Bug: 166474546

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  1. 5f99c12 Mark ab/6881855 as merged by Xin Li · 8 weeks ago master
  2. 6e71720 Copy description from Cargo.toml to METADATA am: 1ea29df5bc by Chih-Hung Hsieh · 3 months ago
  3. 1ea29df Copy description from Cargo.toml to METADATA by Chih-Hung Hsieh · 3 months ago
  4. 250e55c Import 'ryu' crate version 1.0.5 am: ce81bb62a2 am: 261516ff4c am: 803bba2af0 am: 1f6fa1f83c by Yi Kong · 5 months ago
  5. 1f6fa1f Import 'ryu' crate version 1.0.5 am: ce81bb62a2 am: 261516ff4c am: 803bba2af0 by Yi Kong · 5 months ago

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=f320.00.12342.718281828459045f32::MAX
RYU3ns28ns23ns22ns
STD40ns106ns128ns110ns
type=f640.00.12342.718281828459045f64::MAX
RYU3ns50ns35ns32ns
STD39ns105ns128ns202ns

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