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

Features

small — compact tables for Wasm

The core algorithm needs 10^p as a 128-bit normalized mantissa for each exponent p in −348..=347. By default, all 696 entries are stored in a flat lookup table (696 × 16 = 11 KB).

The small feature decomposes the lookup using 10^p = 10^(27q) × 10^r where p = 27q + r and 0 ≤ r < 27. This replaces the single table with:

• POW10_COARSE: 26 entries of (u64, u64) for 10^(27q) — 416 bytes
• POW10_FINE: 27 entries of u64 for 10^r — 216 bytes

Fine entries need only one u64 (not two) because 10^r for r ≤ 26 is exact at 128 bits and the low 64 bits are always zero.

At runtime, prescale multiplies the two factors back together with u128 arithmetic instead of doing a direct table lookup. This reduces Wasm binary size from 14 KB to 4 KB with a modest formatting slowdown (~1.6x), while parsing is unaffected. Still 2.4x faster than ryu for formatting.

fpfmt = { version = "0.2", features = ["small"] }

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	fpfmt small	ryu	stdlib
format (f64 → string)	63 ns	99 ns	239 ns	529 ns
parse (string → f64)	690 ns	700 ns	—	675 ns

cargo bench -p bench
cargo bench -p bench --features small

Wasm size

Configuration	Size
default	14,379 bytes
small	4,175 bytes

cargo build --target wasm32-unknown-unknown --release -p wasm-size
cargo build --target wasm32-unknown-unknown --release -p wasm-size --features small
wc -c target/wasm32-unknown-unknown/release/wasm_size.wasm

License

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