🐉 Żmij

A fast floating-point-to-string conversion library for C and C++

Features

• Round trip guarantee
• Shortest decimal representation similar to Python's default output
• Correct rounding
• Scientific, fixed, and general formatting with a given precision (like printf's %e/%f/%g)
• Locale-independent output
• float, double, and long double support (IEEE 754 binary32/64/128 and x87 80-bit extended)
• High performance
• Small binary size
• Fast compile time
• Optional std::to_chars-style API, several times faster than common standard library implementations
• Safer API than classic dtoa
• Negative zero dependencies
• Small, clean codebase consisting of one source file and a header
• Permissive license

Usage

#include "zmij.h"
#include <stdio.h>

int main() {
  char buf[zmij::double_buffer_size + 1];
  auto end = zmij::write(buf, sizeof(buf), 5.0507837461e-27);
  *end = '\0';
  puts(buf);
}

For a std::to_chars-style API, include zmij-to-chars.h, which provides zmij::to_chars returning a {ptr, ec} result (usable in C++14):

#include "zmij-to-chars.h"

char buf[zmij::double_buffer_size];
auto result = zmij::to_chars(buf, buf + sizeof(buf), 5.0507837461e-27);
// result.ec == std::errc() on success; result.ptr points past the output.

Performance

On an Apple M5 Max running macOS, compiled with Clang 21.0, Żmij is more than 7x faster than Ryū, used by multiple C++ standard library implementations, ~13x faster than double-conversion and ~100x faster than sprintf on dtoa-benchmark.

Conversion time (smaller is better):

ostringstream and sprintf are left out of the charts to keep the faster methods readable.

On an AMD EPYC 7C13 (Milan) running Linux, compiled with GCC 13.3, Żmij is ~3.8x faster than Ryū, ~7x faster than double-conversion and ~34x faster than sprintf.

Conversion time (smaller is better):

ostringstream and sprintf are left out of the charts to keep the faster methods readable.

Compile time

Compile time is ~135ms by default and ~180ms with optimizations enabled as measured by

% time c++ -c zmij.cc [-O2]

taking the best of 3 runs.

Languages

• C++: https://github.com/vitaut/zmij/blob/main/zmij.cc (reference implementation)
• C: https://github.com/vitaut/zmij/blob/main/zmij.c
• Rust: https://github.com/dtolnay/zmij
• Zig: https://github.com/de-sh/zmij

Name

Żmij (pronounced roughly zhmeey or more precisely /ʐmij/) is a Polish word that refers to a mythical dragon- or serpent-like creature, continuing the dragon theme started by Steele and White.

A nice bonus is that the name even contains a "floating point" in its first letter. And to quote Aras Pranckevičius, "Żmij is also literally a beast."

Acknowledgements

We would like to express our gratitude to the individuals who have made Żmij possible:

• Victor Zverovich (@vitaut) - Original author and maintainer of Żmij.

• Tobias Schlüter (@TobiSchluter) - Contributed significant performance and portability improvements, including SIMD/SSE support and core algorithm refinements that enhance execution speed and cross-platform compatibility.

• Dougall Johnson (@dougallj) – Authored the NEON implementation and contributed many optimization ideas, substantially improving performance on ARM platforms.

• Alex Guteniev (@AlexGuteniev) - Contributed multiple fixes and improvements across build systems, platform compatibility, and testing infrastructure.

• Xiang JunBo (@xjb714) - Contributed high-performance BCD digit extraction algorithm and additional optimization ideas used across scalar and SIMD code paths. The double path uses xjb's $10^{-k-1}$ scaling to eliminate a division from the critical path.

• David Tolnay (@dtolnay) - Created and maintains the Rust port of Żmij, expanding the algorithm's reach and adoption in the Rust ecosystem.

• Raffaello Giulietti - Author of the Schubfach algorithm, whose work forms a foundational basis for Żmij.

• Yaoyuan Guo (@ibireme) - Author of the yy algorithm, whose ideas influenced key optimizations used in Żmij.

• Junekey Jeon (@jk-jeon) - Author of the Dragonbox algorithm, which informed design and benchmarking comparisons for Żmij, as well as the to_decimal API.

• Russ Cox (@rsc) - Described the guard-bit rerounding technique used in Żmij's fixed-precision conversion.

• Community contributors who provided feedback, issues, suggestions, and occasional commits, helping improve the robustness and performance of Żmij.