A double-to-string conversion algorithm based on Schubfach and yy with implementations in C and C++

• Round trip guarantee
• Shortest decimal representation
• Correct rounding
• High performance
• Fast compile time
• IEEE 754 double and float support
• Safer API than classic dtoa
• Negative zero dependencies
• Small, clean codebase consisting of one source file and one header
• Permissive license

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

int main() {
  char buf[zmij::double_buffer_size];
  zmij::write(buf, sizeof(buf), 6.62607015e-34);
  puts(buf);
}

More than 3x faster than Ryu used by multiple C++ standard library implementations and ~2x faster than Schubfach on dtoa-benchmark run on Apple M1.

Compile time is ~135ms by default and ~155ms with optimizations enabled as measured by

% time c++ -c -std=c++20 zmij.cc [-O2]

taking the best of 3 runs.

• 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

• 1 instead of 3 multiplications by powers of 10 in the common case
• Faster logarithm approximations
• Faster division and modulo
• Fewer conditional branches
• More efficient significand and exponent output
• Improved storage of powers of 10

Żmij (pronounced roughly zhmeey or more precisely /ʐmij/) is a Polish word that refers to a mythical dragon- or serpent-like creature. This continues the dragon theme started by Steele and White. Nice feature of this name is that it has a floating point in the first letter.