What is "zpaq"

ZPAQ is an open source ( GPL) command line archiver for Windows and Linux. It uses a journaling or append-only format which can be rolled back to an earlier state to retrieve older versions of files and directories. It supports fast incremental update by adding only files whose last-modified date has changed since the previous update. It compresses using deduplication and several algorithms ( LZ77, BWT, and context mixing) depending on the data type and the selected compression level. To preserve forward and backward compatibility between versions as the compression algorithm is improved, it stores the decompression algorithm in the archive. The ZPAQ source code includes a public domain API, libzpaq, which provides compression and decompression services to C++ applications. The format is believed to be unencumbered by patents.