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248 lines
11 KiB
Markdown
248 lines
11 KiB
Markdown
3 years ago
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## fast_float number parsing library: 4x faster than strtod
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The fast_float library provides fast header-only implementations for the C++ from_chars
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functions for `float` and `double` types. These functions convert ASCII strings representing
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decimal values (e.g., `1.3e10`) into binary types. We provide exact rounding (including
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round to even). In our experience, these `fast_float` functions many times faster than comparable number-parsing functions from existing C++ standard libraries.
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Specifically, `fast_float` provides the following two functions with a C++17-like syntax (the library itself only requires C++11):
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```C++
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from_chars_result from_chars(const char* first, const char* last, float& value, ...);
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from_chars_result from_chars(const char* first, const char* last, double& value, ...);
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```
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The return type (`from_chars_result`) is defined as the struct:
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```C++
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struct from_chars_result {
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const char* ptr;
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std::errc ec;
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};
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```
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It parses the character sequence [first,last) for a number. It parses floating-point numbers expecting
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a locale-independent format equivalent to the C++17 from_chars function.
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The resulting floating-point value is the closest floating-point values (using either float or double),
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using the "round to even" convention for values that would otherwise fall right in-between two values.
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That is, we provide exact parsing according to the IEEE standard.
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Given a successful parse, the pointer (`ptr`) in the returned value is set to point right after the
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parsed number, and the `value` referenced is set to the parsed value. In case of error, the returned
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`ec` contains a representative error, otherwise the default (`std::errc()`) value is stored.
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The implementation does not throw and does not allocate memory (e.g., with `new` or `malloc`).
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It will parse infinity and nan values.
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Example:
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``` C++
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#include "fast_float/fast_float.h"
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#include <iostream>
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int main() {
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const std::string input = "3.1416 xyz ";
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double result;
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auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
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if(answer.ec != std::errc()) { std::cerr << "parsing failure\n"; return EXIT_FAILURE; }
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std::cout << "parsed the number " << result << std::endl;
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return EXIT_SUCCESS;
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}
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```
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Like the C++17 standard, the `fast_float::from_chars` functions take an optional last argument of
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the type `fast_float::chars_format`. It is a bitset value: we check whether
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`fmt & fast_float::chars_format::fixed` and `fmt & fast_float::chars_format::scientific` are set
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to determine whether we allow the fixed point and scientific notation respectively.
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The default is `fast_float::chars_format::general` which allows both `fixed` and `scientific`.
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The library seeks to follow the C++17 (see [20.19.3](http://eel.is/c++draft/charconv.from.chars).(7.1)) specification.
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* The `from_chars` function does not skip leading white-space characters.
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* [A leading `+` sign](https://en.cppreference.com/w/cpp/utility/from_chars) is forbidden.
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* It is generally impossible to represent a decimal value exactly as binary floating-point number (`float` and `double` types). We seek the nearest value. We round to an even mantissa when we are in-between two binary floating-point numbers.
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Furthermore, we have the following restrictions:
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* We only support `float` and `double` types at this time.
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* We only support the decimal format: we do not support hexadecimal strings.
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* For values that are either very large or very small (e.g., `1e9999`), we represent it using the infinity or negative infinity value.
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We support Visual Studio, macOS, Linux, freeBSD. We support big and little endian. We support 32-bit and 64-bit systems.
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We assume that the rounding mode is set to nearest (`std::fegetround() == FE_TONEAREST`).
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## Using commas as decimal separator
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The C++ standard stipulate that `from_chars` has to be locale-independent. In
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particular, the decimal separator has to be the period (`.`). However,
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some users still want to use the `fast_float` library with in a locale-dependent
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manner. Using a separate function called `from_chars_advanced`, we allow the users
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to pass a `parse_options` instance which contains a custom decimal separator (e.g.,
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the comma). You may use it as follows.
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```C++
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#include "fast_float/fast_float.h"
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#include <iostream>
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int main() {
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const std::string input = "3,1416 xyz ";
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double result;
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fast_float::parse_options options{fast_float::chars_format::general, ','};
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auto answer = fast_float::from_chars_advanced(input.data(), input.data()+input.size(), result, options);
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if((answer.ec != std::errc()) || ((result != 3.1416))) { std::cerr << "parsing failure\n"; return EXIT_FAILURE; }
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std::cout << "parsed the number " << result << std::endl;
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return EXIT_SUCCESS;
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}
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```
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You can parse delimited numbers:
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```C++
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const std::string input = "234532.3426362,7869234.9823,324562.645";
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double result;
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auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
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if(answer.ec != std::errc()) {
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// check error
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}
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// we have result == 234532.3426362.
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if(answer.ptr[0] != ',') {
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// unexpected delimiter
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}
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answer = fast_float::from_chars(answer.ptr + 1, input.data()+input.size(), result);
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if(answer.ec != std::errc()) {
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// check error
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}
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// we have result == 7869234.9823.
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if(answer.ptr[0] != ',') {
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// unexpected delimiter
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}
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answer = fast_float::from_chars(answer.ptr + 1, input.data()+input.size(), result);
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if(answer.ec != std::errc()) {
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// check error
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}
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// we have result == 324562.645.
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```
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## Relation With Other Work
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The fast_float library is part of:
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- GCC (as of version 12): the `from_chars` function in GCC relies on fast_float.
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- [WebKit](https://github.com/WebKit/WebKit), the engine behind Safari (Apple's web browser)
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The fastfloat algorithm is part of the [LLVM standard libraries](https://github.com/llvm/llvm-project/commit/87c016078ad72c46505461e4ff8bfa04819fe7ba).
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There is a [derived implementation part of AdaCore](https://github.com/AdaCore/VSS).
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The fast_float library provides a performance similar to that of the [fast_double_parser](https://github.com/lemire/fast_double_parser) library but using an updated algorithm reworked from the ground up, and while offering an API more in line with the expectations of C++ programmers. The fast_double_parser library is part of the [Microsoft LightGBM machine-learning framework](https://github.com/microsoft/LightGBM).
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## Reference
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- Daniel Lemire, [Number Parsing at a Gigabyte per Second](https://arxiv.org/abs/2101.11408), Software: Practice and Experience 51 (8), 2021.
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## Other programming languages
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- [There is an R binding](https://github.com/eddelbuettel/rcppfastfloat) called `rcppfastfloat`.
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- [There is a Rust port of the fast_float library](https://github.com/aldanor/fast-float-rust/) called `fast-float-rust`.
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- [There is a Java port of the fast_float library](https://github.com/wrandelshofer/FastDoubleParser) called `FastDoubleParser`. It used for important systems such as [Jackson](https://github.com/FasterXML/jackson-core).
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- [There is a C# port of the fast_float library](https://github.com/CarlVerret/csFastFloat) called `csFastFloat`.
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## Users
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The fast_float library is used by [Apache Arrow](https://github.com/apache/arrow/pull/8494) where it multiplied the number parsing speed by two or three times. It is also used by [Yandex ClickHouse](https://github.com/ClickHouse/ClickHouse) and by [Google Jsonnet](https://github.com/google/jsonnet).
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## How fast is it?
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It can parse random floating-point numbers at a speed of 1 GB/s on some systems. We find that it is often twice as fast as the best available competitor, and many times faster than many standard-library implementations.
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<img src="http://lemire.me/blog/wp-content/uploads/2020/11/fastfloat_speed.png" width="400">
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```
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$ ./build/benchmarks/benchmark
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# parsing random integers in the range [0,1)
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volume = 2.09808 MB
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netlib : 271.18 MB/s (+/- 1.2 %) 12.93 Mfloat/s
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doubleconversion : 225.35 MB/s (+/- 1.2 %) 10.74 Mfloat/s
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strtod : 190.94 MB/s (+/- 1.6 %) 9.10 Mfloat/s
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abseil : 430.45 MB/s (+/- 2.2 %) 20.52 Mfloat/s
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fastfloat : 1042.38 MB/s (+/- 9.9 %) 49.68 Mfloat/s
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```
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See https://github.com/lemire/simple_fastfloat_benchmark for our benchmarking code.
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## Video
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[](http://www.youtube.com/watch?v=AVXgvlMeIm4)<br />
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## Using as a CMake dependency
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This library is header-only by design. The CMake file provides the `fast_float` target
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which is merely a pointer to the `include` directory.
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If you drop the `fast_float` repository in your CMake project, you should be able to use
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it in this manner:
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```cmake
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add_subdirectory(fast_float)
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target_link_libraries(myprogram PUBLIC fast_float)
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```
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Or you may want to retrieve the dependency automatically if you have a sufficiently recent version of CMake (3.11 or better at least):
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```cmake
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FetchContent_Declare(
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fast_float
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GIT_REPOSITORY https://github.com/lemire/fast_float.git
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GIT_TAG tags/v1.1.2
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GIT_SHALLOW TRUE)
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FetchContent_MakeAvailable(fast_float)
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target_link_libraries(myprogram PUBLIC fast_float)
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```
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You should change the `GIT_TAG` line so that you recover the version you wish to use.
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## Using as single header
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The script `script/amalgamate.py` may be used to generate a single header
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version of the library if so desired.
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Just run the script from the root directory of this repository.
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You can customize the license type and output file if desired as described in
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the command line help.
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You may directly download automatically generated single-header files:
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https://github.com/fastfloat/fast_float/releases/download/v3.4.0/fast_float.h
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## Credit
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Though this work is inspired by many different people, this work benefited especially from exchanges with
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Michael Eisel, who motivated the original research with his key insights, and with Nigel Tao who provided
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invaluable feedback. Rémy Oudompheng first implemented a fast path we use in the case of long digits.
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The library includes code adapted from Google Wuffs (written by Nigel Tao) which was originally published
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under the Apache 2.0 license.
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## License
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<sup>
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Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
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2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
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</sup>
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<br>
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<sub>
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in this repository by you, as defined in the Apache-2.0 license,
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shall be dual licensed as above, without any additional terms or conditions.
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</sub>
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