mirror of
https://github.com/RetroDECK/Duckstation.git
synced 2024-11-30 01:25:51 +00:00
248 lines
11 KiB
Markdown
248 lines
11 KiB
Markdown
## fast_float number parsing library: 4x faster than strtod
|
|
|
|
The fast_float library provides fast header-only implementations for the C++ from_chars
|
|
functions for `float` and `double` types. These functions convert ASCII strings representing
|
|
decimal values (e.g., `1.3e10`) into binary types. We provide exact rounding (including
|
|
round to even). In our experience, these `fast_float` functions many times faster than comparable number-parsing functions from existing C++ standard libraries.
|
|
|
|
Specifically, `fast_float` provides the following two functions with a C++17-like syntax (the library itself only requires C++11):
|
|
|
|
```C++
|
|
from_chars_result from_chars(const char* first, const char* last, float& value, ...);
|
|
from_chars_result from_chars(const char* first, const char* last, double& value, ...);
|
|
```
|
|
|
|
The return type (`from_chars_result`) is defined as the struct:
|
|
```C++
|
|
struct from_chars_result {
|
|
const char* ptr;
|
|
std::errc ec;
|
|
};
|
|
```
|
|
|
|
It parses the character sequence [first,last) for a number. It parses floating-point numbers expecting
|
|
a locale-independent format equivalent to the C++17 from_chars function.
|
|
The resulting floating-point value is the closest floating-point values (using either float or double),
|
|
using the "round to even" convention for values that would otherwise fall right in-between two values.
|
|
That is, we provide exact parsing according to the IEEE standard.
|
|
|
|
|
|
Given a successful parse, the pointer (`ptr`) in the returned value is set to point right after the
|
|
parsed number, and the `value` referenced is set to the parsed value. In case of error, the returned
|
|
`ec` contains a representative error, otherwise the default (`std::errc()`) value is stored.
|
|
|
|
The implementation does not throw and does not allocate memory (e.g., with `new` or `malloc`).
|
|
|
|
It will parse infinity and nan values.
|
|
|
|
Example:
|
|
|
|
``` C++
|
|
#include "fast_float/fast_float.h"
|
|
#include <iostream>
|
|
|
|
int main() {
|
|
const std::string input = "3.1416 xyz ";
|
|
double result;
|
|
auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
|
|
if(answer.ec != std::errc()) { std::cerr << "parsing failure\n"; return EXIT_FAILURE; }
|
|
std::cout << "parsed the number " << result << std::endl;
|
|
return EXIT_SUCCESS;
|
|
}
|
|
```
|
|
|
|
|
|
Like the C++17 standard, the `fast_float::from_chars` functions take an optional last argument of
|
|
the type `fast_float::chars_format`. It is a bitset value: we check whether
|
|
`fmt & fast_float::chars_format::fixed` and `fmt & fast_float::chars_format::scientific` are set
|
|
to determine whether we allow the fixed point and scientific notation respectively.
|
|
The default is `fast_float::chars_format::general` which allows both `fixed` and `scientific`.
|
|
|
|
The library seeks to follow the C++17 (see [20.19.3](http://eel.is/c++draft/charconv.from.chars).(7.1)) specification.
|
|
* The `from_chars` function does not skip leading white-space characters.
|
|
* [A leading `+` sign](https://en.cppreference.com/w/cpp/utility/from_chars) is forbidden.
|
|
* 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.
|
|
|
|
Furthermore, we have the following restrictions:
|
|
* We only support `float` and `double` types at this time.
|
|
* We only support the decimal format: we do not support hexadecimal strings.
|
|
* For values that are either very large or very small (e.g., `1e9999`), we represent it using the infinity or negative infinity value.
|
|
|
|
We support Visual Studio, macOS, Linux, freeBSD. We support big and little endian. We support 32-bit and 64-bit systems.
|
|
|
|
We assume that the rounding mode is set to nearest (`std::fegetround() == FE_TONEAREST`).
|
|
|
|
## Using commas as decimal separator
|
|
|
|
|
|
The C++ standard stipulate that `from_chars` has to be locale-independent. In
|
|
particular, the decimal separator has to be the period (`.`). However,
|
|
some users still want to use the `fast_float` library with in a locale-dependent
|
|
manner. Using a separate function called `from_chars_advanced`, we allow the users
|
|
to pass a `parse_options` instance which contains a custom decimal separator (e.g.,
|
|
the comma). You may use it as follows.
|
|
|
|
```C++
|
|
#include "fast_float/fast_float.h"
|
|
#include <iostream>
|
|
|
|
int main() {
|
|
const std::string input = "3,1416 xyz ";
|
|
double result;
|
|
fast_float::parse_options options{fast_float::chars_format::general, ','};
|
|
auto answer = fast_float::from_chars_advanced(input.data(), input.data()+input.size(), result, options);
|
|
if((answer.ec != std::errc()) || ((result != 3.1416))) { std::cerr << "parsing failure\n"; return EXIT_FAILURE; }
|
|
std::cout << "parsed the number " << result << std::endl;
|
|
return EXIT_SUCCESS;
|
|
}
|
|
```
|
|
|
|
You can parse delimited numbers:
|
|
```C++
|
|
const std::string input = "234532.3426362,7869234.9823,324562.645";
|
|
double result;
|
|
auto answer = fast_float::from_chars(input.data(), input.data()+input.size(), result);
|
|
if(answer.ec != std::errc()) {
|
|
// check error
|
|
}
|
|
// we have result == 234532.3426362.
|
|
if(answer.ptr[0] != ',') {
|
|
// unexpected delimiter
|
|
}
|
|
answer = fast_float::from_chars(answer.ptr + 1, input.data()+input.size(), result);
|
|
if(answer.ec != std::errc()) {
|
|
// check error
|
|
}
|
|
// we have result == 7869234.9823.
|
|
if(answer.ptr[0] != ',') {
|
|
// unexpected delimiter
|
|
}
|
|
answer = fast_float::from_chars(answer.ptr + 1, input.data()+input.size(), result);
|
|
if(answer.ec != std::errc()) {
|
|
// check error
|
|
}
|
|
// we have result == 324562.645.
|
|
```
|
|
|
|
|
|
## Relation With Other Work
|
|
|
|
The fast_float library is part of:
|
|
|
|
- GCC (as of version 12): the `from_chars` function in GCC relies on fast_float.
|
|
- [WebKit](https://github.com/WebKit/WebKit), the engine behind Safari (Apple's web browser)
|
|
|
|
|
|
The fastfloat algorithm is part of the [LLVM standard libraries](https://github.com/llvm/llvm-project/commit/87c016078ad72c46505461e4ff8bfa04819fe7ba).
|
|
|
|
There is a [derived implementation part of AdaCore](https://github.com/AdaCore/VSS).
|
|
|
|
|
|
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).
|
|
|
|
## Reference
|
|
|
|
- Daniel Lemire, [Number Parsing at a Gigabyte per Second](https://arxiv.org/abs/2101.11408), Software: Practice and Experience 51 (8), 2021.
|
|
|
|
## Other programming languages
|
|
|
|
- [There is an R binding](https://github.com/eddelbuettel/rcppfastfloat) called `rcppfastfloat`.
|
|
- [There is a Rust port of the fast_float library](https://github.com/aldanor/fast-float-rust/) called `fast-float-rust`.
|
|
- [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).
|
|
- [There is a C# port of the fast_float library](https://github.com/CarlVerret/csFastFloat) called `csFastFloat`.
|
|
|
|
|
|
## Users
|
|
|
|
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).
|
|
|
|
|
|
## How fast is it?
|
|
|
|
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.
|
|
|
|
<img src="http://lemire.me/blog/wp-content/uploads/2020/11/fastfloat_speed.png" width="400">
|
|
|
|
```
|
|
$ ./build/benchmarks/benchmark
|
|
# parsing random integers in the range [0,1)
|
|
volume = 2.09808 MB
|
|
netlib : 271.18 MB/s (+/- 1.2 %) 12.93 Mfloat/s
|
|
doubleconversion : 225.35 MB/s (+/- 1.2 %) 10.74 Mfloat/s
|
|
strtod : 190.94 MB/s (+/- 1.6 %) 9.10 Mfloat/s
|
|
abseil : 430.45 MB/s (+/- 2.2 %) 20.52 Mfloat/s
|
|
fastfloat : 1042.38 MB/s (+/- 9.9 %) 49.68 Mfloat/s
|
|
```
|
|
|
|
See https://github.com/lemire/simple_fastfloat_benchmark for our benchmarking code.
|
|
|
|
|
|
## Video
|
|
|
|
[](http://www.youtube.com/watch?v=AVXgvlMeIm4)<br />
|
|
|
|
## Using as a CMake dependency
|
|
|
|
This library is header-only by design. The CMake file provides the `fast_float` target
|
|
which is merely a pointer to the `include` directory.
|
|
|
|
If you drop the `fast_float` repository in your CMake project, you should be able to use
|
|
it in this manner:
|
|
|
|
```cmake
|
|
add_subdirectory(fast_float)
|
|
target_link_libraries(myprogram PUBLIC fast_float)
|
|
```
|
|
|
|
Or you may want to retrieve the dependency automatically if you have a sufficiently recent version of CMake (3.11 or better at least):
|
|
|
|
```cmake
|
|
FetchContent_Declare(
|
|
fast_float
|
|
GIT_REPOSITORY https://github.com/lemire/fast_float.git
|
|
GIT_TAG tags/v1.1.2
|
|
GIT_SHALLOW TRUE)
|
|
|
|
FetchContent_MakeAvailable(fast_float)
|
|
target_link_libraries(myprogram PUBLIC fast_float)
|
|
|
|
```
|
|
|
|
You should change the `GIT_TAG` line so that you recover the version you wish to use.
|
|
|
|
## Using as single header
|
|
|
|
The script `script/amalgamate.py` may be used to generate a single header
|
|
version of the library if so desired.
|
|
Just run the script from the root directory of this repository.
|
|
You can customize the license type and output file if desired as described in
|
|
the command line help.
|
|
|
|
You may directly download automatically generated single-header files:
|
|
|
|
https://github.com/fastfloat/fast_float/releases/download/v3.4.0/fast_float.h
|
|
|
|
## Credit
|
|
|
|
Though this work is inspired by many different people, this work benefited especially from exchanges with
|
|
Michael Eisel, who motivated the original research with his key insights, and with Nigel Tao who provided
|
|
invaluable feedback. Rémy Oudompheng first implemented a fast path we use in the case of long digits.
|
|
|
|
The library includes code adapted from Google Wuffs (written by Nigel Tao) which was originally published
|
|
under the Apache 2.0 license.
|
|
|
|
## License
|
|
|
|
<sup>
|
|
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
|
|
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
|
|
</sup>
|
|
|
|
<br>
|
|
|
|
<sub>
|
|
Unless you explicitly state otherwise, any contribution intentionally submitted
|
|
for inclusion in this repository by you, as defined in the Apache-2.0 license,
|
|
shall be dual licensed as above, without any additional terms or conditions.
|
|
</sub>
|