Ultra-fast Vietnamese input method engine (Telex, VNI) written in Rust.

A no_std / no-alloc compatible library with zero external dependencies. Designed for sub-microsecond latency per keystroke through incremental state updates, positive syllable validation, and 100% stack-allocated hot paths.

macOS implementation: See UVieKey for the macOS menu bar app that uses this engine.

• Telex & VNI: Full support for both popular input methods.
• Modern orthography: Optional tone placement per new standard (e.g. hoas → hoá).
• Relaxed coda mode: Optional lenient coda validation for flexible typing.
• Per-character state: Each keystroke gets its own state entry; transforms are bit-flips, not multi-pass reordering.
• Validate raw keystrokes: Checks raw ASCII sequence against positive syllable tables before any transform. English passthrough is automatic.
• Diff-based API: Returns (backspace_count, suffix_to_type) per keystroke for minimal screen updates.
• O(1) backspace: Snapshot stack mechanism eliminates O(n²) rebuild on backspace.
• No heap in hot path: All buffer types (StackStr, CharVec, SylBuf) are stack-allocated — zero allocation per keystroke.
• Zero dependencies: No heapless, no external crates — pure Rust, no_std-compatible out of the box.
• no_std compatible: Works in embedded or constrained environments with --no-default-features.

The engine uses an incremental, stateful model with per-character state buffers. See src/ARCHITECT.md for detailed design rationale, state model, and per-keystroke flow.

use uvie::{UltraFastViEngine, InputMethod};

let mut engine = UltraFastViEngine::new();
engine.set_input_method(InputMethod::Telex);

// Feed keystrokes
let result = engine.feed('t');  // "t"
let result = engine.feed('i');  // "ti"
let result = engine.feed('e');  // "tie"
let result = engine.feed('s');  // "tiế"

// Commit word (e.g. on space)
engine.commit();

The library compiles to staticlib and cdylib. The C API provides:

• uvie_engine_new() / uvie_engine_free()
• uvie_feed(engine, key, &backspace_count, suffix, suffix_len)
• uvie_backspace(engine, &backspace_count, suffix, suffix_len)
• uvie_set_mode(engine, method)
• Configuration toggles (modern orthography, etc.)

See src/ffi.rs for the full C API.

# Build release library
cargo build --release

# Run benchmarks
cargo bench

# Run tests
cargo test

For no_std builds:

cargo build --release --no-default-features

GitHub releases provide ready-to-link static libraries:

See the release workflow for build details.

Apple Silicon (cargo bench), comparison against the vi crate:

Optimizations: single-pass char counting, mem::swap/mem::take instead of String::clone, StackStr<N> stack-allocated UTF-8 buffer, version-based partition_syllable() cache, branch prediction reordering, O(1) backspace via snapshot stack, and thread_local scratch engine for V-C-V splits.

Full report: thuupx.github.io/uvie-rs/criterion/report/

MIT OR Apache-2.0