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<span class="eyebrow">C++11 · header-only · triple-licensed</span>
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<h1>Parse floating-point numbers <span class="grad">at a gigabyte per second</span>.</h1>
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<p class="lede">
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<strong>fast_float</strong> is a header-only C++ implementation of
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<code>std::from_chars</code> for <code>float</code>, <code>double</code>, and integer types.
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Exact IEEE rounding, no allocations, no exceptions — often <strong>many times faster</strong>
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than your standard library.
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<span class="dot"></span> Latest release: <code data-version>v{{VERSION}}</code>
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<section id="features" class="section">
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<div class="container">
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<h2>Why fast_float?</h2>
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<p class="section-lede">A drop-in <code>from_chars</code> built for performance-critical code paths.</p>
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<div class="grid features">
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<article class="card">
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<h3>Blazing fast</h3>
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<p>Often <strong>4× faster</strong> than the best competitor and many times faster than typical standard-library implementations. Sustains <strong>1 GB/s</strong> on commodity hardware.</p>
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</article>
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<article class="card">
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<h3>Exact rounding</h3>
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<p>Returns the closest IEEE 754 <code>float</code> or <code>double</code> with round-to-nearest, ties-to-even — bit-for-bit correct.</p>
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</article>
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<article class="card">
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<h3>Header-only</h3>
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<p>Just drop in <code>fast_float.h</code> or use it via CMake, Conan, vcpkg, xmake, or Homebrew. Requires only C++11.</p>
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</article>
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<article class="card">
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<h3>No surprises</h3>
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<p>Does not allocate, does not throw, locale-independent. The interface mirrors C++17 <code>std::from_chars</code>.</p>
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</article>
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<article class="card">
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<h3>Integers too</h3>
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<p>Parses every standard integer type in bases 2–36, plus <code>bool</code>. The same fast, allocation-free interface.</p>
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</article>
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<article class="card">
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<h3>Unicode & formats</h3>
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<p>UTF-8, UTF-16, and UTF-32 inputs. JSON, Fortran, and custom decimal separators via <code>from_chars_advanced</code>.</p>
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</article>
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<article class="card">
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<h3>constexpr-ready</h3>
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<p>In C++20, parse strings at compile time with <code>consteval</code> — zero runtime cost.</p>
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</article>
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<article class="card">
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<h3>Portable</h3>
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<p>Visual Studio, GCC, Clang, MSYS2. Linux, macOS, FreeBSD, Windows. x86-64, ARM, RISC-V, s390x. 32-bit and 64-bit.</p>
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</article>
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</div>
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</div>
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</section>
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<section id="performance" class="section alt">
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<div class="container">
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<h2>Performance</h2>
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<p class="section-lede">
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Parsing random floating-point numbers, measured in megabytes per second
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(higher is better). Source: project benchmark suite on a typical x86-64 box.
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</p>
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<div class="bench">
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<div class="bench-row">
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<span class="bench-label">fast_float</span>
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<div class="bench-bar"><span class="bench-fill primary" style="--w: 100%">1042 MB/s</span></div>
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</div>
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<div class="bench-row">
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<span class="bench-label">abseil</span>
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<div class="bench-bar"><span class="bench-fill" style="--w: 41%">430 MB/s</span></div>
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</div>
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<div class="bench-row">
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<span class="bench-label">netlib</span>
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<div class="bench-bar"><span class="bench-fill" style="--w: 26%">271 MB/s</span></div>
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</div>
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<div class="bench-row">
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<span class="bench-label">double-conversion</span>
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<div class="bench-bar"><span class="bench-fill" style="--w: 22%">225 MB/s</span></div>
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</div>
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<div class="bench-row">
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<span class="bench-label">strtod</span>
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<div class="bench-bar"><span class="bench-fill" style="--w: 18%">191 MB/s</span></div>
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</div>
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</div>
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<details class="repro">
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<summary>Reproduce these numbers</summary>
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<pre data-lang="bash"><code>cmake -B build -D FASTFLOAT_BENCHMARKS=ON
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cmake --build build
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./build/benchmarks/benchmark</code></pre>
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</details>
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</div>
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</section>
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<section id="quickstart" class="section">
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<div class="container">
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<h2>Quick start</h2>
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<p class="section-lede">Parse a <code>double</code> from a string in three lines.</p>
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<pre data-lang="cpp"><code>#include "fast_float/fast_float.h"
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#include <iostream>
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#include <string>
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int main() {
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std::string input = "3.1416 xyz ";
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double result;
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auto answer = fast_float::from_chars(input.data(),
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input.data() + input.size(),
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result);
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if (answer.ec != std::errc()) {
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std::cerr << "parsing failure\n";
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return EXIT_FAILURE;
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}
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std::cout << "parsed the number " << result << '\n';
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}</code></pre>
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<h3>Integers in any base (2–36)</h3>
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<pre data-lang="cpp"><code>uint64_t value;
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std::string hex = "4f0cedc95a718c";
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auto r = fast_float::from_chars(hex.data(), hex.data() + hex.size(), value, 16);
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// value == 22250738585072012</code></pre>
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<h3>UTF-16 input</h3>
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<pre data-lang="cpp"><code>std::u16string input = u"3.1416 xyz ";
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double result;
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auto r = fast_float::from_chars(input.data(), input.data() + input.size(), result);</code></pre>
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<h3>Comma as decimal separator, Fortran, or JSON</h3>
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<pre data-lang="cpp"><code>// "3,1416" — French-style
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fast_float::parse_options opts{fast_float::chars_format::general, ','};
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fast_float::from_chars_advanced(s.data(), s.data() + s.size(), result, opts);
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// "1d+4" — Fortran exponent
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opts = {fast_float::chars_format::fortran};
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// strict JSON per RFC 8259
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opts = {fast_float::chars_format::json};</code></pre>
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<h3>C++20: parse at compile time</h3>
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<pre data-lang="cpp"><code>consteval double parse(std::string_view s) {
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double v;
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auto r = fast_float::from_chars(s.data(), s.data() + s.size(), v);
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return r.ec == std::errc() ? v : -1.0;
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}
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constexpr double pi = parse("3.1415"); // computed at compile time</code></pre>
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</div>
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</section>
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<section id="install" class="section alt">
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<div class="container">
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<h2>Install</h2>
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<p class="section-lede">Pick the workflow that matches your project.</p>
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<div class="grid install">
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<article class="card">
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<h3>CMake <code>FetchContent</code></h3>
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<pre data-lang="cmake"><code>FetchContent_Declare(
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fast_float
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GIT_REPOSITORY https://github.com/fastfloat/fast_float.git
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GIT_TAG tags/<span data-version>v{{VERSION}}</span>
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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)</code></pre>
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</article>
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<article class="card">
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<h3>CPM</h3>
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<pre data-lang="cmake"><code>CPMAddPackage(
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NAME fast_float
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GITHUB_REPOSITORY "fastfloat/fast_float"
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GIT_TAG <span data-version>v{{VERSION}}</span>)</code></pre>
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</article>
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<article class="card">
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<h3>Single header</h3>
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<p>Download a pre-amalgamated header — no build system required.</p>
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<pre data-lang="bash"><code>curl -LO https://github.com/fastfloat/fast_float/releases/download/<span data-version>v{{VERSION}}</span>/fast_float.h</code></pre>
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</article>
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<article class="card">
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<h3>Package managers</h3>
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<ul class="bullets">
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<li><a href="https://conan.io/center/recipes/fast_float">Conan</a></li>
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<li><a href="https://formulae.brew.sh/formula/fast_float">Homebrew</a> — <code>brew install fast_float</code></li>
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<li><a href="https://xmake.io">xmake</a></li>
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<li>Fedora — <code>dnf install fast_float-devel</code></li>
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<li><a href="https://repology.org/project/fast-float/versions">More distributions</a></li>
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</ul>
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</article>
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</div>
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</div>
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</section>
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<section id="users" class="section">
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<div class="container">
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<h2>Trusted by</h2>
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<p class="section-lede">fast_float ships inside compilers, browsers, databases, and more.</p>
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<ul class="users">
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<li><strong>GCC</strong> — backs <code>std::from_chars</code> since version 12</li>
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<li><strong>Chromium</strong> — Chrome, Edge, and Opera</li>
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<li><strong>WebKit</strong> — Safari</li>
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<li><strong>Ladybird</strong> — independent browser engine</li>
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<li><strong>DuckDB</strong> — in-process analytical database</li>
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<li><strong>Apache Arrow</strong> — 2–3× faster number parsing</li>
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<li><strong>ClickHouse</strong> — OLAP database</li>
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<li><strong>MySQL</strong></li>
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<li><strong>Boost.JSON</strong></li>
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<li><strong>Blender</strong></li>
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<li><strong>Google Jsonnet</strong></li>
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</ul>
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<p class="aside">
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Ports and bindings exist for
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<a href="https://github.com/aldanor/fast-float-rust/">Rust</a>,
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<a href="https://github.com/wrandelshofer/FastDoubleParser">Java</a>,
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<a href="https://github.com/CarlVerret/csFastFloat">C#</a>,
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<a href="https://github.com/kolemannix/ffc.h">C</a>, and
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<a href="https://github.com/eddelbuettel/rcppfastfloat">R</a>.
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</p>
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</div>
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</section>
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<section id="papers" class="section alt">
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<div class="container">
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<h2>The research behind it</h2>
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<ul class="papers">
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<li>
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Daniel Lemire,
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<a href="https://arxiv.org/abs/2101.11408"><strong>Number Parsing at a Gigabyte per Second</strong></a>.
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<em>Software: Practice and Experience</em> 51(8), 2021.
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</li>
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<li>
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Noble Mushtak, Daniel Lemire,
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<a href="https://arxiv.org/abs/2212.06644"><strong>Fast Number Parsing Without Fallback</strong></a>.
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<em>Software: Practice and Experience</em> 53(7), 2023.
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</li>
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||
</ul>
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</div>
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</section>
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<div>
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<strong>fast_float</strong>
|
||
<p>Triple-licensed under Apache 2.0, MIT, and Boost. Use it however you like.</p>
|
||
</div>
|
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<div>
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||
<h4>Project</h4>
|
||
<ul>
|
||
<li><a href="https://github.com/fastfloat/fast_float">GitHub repository</a></li>
|
||
<li><a href="https://github.com/fastfloat/fast_float/releases">Releases</a></li>
|
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<li><a href="https://github.com/fastfloat/fast_float/issues">Issue tracker</a></li>
|
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<li><a href="https://github.com/fastfloat/fast_float/blob/main/SECURITY.md">Security policy</a></li>
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</ul>
|
||
</div>
|
||
<div>
|
||
<h4>Learn more</h4>
|
||
<ul>
|
||
<li><a href="https://arxiv.org/abs/2101.11408">Number Parsing at a Gigabyte per Second</a></li>
|
||
<li><a href="https://www.youtube.com/watch?v=AVXgvlMeIm4">Go Systems 2020 talk</a></li>
|
||
<li><a href="https://github.com/fastfloat/fast_float/blob/main/README.md">README</a></li>
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</ul>
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</div>
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</div>
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<div class="container subfoot">
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<span>Current release <code data-version>v{{VERSION}}</code></span>
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<span>Maintained by the <a href="https://github.com/fastfloat">fast_float</a> contributors.</span>
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