Ryū

Pure Rust implementation of Ryū, an algorithm to quickly convert floating point numbers to decimal strings.

The PLDI'18 paper Ryū: fast float-to-string conversion by Ulf Adams includes a complete correctness proof of the algorithm. The paper is available under the creative commons CC-BY-SA license.

This Rust implementation is a line-by-line port of Ulf Adams' implementation in C, https://github.com/ulfjack/ryu.

Requirements: this crate supports any compiler version back to rustc 1.31; it uses nothing from the Rust standard library so is usable from no_std crates.

[dependencies]
ryu = "1.0"

Example

fn main() {
    let mut buffer = ryu::Buffer::new();
    let printed = buffer.format(1.234);
    assert_eq!(printed, "1.234");
}

Performance

You can run upstream's benchmarks with:

$ git clone https://github.com/ulfjack/ryu c-ryu
$ cd c-ryu
$ bazel run -c opt //ryu/benchmark

And the same benchmark against our implementation with:

$ git clone https://github.com/dtolnay/ryu rust-ryu
$ cd rust-ryu
$ cargo run --example upstream_benchmark --release

These benchmarks measure the average time to print a 32-bit float and average time to print a 64-bit float, where the inputs are distributed as uniform random bit patterns 32 and 64 bits wide.

The upstream C code, the unsafe direct Rust port, and the safe pretty Rust API all perform the same, taking around 21 nanoseconds to format a 32-bit float and 31 nanoseconds to format a 64-bit float.

There is also a Rust-specific benchmark comparing this implementation to the standard library which you can run with:

$ cargo bench

The benchmark shows Ryū approximately 4-10x faster than the standard library across a range of f32 and f64 inputs. Measurements are in nanoseconds per iteration; smaller is better.

type=f32	0.0	0.1234	2.718281828459045	f32::MAX
RYU	3ns	28ns	23ns	22ns
STD	40ns	106ns	128ns	110ns

type=f64	0.0	0.1234	2.718281828459045	f64::MAX
RYU	3ns	50ns	35ns	32ns
STD	39ns	105ns	128ns	202ns

Formatting

This library tends to produce more human-readable output than the standard library's to_string, which never uses scientific notation. Here are two examples:

• ryu: 1.23e40, std: 12300000000000000000000000000000000000000
• ryu: 1.23e-40, std: 0.000000000000000000000000000000000000000123

Both libraries print short decimals such as 0.0000123 without scientific notation.

License

^{Licensed under either of Apache License, Version 2.0 or Boost Software License 1.0 at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}