punycode-rs 0.1.0

A Rust implementation of Punycode (RFC 3492), the ASCII encoding behind internationalized domain names.
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punycode-rs

A Rust implementation of Punycode (RFC 3492) — the ASCII-compatible encoding at the heart of Internationalized Domain Names. It turns Unicode into ASCII and back: münchen ⇄ mnchen-3ya (which IDNA then prefixes as xn--mnchen-3ya).

The implementation is a faithful port of Python's standard-library punycode codec, and is verified against it byte-for-byte (encode and decode, including malformed-input handling).

Usage

use punycode_rs::{encode, decode};

assert_eq!(encode("münchen"), "mnchen-3ya");
assert_eq!(decode("mnchen-3ya").unwrap(), "münchen");

// Pure ASCII gains a trailing '-'.
assert_eq!(encode("abc"), "abc-");

// Decoding malformed input returns an error.
assert!(decode("not ascii é").is_err());

Installation

cargo add punycode-rs

[dependencies]
punycode-rs = "0.1"

Requires a Rust toolchain with 2024-edition support (Rust 1.85 or newer).

How it works

Punycode separates a string into its ASCII "base" (kept verbatim) and its non-ASCII characters, then encodes each non-ASCII character as a single integer delta that records both which character it is and where it gets inserted. Deltas are written as base-36 generalized variable-length integers using an adaptive bias that self-tunes for compactness. Decoding mirrors the process exactly. (See RFC 3492 §3.)

Scope

This crate is raw Punycode — the encoding algorithm itself. It does not include the full IDNA ToASCII/ToUnicode pipeline (the xn-- prefix handling and UTS-46 Unicode mapping), which layers on top and pulls in large Unicode data tables.

License

Licensed under the MIT License.