From 64973fb160635ee2770d07732290c97e06ab0493 Mon Sep 17 00:00:00 2001
From: Khalil Estell <kammce@google.com>
Date: Tue, 7 Jul 2020 15:04:17 -0700
Subject: [PATCH] Remove allocating containers from on_num

Use std::string_view and std::array containers when
FMT_STATIC_THOUSANDS_SEPARATOR is defined.

This will help in the case for embedded system where even using
containers that can potentially allocate results in malloc/free and
new/delete being linked into the binary. In many embedded cases dynamic
library allocation is prohibited and even the cost of linking in these
libraries costs too high of a cost in flash size.

Resolves #1762
---
 include/fmt/core.h   |  7 +++++++
 include/fmt/format.h | 14 ++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 761162eb..0a9f3c7f 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -392,6 +392,13 @@ template <typename Char> class basic_string_view {
   constexpr iterator begin() const { return data_; }
   constexpr iterator end() const { return data_ + size_; }
 
+  constexpr iterator cbegin() const { return data_; }
+  constexpr iterator cend() const { return data_ + size_; }
+
+  constexpr bool empty() const { return size_ == 0; };
+
+  constexpr Char back() const { return data_[size_ - 1]; }
+
   constexpr const Char& operator[](size_t pos) const { return data_[pos]; }
 
   FMT_CONSTEXPR void remove_prefix(size_t n) {
diff --git a/include/fmt/format.h b/include/fmt/format.h
index ddf086f8..795ec538 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -34,6 +34,7 @@
 #define FMT_FORMAT_H_
 
 #include <algorithm>
+#include <array>
 #include <cerrno>
 #include <cmath>
 #include <cstdint>
@@ -1552,13 +1553,18 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
   enum { sep_size = 1 };
 
   void on_num() {
+#if !defined(FMT_STATIC_THOUSANDS_SEPARATOR)
     std::string groups = grouping<Char>(locale);
+#else
+    constexpr const char kDefaultGroups[] = "\03";
+    basic_string_view<Char> groups(kDefaultGroups, sizeof(kDefaultGroups));
+#endif
     if (groups.empty()) return on_dec();
     auto sep = thousands_sep<Char>(locale);
     if (!sep) return on_dec();
     int num_digits = count_digits(abs_value);
     int size = num_digits, n = num_digits;
-    std::string::const_iterator group = groups.cbegin();
+    auto group = groups.cbegin();
     while (group != groups.cend() && num_digits > *group && *group > 0 &&
            *group != max_value<char>()) {
       size += sep_size;
@@ -1568,9 +1574,9 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
     if (group == groups.cend()) size += sep_size * ((n - 1) / groups.back());
     char digits[40];
     format_decimal(digits, abs_value, num_digits);
-    basic_memory_buffer<Char> buffer;
+    // TODO(kammce): remove the * 2!
+    std::array<Char, inline_buffer_size * 2> buffer;
     size += prefix_size;
-    buffer.resize(size);
     basic_string_view<Char> s(&sep, sep_size);
     // Index of a decimal digit with the least significant digit having index 0.
     int digit_index = 0;
@@ -1590,7 +1596,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
                               make_checked(p, s.size()));
     }
     if (prefix_size != 0) p[-1] = static_cast<Char>('-');
-    write(out, basic_string_view<Char>(buffer.data(), buffer.size()), specs);
+    write(out, basic_string_view<Char>(buffer.data(), size), specs);
   }
 
   void on_chr() { *out++ = static_cast<Char>(abs_value); }