copia

copia is a pure-Rust tool that provides both a library and a command-line interface for file synchronization. It serves as a modern, dependency-free alternative to rsync, implementing a rolling-checksum and BLAKE3 delta-transfer engine without C dependencies or unsafe code. The tool is designed for reliability, using content hashes as the sole source of truth for file changes and conflicts. It supports three main layers of synchronization: L1 for incremental one-way mirroring (local or remote via SSH), L2 for two-way bidirectional sync with safe conflict resolution that preserves both versions of divergent files, and L3 for a hub-and-spoke model where multiple clients can push to a central hub concurrently. All writes are atomic, and the project includes machine-proved safety invariants using formal verification methods like Kani and Lean 4 to guarantee that data is never lost and concurrent updates are handled safely.