fpfmt

Rust port of rsc/fpfmt, a floating-point formatting algorithm.

Provides shortest-representation and fixed-width formatting of f64 values, as well as parsing decimal strings back to f64.

API

• short(f: f64) -> (u64, i32) — shortest decimal that round-trips back to f
• parse(d: u64, p: i32) -> f64 — round d * 10^p to nearest f64
• parse_text(s: &[u8]) -> Option<f64> — parse a decimal string to f64
• fixed_width(f: f64, n: i32) -> (u64, i32) — n-digit decimal form
• fmt_float(s: &mut [u8], d: u64, p: i32, nd: i32) -> usize — format into buffer
• digits(d: u64) -> i32 — number of decimal digits

Tools

Regenerate the power-of-10 table:

cargo run -p pow10gen

Benchmarks

Formatting and parsing 8 representative f64 values (1.0, 0.1, 3.14, PI, E, 1e23, 5e-324, 1.7976931348623157e308).

Measured on Apple M3 Pro, macOS 15.7.3 (aarch64):

Task	fpfmt	ryu	stdlib
format (f64 → string)	63 ns	164 ns	535 ns
parse (string → f64)	738 ns	—	702 ns

cargo bench -p bench

Wasm size

32,970 bytes for short + parse as a cdylib (wasm32-unknown-unknown, -Oz).

RUSTFLAGS="-C opt-level=s" cargo build --target wasm32-unknown-unknown --release -p wasm-size
RUSTFLAGS="-C opt-level=z" cargo build --target wasm32-unknown-unknown --release -p wasm-size
wc -c target/wasm32-unknown-unknown/release/wasm_size.wasm

License

Same as the original: BSD-style. See LICENSE.