Deflate64

Deflate64 is a compression method used in ZIP archives as an extension of the standard Deflate algorithm. It is designated as the Deflate64 compression method (commonly referenced as method 9 in ZIP specifications) and was introduced to provide improved compression efficiency for certain data types and to better support large archives when used in conjunction with ZIP64.

Technically, Deflate64 builds on Deflate by employing a larger sliding window, typically around 64 kilobytes, and enhanced distance and length code sets. Like Deflate, it still relies on LZ77-style matching and Huffman coding, but the expanded dictionary and improved encoding enable longer matches and potentially better compression ratios for some files, especially large binary data and executables.

Adoption and compatibility vary. Deflate64 is not universally supported by all ZIP tools. Some decompression programs and libraries implement Deflate64, while others support only the standard Deflate method. Files compressed with Deflate64 may require compatible software to extract correctly. When possible, ZIP64 is used in conjunction with Deflate64 to handle very large archives and file sizes.

Limitations and considerations include compatibility concerns with older or basic ZIP tools that do not recognize the Deflate64 method. Because it is not as widely supported as the standard Deflate, users may encounter extraction issues on platforms lacking Deflate64 support. For cross-platform interoperability, many users opt for standard Deflate or rely on ZIP64 for large archives.

See also: Deflate, ZIP, ZIP64, LZ77, Huffman coding.