////
Copyright 2022 Peter Dimov
Copyright 2023-2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= CharConv: An Implementation of <charconv> in {cpp}11
Matt Borland
:toc: left
:toclevels: 4
:idprefix:
:listing-caption: Code Example
:docinfo: private-footer
:source-highlighter: rouge
:source-language: c++

:leveloffset: +1

////
Copyright 2022 Peter Dimov
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#overview]
= Overview
:idprefix: overview_

== Description

Boost.Charconv converts character buffers to numbers, and numbers to character buffers.
It is a small library of two overloaded functions to do the heavy lifting, plus several supporting enums, structures, templates, and constants, with a particular focus on performance and consistency
across the supported development environments.

Why should I be interested in this Library? Charconv is locale-independent, non-allocating^1^, non-throwing and only requires a minimum of C++ 11.
It provides functionality similar to that found in `std::printf` or `std::strtod` with <<benchmark_results_, substantial performance increases>>.
This library can also be used in place of the standard library `<charconv>` if unavailable with your toolchain.
Currently only https://en.cppreference.com/w/cpp/compiler_support/17.html[GCC 11+ and MSVC 19.24+] support both integer and floating-point conversions in their implementation of `<charconv>`. +
If you are using either of those compilers, Boost.Charconv is at least as performant as `<charconv>`, and can be up to several times faster.
See: <<Benchmarks>>

^1^ The one edge case where allocation may occur is you are parsing a string to an 80 or 128-bit `long double` or `__float128`, and the string is over 1024 bytes long.

== Supported Compilers / OS

Boost.Charconv is tested on Ubuntu, macOS, and Windows with the following compilers:

* GCC 5 or later
* Clang 3.8 or later
* Visual Studio 2015 (14.0) or later

Tested on https://github.com/boostorg/charconv/actions[GitHub Actions] and https://drone.cpp.al/boostorg/charconv[Drone].

////
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= Getting Started
:idprefix: build_

== B2

Run the following commands to clone the latest versions of Boost and Charconv, prepare the Boost.Build system for use, and build the libraries with C++11 as the default standard:
[source, bash]
----
git clone https://github.com/boostorg/boost
cd boost
git submodule update --init
cd ..
./bootstrap
./b2 cxxstd=11
----

To install the development environment, run:

[source, bash]
----
sudo ./b2 install cxxstd=11
----

The value of cxxstd must be at least 11. https://www.boost.org/doc/libs/1_84_0/tools/build/doc/html/index.html[See the b2 documentation] under `cxxstd` for all valid values.

== `__float128` and `std::float128_t` Support

If using B2 or CMake the build system will automatically define `BOOST_CHARCONV_HAS_QUADMATH` and link against it if the build system can successfully run a small test case.
If you are using another build system and you want support for these types you will have to define `BOOST_CHARCONV_HAS_QUADMATH`, and link against https://gcc.gnu.org/onlinedocs/libquadmath/[libquadmath].

IMPORTANT: libquadmath is only available on supported platforms (e.g. Linux with x86, x86_64, PPC64, and IA64).

== Dependencies

This library depends on: Boost.Assert, Boost.Config, Boost.Core, and optionally libquadmath (see above).

////
Copyright 2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#basic_usage]
= Basic Usage Examples
:idprefix: basic_usage_

== Usage Examples
[source, c++]
----
#include <boost/charconv.hpp>

const char* buffer = "42";
int v = 0;
boost::charconv::from_chars_result r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
assert(r.ec == std::errc());
assert(v == 42);

char buffer[64];
int v = 123456;
boost::charconv::to_chars_result r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), v);
assert(r.ec == std::errc());
assert(!strncmp(buffer, "123456", 6)); // Strncmp returns 0 on match

----

////
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#api_reference]
= API Reference
:idprefix: api_ref_

== Functions

- <<from_chars_definitions_, `boost::charconv::from_chars`>>
- <<from_chars_definitions_, `boost::charconv::from_chars_erange`>>
- <<to_chars_definitions_, `boost::charconv::to_chars`>>

== Structures

- <<from_chars_definitions_, `boost::charconv::from_chars_result`>>
- <<to_chars_definitions_, `boost::charconv::to_chars_result`>>

== Enums

- <<chars_format_defintion_,`boost::charconv::chars_format`>>

== Constants

- <<limits_definitions_, `boost::charconv::limits::digits`>>
- <<limits_definitions_, `boost::charconv::limits::digits10`>>

== Macros

- <<integral_usage_notes_, `BOOST_CHARCONV_CONSTEXPR`>>
- <<run_benchmarks_, `BOOST_CHARCONV_RUN_BENCHMARKS`>>

////
Copyright 2023 - 2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= from_chars
:idprefix: from_chars_

== from_chars overview

`from_chars` is a set of functions that parse a string from `[first, last)` in an attempt to convert the string into `value` according to the `chars_format` specified (if applicable).
The parsing of number is locale-independent (e.g. equivalent to the "C" locale).
The result of `from_chars` is `from_chars_result` which on success returns `ptr == last` and `ec == std::errc()`, and on failure returns `ptr` equal to the last valid character parsed or `last` on underflow/overflow, and `ec == std::errc::invalid_argument` or `std::errc::result_out_of_range` respectively. `from_chars` does not require the character sequence to be null terminated.

== Definitions
[#from_chars_definitions_]

[source, c++]
----
namespace boost { namespace charconv {

struct from_chars_result
{
    const char* ptr;
    std::errc ec;

    friend constexpr bool operator==(const from_chars_result& lhs, const from_chars_result& rhs) noexcept = default;
    constexpr explicit operator bool() const noexcept { return ec == std::errc{}; }
}

template <typename Integral>
BOOST_CXX14_CONSTEXPR from_chars_result from_chars(const char* first, const char* last, Integral& value, int base = 10) noexcept;

template <typename Integral>
BOOST_CXX14_CONSTEXPR from_chars_result from_chars(boost::core::string_view sv, Integral& value, int base = 10) noexcept;

BOOST_CXX14_CONSTEXPR from_chars_result from_chars<bool>(const char* first, const char* last, bool& value, int base) = delete;

template <typename Real>
from_chars_result from_chars(const char* first, const char* last, Real& value, chars_format fmt = chars_format::general) noexcept;

template <typename Real>
from_chars_result from_chars(boost::core::string_view sv, Real& value, chars_format fmt = chars_format::general) noexcept;

// See note below Usage notes for from_chars for floating point types

template <typename Real>
from_chars_result from_chars_erange(const char* first, const char* last, Real& value, chars_format fmt = chars_format::general) noexcept;

template <typename Real>
from_chars_result from_chars_erange(boost::core::string_view sv, Real& value, chars_format fmt = chars_format::general) noexcept;

}} // Namespace boost::charconv
----

== from_chars parameters
* `first`, `last` - pointers to a valid range to parse
* `sv` - string view of a valid range to parse.
Compatible with boost::core::string_view, std::string, and std::string_view
* `value` - where the output is stored upon successful parsing
* `base` (integer only) - the integer base to use. Must be between 2 and 36 inclusive
* `fmt` (floating point only) - The format of the buffer. See <<chars_format overview>> for description.

== from_chars_result
* `ptr` - On return from `from_chars` it is a pointer to the first character not matching the pattern, or pointer to `last` if all characters are successfully parsed.
* `ec` - https://en.cppreference.com/w/cpp/error/errc[the error code]. Values returned by `from_chars` are:

|===
|Return Value | Description
| `std::errc()` | Successful Parsing
| `std::errc::invalid_argument` | 1) Parsing a negative into an unsigned type

2) Leading `+` sign

3) Leading space

4) Incompatible formatting (e.g. exponent on `chars_format::fixed`, or p as exponent on value that is not `chars_format::hex`) See <<chars_format overview>>

| `std::errc::result_out_of_range` | 1) Overflow

2) Underflow
|===

* `operator==` - compares the values of ptr and ec for equality

== Usage Notes

=== Usage notes for from_chars for integral types
* All built-in integral types are allowed except bool which is deleted
* These functions have been tested to support `\__int128` and `unsigned __int128`
* from_chars for integral types is constexpr when compiled using `-std=c++14` or newer
** One known exception is GCC 5 which does not support constexpr comparison of `const char*`.
* A valid string must only contain the characters for numbers. Leading spaces are not ignored, and will return `std::errc::invalid_argument`.

=== Usage notes for from_chars for floating point types
* On `std::errc::result_out_of_range` we return ±0 for small values (e.g. 1.0e-99999) or ±HUGE_VAL for large values (e.g. 1.0e+99999) to match the handling of `std::strtod`.
This is a divergence from the standard which states we should return the `value` argument unmodified.

** `from_chars` has an open issue with LWG here: https://cplusplus.github.io/LWG/lwg-active.html#3081.
The standard for <charconv> does not distinguish between underflow and overflow like strtod does.
Let's say you are writing a JSON library, and you replace `std::strtod` with `boost::charconv::from_chars` for performance reasons.
Charconv returns std::errc::result_out_of_range on some conversion.
You would then have to parse the string again yourself to figure out which of the four possible reasons you got `std::errc::result_out_of_range`.
Charconv can give you that information by using `boost::charconv::from_chars_erange` instead of `boost::charconv::from_chars` throughout the code base.
By implementing the resolution to the LWG issue that matches the established strtod behavior I think we are providing the correct behavior without waiting on the committee's decision.

* These functions have been tested to support all built-in floating-point types and those from C++23's `<stdfloat>`
** Long doubles can be 64, 80, or 128-bit, but must be IEEE 754 compliant. An example of a non-compliant, and therefore unsupported, format is `__ibm128`.
** Use of `__float128` or `std::float128_t` requires compiling with `-std=gnu++xx` and linking GCC's `libquadmath`.
This is done automatically when building with CMake.

== Examples

=== Basic usage
==== Integral
[source, c++]
----
const char* buffer = "42";
int v = 0;
from_chars_result r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(v == 42);

std::string str_buffer (buffer);
boost::core::string_view sv(str_buffer);
int v2;
auto r2 = boost::charconv::from_chars(sv, v2);
assert(r);
assert(v2 == v);
----

==== Floating Point
[source, c++]
----
const char* buffer = "1.2345"
double v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(v == 1.2345);

std::string str_buffer(buffer);
double v2;
auto r2 = boost::charconv::from_chars(buffer, v2);
assert(r2);
assert(v == v2);
----

=== Hexadecimal
==== Integral
[source, c++]
----
const char* buffer = "2a";
unsigned v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, 16);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(v == 42);
----
==== Floating Point
[source, c++]
----
const char* buffer = "1.3a2bp-10";
double v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, boost::charconv::chars_format::hex);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(v == 8.0427e-18);
----

=== std::errc::invalid_argument

The below is invalid because a negative value is being parsed into an unsigned integer.

[source, c++]
----
const char* buffer = "-123";
unsigned v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
assert(r.ec == std::errc::invalid_argument);
assert(!r); // Same as above but less verbose. Added in C++26.
----

The below is invalid because a fixed format floating-point value can not have an exponent.

[source, c++]
----
const char* buffer = "-1.573e-3";
double v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v, boost::charconv::chars_format::fixed);
assert(r.ec == std::errc::invalid_argument);
assert(!r); // Same as above but less verbose. Added in C++26.
----
Note: In the event of `std::errc::invalid_argument`, v is not modified by `from_chars`

=== std::errc::result_out_of_range
[source, c++]
----
const char* buffer = "1234";
unsigned char v = 0;
auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);
assert(r.ec == std::errc::result_out_of_range);
assert(!r); // Same as above but less verbose. Added in C++26.
assert(v == 0)
----
Note: In the event of `std::errc::result_out_of_range`, v is not modified by `from_chars`

////
Copyright 2023-2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= to_chars
:idprefix: to_chars_

== to_chars overview

`to_chars` is a set of functions that attempts to convert `value` into a character buffer specified by `[first, last)`.
The result of `to_chars` is `to_chars_result` which on success returns `ptr` equal to one-past-the-end of the characters written and `ec == std::errc()` and on failure returns `std::errc::value_too_large` and `ptr == last`.
`to_chars` does not null-terminate the returned characters.

== Definitions
[#to_chars_definitions_]

[source, c++]
----
namespace boost { namespace charconv {

struct to_chars_result
{
    char* ptr;
    std::errc ec;

    friend constexpr bool operator==(const to_chars_result& lhs, const to_chars_result& rhs) noexcept; = default;
    constexpr explicit operator bool() const noexcept { return ec == std::errc{}; }
};

template <typename Integral>
BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars(char* first, char* last, Integral value, int base = 10) noexcept;

template <typename Integral>
BOOST_CHARCONV_CONSTEXPR to_chars_result to_chars<bool>(char* first, char* last, Integral value, int base) noexcept = delete;

template <typename Real>
to_chars_result to_chars(char* first, char* last, Real value, chars_format fmt = chars_format::general, int precision) noexcept;

}} // Namespace boost::charconv
----

== to_chars parameters
* `first, last` - pointers to the beginning and end of the character buffer
* `value` - the value to be parsed into the buffer
* `base` (integer only) - the integer base to use. Must be between 2 and 36 inclusive
* `fmt` (float only) - the floating point format to use.
See <<chars_format overview>> for description.
* `precision` (float only) - the number of decimal places required

== to_chars_result
* `ptr` - On return from `to_chars` points to one-past-the-end of the characters written on success or `last` on failure
* `ec` - https://en.cppreference.com/w/cpp/error/errc[the error code]. Values returned by `to_chars` are:
|===
|Return Value | Description
|`std::errc()` | Successful Parsing
| `std::errc::value_too_large` | 1) Overflow

2) Underflow
|===

* `operator==` - compares the value of ptr and ec for equality

== Usage Notes

=== Usage notes for to_chars for integral types
[#integral_usage_notes_]
* All built-in integral types are allowed except bool which is deleted
* from_chars for integral type is constexpr (BOOST_CHARCONV_CONSTEXPR is defined) when:
** compiled using `-std=c++14` or newer 
** using a compiler with `\__builtin_ is_constant_evaluated`
* These functions have been tested to support `\__int128` and `unsigned __int128`

=== Usage notes for to_chars for floating point types
* The following will be returned when handling different values of `NaN`
** `qNaN` returns "nan"
** `-qNaN` returns "-nan(ind)"
** `sNaN` returns "nan(snan)"
** `-sNaN` returns "-nan(snan)"
* These functions have been tested to support all built-in floating-point types and those from C++23's `<stdfloat>`
** Long doubles can be 64, 80, or 128-bit, but must be IEEE 754 compliant. An example of a non-compliant, and therefore unsupported, format is `ibm128`.
** Use of `__float128` or `std::float128_t` requires compiling with `-std=gnu++xx` and linking GCC's `libquadmath`.
This is done automatically when building with CMake.

== Examples

=== Basic Usage
==== Integral
[source, c++]
----
char buffer[64] {};
int v = 42;
to_chars_result r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer) - 1, v);
assert(r.ec == std::errc());
assert(!strcmp(buffer, "42")); // strcmp returns 0 on match
----
==== Floating Point
[source, c++]
----
char buffer[64] {};
double v = 1e300;
to_chars_result r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer) - 1, v);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(!strcmp(buffer, "1e+300"));
----

=== Hexadecimal
==== Integral
[source, c++]
----
char buffer[64] {};
int v = 42;
to_chars_result r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer) - 1, v, 16);
assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(!strcmp(buffer, "2a")); // strcmp returns 0 on match
----
==== Floating Point
[source, c++]
----
char buffer_u[64] {};
double u = -1.08260383390082950e+20;

char buffer_v[64] {};
double v = -1.08260383390082946e+20;

to_chars(buffer_u, buffer_u + sizeof(buffer_u) - 1, u, chars_format::hex);
to_chars(buffer_v, buffer_v + sizeof(buffer_v) - 1, v, chars_format::hex);

std::cout << "U: " << buffer_u << "\nV: " << buffer_v << std::endl;

// U: -1.779a8946bb5fap+66
// V: -1.779a8946bb5f9p+66
//
// With hexfloats we can see the ULP distance between U and V is a - 9 == 1.

----

=== std::errc::value_too_large
==== Integral
[source, c++]
----
char buffer[3] {};
int v = -1234;
to_chars_result r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer) - 1, v, 16);
assert(r.ec == std::errc::value_too_large);
assert(!r); // Same as above but less verbose. Added in C++26.
----
==== Floating Point
[source, c++]
----
char buffer[3] {};
double v = 1.2345;
auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer) - 1, v);
assert(r.ec == std::errc::value_too_large);
assert(!r); // Same as above but less verbose. Added in C++26.
----

In the event of `std::errc::value_too_large`, to_chars_result.ptr is equal to `last`

////
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= chars_format
:idprefix: chars_format_

== chars_format overview

`boost::charconv::chars_format` is an `enum class` used to define the format of floating point types with `from_chars` and `to_chars`.

== Definition
[#chars_format_defintion_]

[source, c++]
----
namespace boost { namespace charconv {

enum class chars_format : unsigned
{
    scientific = 1 << 0,
    fixed = 1 << 1,
    hex = 1 << 2,
    general = fixed | scientific
};

}} // Namespace boost::charconv
----

== Formats

=== Scientific Format
Scientific format will be of the form `1.3e+03`.
The integer part will be between 0 and 9 inclusive. The fraction and exponent will always appear.
The exponent will always have a minimum of 2 digits.

=== Fixed Format
Fixed format will be of the form `2.30` or `3090`. An exponent will not appear with this format.
If the precision of `to_chars` exceeds that of the type (e.g. `std::numeric_limits<double>::chars10`), 0s will be appended to the end of the significant digits.

=== Hex Format
Hex format will be of the form `1.0cp+05`. The integer part will always be 0 or 1.
The exponent will be with a p instead of an e as used with base 10 formats, because e is a valid hex value.
*Note: Every binary floating-point number has a unique representation as a hexfloat, but not every hexfloat has a unique representation as a binary floating-point number.*
This is due to the fact that the number of bits in the significand of an IEEE754 binary32 and binary64 are not divisible by 4.

==== Hexfloat Use Cases
For those unfamiliar with hexfloats, they are valuable in specific instances:

* Precision control: Hexfloats can offer finer control over the precision of floating-point values.
In hexadecimal notation, each digit represents four bits (one hexit), allowing you to directly manipulate the precision of the number by specifying a certain number of hexadecimal digits.
This can be useful when you need to precisely control the level of accuracy required for your calculations.

* Bit-level representation: Hexfloats provide a direct representation of the underlying bits of a floating-point number.
Each hexadecimal digit corresponds to a specific group of bits, making it easier to visualize and understand the internal structure of the floating-point value.
This can be helpful for debugging or analyzing floating-point arithmetic operations (e.g. Computing https://en.wikipedia.org/wiki/Unit_in_the_last_place[ULP] distances).

=== General
General format will be the shortest representation of a number in either fixed or general format (e.g. `1234` instead of `1.234e+03`.

////
Copyright 2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= Limits
:idprefix: limits_

== Limits overview

The contents of `<boost/charconv/limits.hpp>` are designed to help the user optimize the size of the buffer required for `to_chars`.

== Definitions
[#limits_definitions_]

[source, c++]
----
namespace boost { namespace charconv {

template <typename T>
constexpr int limits<T>::max_chars10;

template <typename T>
constexpr int limits<T>::max_chars;

}} // Namespace boost::charconv
----

=== max_chars10

The minimum size of the buffer that needs to be
passed to `to_chars` to guarantee successful conversion for all values of type T, when either no base is passed, or base 10 is passed.

=== max_chars

The minimum size of the buffer that needs to be  passed to `to_chars` to guarantee successful conversion for all values of type T, for any value of base.

== Examples

The following two examples are for `max_chars10` to optimize the buffer size with `to_chars` for an integral type and a floating-point type respectively.

[source, c++]
----
char buffer [boost::charconv::limits<std::int32_t>::max_chars10;
auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), std::numeric_limits<std::int32_t>::max());

assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(!strcmp(buffer, "2147483647")); // strcmp returns 0 on match
----

[source, c++]
----
char buffer [boost::charconv::limits<float>::max_chars10;
auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), std::numeric_limits<float>::max());

assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(!strcmp(buffer, "3.40282347e+38")); // strcmp returns 0 on match
----

The following example is a usage of `max_chars` when used to serialize an integer in binary (base = 2).

[source, c++]
----
char buffer [boost::charconv::limits<std::uint16_t>::max_chars;
auto r = boost::charconv::to_chars(buffer, buffer + sizeof(buffer), std::numeric_limits<std::uint16_t>::max(), 2);

assert(r.ec == std::errc());
assert(r); // Same as above but less verbose. Added in C++26.
assert(!strcmp(buffer, "1111111111111111")); // strcmp returns 0 on match
----

////
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

= Benchmarks
:idprefix: benchmarks

This section describes a range of performance benchmarks that have been run comparing this library with the standard library, and how to run your own benchmarks if required.

The values are relative to the performance of `std::printf` and `std::strtoX`.
Larger numbers are more performant (e.g. 2.00 means twice as fast, and 0.50 means it takes twice as long).
`std::printf` and `std::strtoX` are always listed first as they will be the reference value.

== How to run the Benchmarks
[#run_benchmarks_]

To run the benchmarks yourself, navigate to the test folder and define `BOOST_CHARCONV_RUN_BENCHMARKS` when running the tests.
An example on Linux with b2: `../../../b2 cxxstd=20 toolset=gcc-13 define=BOOST_CHARCONV_RUN_BENCHMARKS STL_benchmark linkflags="-lfmt" -a release` .

Additionally, you will need the following:

* A compiler with full `<charconv>` support:
** GCC 11 or newer
** MSVC 19.24 or newer
* https://github.com/google/double-conversion[libdouble-conversion]
* https://github.com/fmtlib/fmt[{fmt}]

== Results
[#benchmark_results_]

=== x86_64 Linux

Data in tables 1 - 4 were run on Ubuntu 23.04 with x86_64 architecture using GCC 13.1.0 with libstdc++.

==== Floating Point

.to_chars floating point with the shortest representation
|===
|Function|Relative Performance (float / double)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|0.56 / 0.49
|Boost.spirit.karma
|1.70 / 2.62
|std::to_chars
|4.01 / 6.03
|Boost.Charconv.to_chars
|4.46 / 6.20
|Google double-conversion
|1.26 / 1.91
|{fmt}
|2.52 / 3.63
|===

.from_chars floating point with scientific formatting
|===
|Function|Relative Performance (float / double)

|std::strto(f/d)
|1.00 / 1.00
|Boost.lexical_cast
|0.33 / 0.42
|Boost.spirit.qi
|3.17 / 4.65
|std::from_chars
|3.23 / 5.77
|Boost.Charconv.from_chars
|3.28 / 5.75
|Google double-conversion
|1.16 / 1.30
|===

==== Integral

.to_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|1.80 / 1.38
|Boost.spirit.karma
|2.81 / 1.62
|std::to_chars
|4.06 / 2.45
|Boost.Charconv.to_chars
|4.13 / 2.48
|{fmt}
|2.88 / 2.21
|===

.from_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::strto(ul,ull)
|1.00 / 1.00
|Boost.lexical_cast
|0.53 / 0.52
|Boost.spirit.qi
|2.24 / 1.49
|std::from_chars
|1.97 / 1.68
|Boost.Charconv.from_chars
|2.54 / 1.78
|===

=== x86_64 Windows

Data in tables 5 - 8 were run on Windows 11 with x86_64 architecture using MSVC 14.3 (V17.7.0).

==== Floating Point

.to_chars floating point with the shortest representation
|===
|Function|Relative Performance (float / double)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|0.50 / 0.70
|Boost.spirit.karma
|2.23 / 7.58
|std::to_chars
|5.58 / 15.77
|Boost.Charconv.to_chars
|5.62 / 15.26
|===

.from_chars floating point with scientific formatting
|===
|Function|Relative Performance (float / double)

|std::strto(f/d)
|1.00 / 1.00
|Boost.lexical_cast
|0.14 / 0.20
|Boost.spirit.qi
|2.03 / 4.58
|std::from_chars
|1.01 / 1.23
|Boost.Charconv.from_chars
|2.06 / 5.21
|===

==== Integral

.to_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|0.68 / 0.68
|Boost.spirit.karma
|2.75 / 1.67
|std::to_chars
|2.75 / 2.10
|Boost.Charconv.to_chars
|2.75 / 2.06
|===

.from_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::strto(ul,ull)
|1.00 / 1.00
|Boost.lexical_cast
|0.46 / 0.39
|Boost.spirit.qi
|1.94 / 1.63
|std::from_chars
|2.43 / 2.18
|Boost.Charconv.from_chars
|2.68 / 2.27
|===

=== ARM MacOS

Data in tables 9-12 were run on MacOS Ventura 13.5.2 with M1 Pro architecture using Homebrew GCC 13.2.0 with libstdc++.

==== Floating Point

.to_chars floating point with the shortest representation
|===
|Function|Relative Performance (float / double)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|0.58 / 0.16
|Boost.spirit.karma
|1.39 / 1.22
|std::to_chars
|6.78 / 6.47
|Boost.Charconv.to_chars
|7.25 / 6.86
|Google double-conversion
|2.26 / 2.16
|{fmt}
|3.78 / 3.38
|===

.from_chars floating point with scientific formatting
|===
|Function|Relative Performance (float / double)

|std::strto(f/d)
|1.00 / 1.00
|Boost.lexical_cast
|0.06 / 0.06
|Boost.spirit.qi
|1.12 / 1.06
|std::from_chars
|1.32 / 1.65
|Boost.Charconv.from_chars
|1.28 / 1.63
|Google double-conversion
|0.45 / 0.32

|===

==== Integral

.to_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::printf
|1.00 / 1.00
|Boost.lexical_cast
|2.08 / 1.75
|Boost.spirit.karma
|4.17 / 2.06
|std::to_chars
|6.25 / 4.12
|Boost.Charconv.to_chars
|6.25 / 4.12
|{fmt}
|5.29 / 3.47
|===

.from_chars base 10 integers
|===
|Function|Relative Performance (uint32_t / uint64_t)

|std::strto(ul,ull)
|1.00 / 1.00
|Boost.lexical_cast
|0.56 / 0.54
|Boost.spirit.qi
|1.39 / 1.33
|std::from_chars
|1.92 / 1.65
|Boost.Charconv.from_chars
|2.27 / 1.65
|===

////
Copyright 2023 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#sources]
= Sources

The following papers and blog posts serve as the basis for the algorithms used in the library:

:idprefix:
:linkattrs:

- J.R. Parker https://dl.acm.org/doi/abs/10.1002/spe.4380150804[_A General Character to Integer Conversion Method_], Software: Practice and Experience 15 (8), 1985.

- Junekey Jeon, https://jk-jeon.github.io/posts/2022/02/jeaiii-algorithm/[_Faster integer formatting - James Anhalt (jeaiii)’s algorithm_]

- Junekey Jeon, https://github.com/jk-jeon/dragonbox/blob/master/other_files/Dragonbox.pdf[_Dragonbox: A New Floating-Point Binary-to-Decimal Conversion Algorithm_]

- Junekey Jeon, https://jk-jeon.github.io/posts/2022/12/fixed-precision-formatting/[_Fixed-precision formatting of floating-point numbers_]

- William D. Clinger, https://dl.acm.org/doi/pdf/10.1145/93542.93557[_How to Read Floating Point Numbers Accurately_], 1990

- Daniel Lemire, https://arxiv.org/abs/2101.11408[_Number Parsing at a Gigabyte per Second_], Software: Practice and Experience 51 (8), 2021.

- Noble Mushtak, Daniel Lemire, https://arxiv.org/abs/2212.06644[_Fast Number Parsing Without Fallback_], Software: Practice and Experience (to appear)

- Ulf Adams, https://dl.acm.org/doi/10.1145/3360595[_Ryū revisited: printf floating point conversion_], Proceedings of the ACM on Programming Languages Volume 3, 2019

////
Copyright 2024 Matt Borland
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#acknowledgments]
= Acknowledgments
:idprefix: ack_

Special thanks to the following people (non-inclusive list):

    - Peter Dimov for providing technical guidance and contributing to the library throughout development.
    - Junekey Jeon for developing and answering my questions about his integer-formatting, Dragonbox, and Floff.
    - Chris Kormanyos for serving as the library review manager.
    - Stephan T. Lavavej for providing the basis for the benchmarks.
    - All that reviewed the library and provided feedback to make it better.

////
Copyright 2022 Peter Dimov
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#copyright]
= Copyright and License
:idprefix:

This documentation is copyright 2022-2023 Peter Dimov and Matt Borland and is distributed under
the http://www.boost.org/LICENSE_1_0.txt[Boost Software License, Version 1.0].

:leveloffset: -1

