json/docs/mkdocs/docs/features/binary_formats/bjdata.md

# BJData

The [BJData format](https://neurojson.org) was derived from and improved upon
[Universal Binary JSON(UBJSON)](https://ubjson.org) specification (Draft 12).
Specifically, it introduces an optimized array container for efficient storage
of N-dimensional packed arrays (**ND-arrays**); it also adds 4 new type markers -
`[u] - uint16`, `[m] - uint32`, `[M] - uint64` and `[h] - float16` - to
unambigiously map common binary numeric types; furthermore, it uses little-endian
(LE) to store all numerics instead of big-endian (BE) as in UBJSON to avoid
unnecessary conversions on commonly available platforms.

Compared to other binary-JSON-like formats such as MessagePack and CBOR, both BJData and
UBJSON demonstrate a rare combination of being both binary and **quasi-human-readable**. This
is because all semantic elements in BJData and UBJSON, including the data-type markers
and name/string types are directly human-readable. Data stored in the BJData/UBJSON format
are not only compact in size, fast to read/write, but also can be directly searched
or read using simple processing.

!!! abstract "References"

	- [BJData Specification](https://neurojson.org/bjdata/draft2)

## Serialization

The library uses the following mapping from JSON values types to BJData types according to the BJData specification:

| JSON value type | value/range                               | BJData type    | marker |
|-----------------|-------------------------------------------|----------------|--------|
| null            | `null`                                    | null           | `Z`    |
| boolean         | `true`                                    | true           | `T`    |
| boolean         | `false`                                   | false          | `F`    |
| number_integer  | -9223372036854775808..-2147483649         | int64          | `L`    |
| number_integer  | -2147483648..-32769                       | int32          | `l`    |
| number_integer  | -32768..-129                              | int16          | `I`    |
| number_integer  | -128..127                                 | int8           | `i`    |
| number_integer  | 128..255                                  | uint8          | `U`    |
| number_integer  | 256..32767                                | int16          | `I`    |
| number_integer  | 32768..65535                              | uint16         | `u`    |
| number_integer  | 65536..2147483647                         | int32          | `l`    |
| number_integer  | 2147483648..4294967295                    | uint32         | `m`    |
| number_integer  | 4294967296..9223372036854775807           | int64          | `L`    |
| number_integer  | 9223372036854775808..18446744073709551615 | uint64         | `M`    |
| number_unsigned | 0..127                                    | int8           | `i`    |
| number_unsigned | 128..255                                  | uint8          | `U`    |
| number_unsigned | 256..32767                                | int16          | `I`    |
| number_unsigned | 32768..65535                              | uint16         | `u`    |
| number_unsigned | 65536..2147483647                         | int32          | `l`    |
| number_unsigned | 2147483648..4294967295                    | uint32         | `m`    |
| number_unsigned | 4294967296..9223372036854775807           | int64          | `L`    |
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64         | `M`    |
| number_float    | *any value*                               | float64        | `D`    |
| string          | *with shortest length indicator*          | string         | `S`    |
| array           | *see notes on optimized format/ND-array*  | array          | `[`    |
| object          | *see notes on optimized format*           | map            | `{`    |

!!! success "Complete mapping"

	The mapping is **complete** in the sense that any JSON value type can be converted to a BJData value.

	Any BJData output created by `to_bjdata` can be successfully parsed by `from_bjdata`.

!!! warning "Size constraints"

	The following values can **not** be converted to a BJData value:

      - strings with more than 18446744073709551615 bytes (theoretical)

!!! info "Unused BJData markers"

	The following markers are not used in the conversion:

    - `Z`: no-op values are not created.
    - `C`: single-byte strings are serialized with `S` markers.

!!! info "NaN/infinity handling"

	If NaN or Infinity are stored inside a JSON number, they are
    serialized properly. This behavior differs from the `dump()`
    function which serializes NaN or Infinity to `null`.


!!! info "Endianness"

	A breaking difference between BJData and UBJSON is the endianness
    of numerical values. In BJData, all numerical data types (integers
    `UiuImlML` and floating-point values `hdD`) are stored in the little-endian (LE)
    byte order as opposed to big-endian as used by UBJSON. To adopt LE
    to store numeric records avoids unnecessary byte swapping on most modern
    computers where LE is used as the default byte order.

!!! info "Optimized formats"

	The optimized formats for containers are supported: Parameter
    `use_size` adds size information to the beginning of a container and
    removes the closing marker. Parameter `use_type` further checks
    whether all elements of a container have the same type and adds the
    type marker to the beginning of the container. The `use_type`
    parameter must only be used together with `use_size = true`.

    Note that `use_size = true` alone may result in larger representations -
    the benefit of this parameter is that the receiving side is
    immediately informed on the number of elements of the container.

!!! info "ND-array optimized format"

	BJData extends UBJSON's optimized array **size** marker to support
    ND-array of uniform numerical data types (referred to as the *packed array*).
    For example, 2-D `uint8` integer array `[[1,2],[3,4],[5,6]]` that can be stored
    as nested optimized array in UBJSON `[ [$U#i2 1 2 [$U#i2 3 4 [$U#i2 5 6 ]`,
    can be further compressed in BJData and stored as `[$U#[$i#i2 2 3 1 2 3 4 5 6`
    or `[$U#[i2 i3] 1 2 3 4 5 6`.

    In order to maintain the type and dimension information of an ND-array,
    when this library parses a BJData ND-array via `from_bjdata`, it converts the
    data into a JSON object, following the **annotated array format** as defined in the
    [JData specification (Draft 3)](https://github.com/NeuroJSON/jdata/blob/master/JData_specification.md#annotated-storage-of-n-d-arrays).
    For example, the above 2-D `uint8` array can be parsed and accessed as

    ```json
    {
        "_ArrayType_": "uint8",
        "_ArraySize_": [2,3],
        "_ArrayData_": [1,2,3,4,5,6]
    }
    ```

    In the reversed direction, when `to_bjdata` detects a JSON object in the
    above form, it automatically converts such object into a BJData ND-array
    to generate compact output. The only exception is that when the 1-D dimensional
    vector stored in `"_ArraySize_"` contains a single integer, or two integers with
    one being 1, a regular 1-D optimized array is generated.

    The current version of this library has not yet supported automatic
    recognition and conversion from a nested JSON array input to a BJData ND-array.

!!! info "Restrictions in optimized data types for arrays and objects"

	Due to diminished space saving, hampered readability, and increased
    security risks, in BJData, the allowed data types following the `$` marker
    in an optimized array and object container are restricted to
    **non-zero-fixed-length** data types. Therefore, the valid optimized
    type markers can only be one of `UiuImlMLhdDC`. This also means other
    variable (`[{SH`) or zero-length types (`TFN`) can not be used in an
    optimized array or object in BJData.

!!! info "Binary values"

	If the JSON data contains the binary type, the value stored is a list
    of integers, as suggested by the BJData documentation.  In particular,
    this means that serialization and the deserialization of a JSON
    containing binary values into BJData and back will result in a
    different JSON object.


??? example

    ```cpp
    --8<-- "examples/to_bjdata.cpp"
    ```

    Output:

    ```c
    --8<-- "examples/to_bjdata.output"
    ```

## Deserialization

The library maps BJData types to JSON value types as follows:

| BJData type | JSON value type                         | marker |
|-------------|-----------------------------------------|--------|
| no-op       | *no value, next value is read*          | `N`    |
| null        | `null`                                  | `Z`    |
| false       | `false`                                 | `F`    |
| true        | `true`                                  | `T`    |
| float16     | number_float                            | `h`    |
| float32     | number_float                            | `d`    |
| float64     | number_float                            | `D`    |
| uint8       | number_unsigned                         | `U`    |
| int8        | number_integer                          | `i`    |
| uint16      | number_unsigned                         | `u`    |
| int16       | number_integer                          | `I`    |
| uint32      | number_unsigned                         | `m`    |
| int32       | number_integer                          | `l`    |
| uint64      | number_unsigned                         | `M`    |
| int64       | number_integer                          | `L`    |
| string      | string                                  | `S`    |
| char        | string                                  | `C`    |
| array       | array (optimized values are supported)  | `[`    |
| ND-array    | object (in JData annotated array format)|`[$.#[.`|
| object      | object (optimized values are supported) | `{`    |

!!! success "Complete mapping"

	The mapping is **complete** in the sense that any BJData value can be converted to a JSON value.


??? example

    ```cpp
    --8<-- "examples/from_bjdata.cpp"
    ```

    Output:

    ```json
    --8<-- "examples/from_bjdata.output"
    ```