vlen

The fastest-decoding self-delimiting varint for Rust. Integers and floats up to 128 bits, smaller values in fewer bytes, zero dependencies, no unsafe code by default, no_std.

use vlen::{Decode, Encode};

let mut buf = [0u8; 5];
let len = 12345u32.encode(&mut buf)?;          // 2 bytes
let (value, _) = u32::decode(&buf[..len])?;    // 12345
# Ok::<(), vlen::Error>(())

Why vlen

It decodes faster than every varint we could find to compare against — including on adversarial input. The length lives in the first byte instead of continuation bits spread across the value, so decoding is one predicted branch:

Encode and single-value results from the same run, with honest losses included:

Methodology: every codec through its fastest public in-memory API over identical data, from one run of benches/comparison.rs. The bulk rows and the chart all use vlen's validating API. The single-value rows use its infallible array API; through the validating slice API — the same work the other crates always do — vlen measures 0.77 ns decode / 1.58 ns encode, level with prost's validating decode (0.80 ns). stream-vbyte runs its scalar kernels (its SSE4.1 decoder is faster on x86_64) and is a control-stream format rather than a self-delimiting varint; prost and vint64 are 64-bit codecs fed the same values widened to u64.

Compression matches LEB128 byte-for-byte below 2^28 — where most varint data lives — and caps at 9 bytes for u64, where LEB128 needs up to 10.

It is safe to point at untrusted bytes. Default builds contain no unsafe code (#![deny(unsafe_code)]); the opt-in simd feature adds two small, audited NEON/SSE2 kernels whose soundness holds by construction (baseline target features, array-derived pointers). The checked API returns typed errors — never panics, never desynchronizes on truncated input, invalid prefixes, or out-of-range values — and decoding needs only the bytes a value actually occupies.

It runs everywhere, at compile time too. The core is dependency- free no_std - the checked codec compiles to ~500 bytes on Cortex-M with no panic paths and no memcpy dependency - and every array-based codec function is const fn:

const LEN: usize = {
    let mut buf = [0u8; 5];
    vlen::encode_u32(&mut buf, 12345)
};

One wire format across all widths. A value encoded as u16 produces the same bytes as u32, u64, or u128, and decodes at any width that can hold it — no cross-type surprises when a field grows.

Bulk operations that exploit your data's shape. The specialized bulk functions for u32, u64, i32, and i64 detect runs of similarly-sized values and move them without per-value length arithmetic — up to 4x faster than the per-value loop on small-value streams, and the signed variants are built for delta-encoded data (smooth delta streams decode ~1.8x faster than the generic loop) — in portable safe Rust on every architecture. Streaming callers get the same treatment: the run-accelerated iterators (decode_iter_u32 and friends) iterate small-value streams ~4.6x faster than the generic decode_iter, which handles any type:

use vlen::{bulk_encode, decode_iter};

let values = [1u64, 250, 70_000, u64::MAX];
let mut buf = [0u8; 36];
let len = bulk_encode(&mut buf, &values)?;

let decoded: Result<Vec<u64>, vlen::Error> =
    decode_iter(&buf[..len]).collect();
assert_eq!(decoded?, values);
# Ok::<(), vlen::Error>(())

Serde that respects your format. With the serde feature, the Vlen* wrapper types hand binary formats (postcard, bincode, ...) the raw encoded bytes and human-readable formats (JSON, ...) base64 — allocation-free either way, hostile input rejected with errors.

Features

MSRV: 1.85. Tested in CI on x86_64 and aarch64, stable and MSRV, with clippy, rustfmt, and no_std builds gating every change.

When something else fits better

If your workload is purely columnar bulk u32 compression — no streaming, no self-delimiting values — a control-stream format like stream-vbyte encodes unpredictably interleaved sizes faster (its lengths live in a separate control stream, so per-value size changes cost it nothing). vlen is built for the general case: self-delimiting streams you can read value by value.

Learn more

• API documentation
• DESIGN.md — wire format specification and performance design notes
• CHANGELOG.md — release notes and migration guides

License

MPL-2.0 — see LICENSE. Based on the original vu128 implementation by John Millikin (attribution in LICENSE-VU128.txt), with performance improvements and enhancements by Harrison Chin.