = json_value(value_t::array);
                    }

                    // read next token
                    get_token();

                    // closing ] -> we are done
                    if (last_token == lexer::token_type::end_array)
                    {
                        get_token();
                        if (callback and not callback(--depth, parse_event_t::array_end, result))
                        {
                            result = basic_json(value_t::discarded);
                        }
                        return result;
                    }

                    // no comma is expected here
                    unexpect(lexer::token_type::value_separator);

                    // otherwise: parse values
                    do
                    {
                        // ugly, but could be fixed with loop reorganization
                        if (last_token == lexer::token_type::value_separator)
                        {
                            get_token();
                        }

                        // parse value
                        auto value = parse_internal(keep);
                        if (keep and not value.is_discarded())
                        {
                            result.push_back(std::move(value));
                        }
                    }
                    while (last_token == lexer::token_type::value_separator);

                    // closing ]
                    expect(lexer::token_type::end_array);
                    get_token();
                    if (keep and callback and not callback(--depth, parse_event_t::array_end, result))
                    {
                        result = basic_json(value_t::discarded);
                    }

                    return result;
                }

                case lexer::token_type::literal_null:
                {
                    get_token();
                    result.m_type = value_t::null;
                    break;
                }

                case lexer::token_type::value_string:
                {
                    const auto s = m_lexer.get_string();
                    get_token();
                    result = basic_json(s);
                    break;
                }

                case lexer::token_type::literal_true:
                {
                    get_token();
                    result.m_type = value_t::boolean;
                    result.m_value = true;
                    break;
                }

                case lexer::token_type::literal_false:
                {
                    get_token();
                    result.m_type = value_t::boolean;
                    result.m_value = false;
                    break;
                }

                case lexer::token_type::value_number:
                {
                    m_lexer.get_number(result);
                    get_token();
                    break;
                }

                default:
                {
                    // the last token was unexpected
                    unexpect(last_token);
                }
            }

            if (keep and callback and not callback(depth, parse_event_t::value, result))
            {
                result = basic_json(value_t::discarded);
            }
            return result;
        }

        typename lexer::token_type get_token()
        {
            last_token = m_lexer.scan();
            return last_token;
        }

        void expect(typename lexer::token_type t) const
        {
            if (t != last_token)
            {
                std::string error_msg = "parse error - unexpected ";
                error_msg += (last_token == lexer::token_type::parse_error ? ("'" +  m_lexer.get_token() + "'") :
                              lexer::token_type_name(last_token));
                error_msg += "; expected " + lexer::token_type_name(t);
                throw std::invalid_argument(error_msg);
            }
        }

        void unexpect(typename lexer::token_type t) const
        {
            if (t == last_token)
            {
                std::string error_msg = "parse error - unexpected ";
                error_msg += (last_token == lexer::token_type::parse_error ? ("'" +  m_lexer.get_token() + "'") :
                              lexer::token_type_name(last_token));
                throw std::invalid_argument(error_msg);
            }
        }

      private:
        int depth = 0;
        parser_callback_t callback;
        typename lexer::token_type last_token = lexer::token_type::uninitialized;
        lexer m_lexer;
    };
};


// presets //

using json = basic_json<>;
}


// nonmember functions //

// specialization of std::swap, and std::hash
namespace std
{
template <>
inline void swap(nlohmann::json& j1,
                 nlohmann::json& j2) noexcept(
                     is_nothrow_move_constructible<nlohmann::json>::value and
                     is_nothrow_move_assignable<nlohmann::json>::value
                 )
{
    j1.swap(j2);
}

template <>
struct hash<nlohmann::json>
{
    std::size_t operator()(const nlohmann::json& j) const
    {
        // a naive hashing via the string representation
        const auto& h = hash<nlohmann::json::string_t>();
        return h(j.dump());
    }
};
}

inline nlohmann::json operator "" _json(const char* s, std::size_t)
{
    return nlohmann::json::parse(reinterpret_cast<const nlohmann::json::string_t::value_type*>(s));
}

// restore GCC/clang diagnostic settings
#if defined(__clang__) || defined(__GNUC__) || defined(__GNUG__)
    #pragma GCC diagnostic pop
#endif

#endif