zsync-rs

A fast, pure Rust implementation of zsync — the efficient file transfer algorithm that downloads only the parts of a file you don't already have.

How it works

zsync uses the rsync rolling checksum algorithm over HTTP. Given an old version of a file, it identifies which blocks are unchanged and downloads only the differences:

Fetches the .zsync control file (block checksums for the target file)
Scans local seed files for matching blocks using rolling checksums
Downloads only the missing blocks via HTTP range requests
Assembles the final file and verifies its SHA-1 checksum

Features

Accurate matching — sequential block chaining, proper rsum_a_mask handling, and zero-padded EOF scanning match the reference C implementation exactly
Fast lookups — flat hash table with a bithash for O(1) negative filtering, matching the C data structure design
Parallel scanning — splits large source files across CPU cores for matching (something the single-threaded C implementations can't do)
Optimized downloads — merges HTTP range requests to minimize round-trips
Multiple seed files — pass multiple -i flags to scan several local files
Self-referential scanning — after seed matching, scans the partially-assembled output to find duplicate target blocks without downloading them
Fuzz-tested — control file parser, rolling checksums, and block matcher are all fuzz-tested with cargo-fuzz

Installation

cargo install --path .

Usage

# Basic: download a file using its .zsync control file
zsync https://example.com/file.AppImage.zsync

# Use a local file as seed to avoid re-downloading unchanged blocks
zsync https://example.com/v2.0/app.AppImage.zsync -i app-v1.9.AppImage

# Multiple seed files for maximum local matching
zsync https://example.com/v3.0/app.zsync -i app-v2.9.AppImage -i app-v2.8.AppImage

# Specify output filename
zsync https://example.com/file.zsync -o output.bin

Library Usage

zsync-rs is library-first — the CLI is a thin wrapper around the public API:

use std::path::Path;
use zsync_rs::ZsyncAssembly;

let mut assembly = ZsyncAssembly::from_url(
    "https://example.com/file.zsync",
    Path::new("output.bin"),
)?;

// Match blocks from a local seed file
assembly.submit_source_file(Path::new("old-version.bin"))?;

// Download whatever's missing
while !assembly.is_complete() {
    assembly.download_missing_blocks()?;
}

// Verify checksum and finalize
assembly.complete()?;

Building

cargo build --release

Testing

cargo test

# Fuzzing (requires nightly)
cargo fuzz run fuzz_control_file
cargo fuzz run fuzz_rsum
cargo fuzz run fuzz_matcher

zsync-rs 0.1.2