1. Algorithm

The deflation algorithm used by zip and gzip is a variation of Lempel-Ziv 1977 [LZ77]. It finds duplicated strings in the input data. The second occurrence of a string is replaced by a pointer to the previous string, in the form of a pair (distance, length). Distances are limited to 32K bytes, and lengths are limited to 258 bytes. When a string does not occur anywhere in the previous 32K bytes, it is emitted as a sequence of literal bytes. (In this description, 'string' must be taken as an arbitrary sequence of bytes, and is not restricted to printable characters.)

Literals or match lengths are compressed with one Huffman tree, and match distances are compressed with another tree. The trees are stored in a compact form at the start of each block. The blocks can have any size (except that the compressed data for one block must fit in available memory). A block is terminated when zip determines that it would be useful to start another block with fresh trees. (This is somewhat similar to compress.)

Duplicated strings are found using a hash table. All input strings of length 3 are inserted in the hash table. A hash index is computed for the next 3 bytes. If the hash chain for this index is not empty, all strings in the chain are compared with the current input string, and the longest match is selected.

The hash chains are searched starting with the most recent strings, to favor small distances and thus take advantage of the Huffman encoding. The hash chains are singly linked. There are no deletions from the hash chains, the algorithm simply discards matches that are too old.

To avoid a worst-case situation, very long hash chains are arbitrarily truncated at a certain length, determined by a runtime option (zip -1 to -9). So zip does not always find the longest possible match but generally finds a match which is long enough.

zip also defers the selection of matches with a lazy evaluation mechanism. After a match of length N has been found, zip searches for a longer match at the next input byte. If a longer match is found, the previous match is truncated to a length of one (thus producing a single literal byte) and the longer match is emitted afterwards. Otherwise, the original match is kept, and the next match search is attempted only N steps later.

The lazy match evaluation is also subject to a runtime parameter. If the current match is long enough, zip reduces the search for a longer match, thus speeding up the whole process. If compression ratio is more important than speed, zip attempts a complete second search even if the first match is already long enough.

The lazy match evaluation is not performed for the fastest compression modes (speed options -1 to -3). For these fast modes, new strings are inserted in the hash table only when no match was found, or when the match is not too long. This degrades the compression ratio but saves time since there are both fewer insertions and fewer searches.

2. gzip file format

The gzip file format was standardized in Internet RFC 1952 [RFC1952]. This section briefly describes the format and comments on some implementation details.

The pkzip format imposes a lot of overhead in various headers, which are useful for an archiver but not necessary when only one file is compressed. gzip uses a much simpler structure. Numbers are in little endian format, and bit 0 is the least significant bit. A gzip file is a sequence of compressed members. Each member has the following structure:

2 bytes magic header 0x1f, 0x8b (\037 \213)
1 byte compression method (0..7 reserved, 8 = deflate)
1 byte flags
bit 0 set: file probably ascii text
bit 1 set: header CRC-16 present
bit 2 set: extra field present
bit 3 set: original file name present
bit 4 set: file comment present
bit 5,6,7: reserved
4 bytes file modification time in Unix format
1 byte extra flags (depend on compression method)
1 byte operating system on which compression took place
2 bytes optional part number (second part=1)
2 bytes optional extra field length
? bytes optional extra field
? bytes optional original file name, zero terminated
? bytes optional file comment, zero terminated
2 bytes optional 16-bit header CRC
? bytes compressed data
4 bytes crc32
4 bytes uncompressed input size modulo 2^32

The format was designed to allow single pass compression without any backwards seek, and without a priori knowledge of the uncompressed input size or the available size on the output media. If input does not come from a regular disk file, the file modification time is set to the time at which compression started.

The timestamp is useful mainly when one gzip file is transferred over a network. In this case it would not help to keep ownership attributes. In the local case, the ownership attributes are preserved by gzip when compressing/decompressing the file. A timestamp of zero is ignored.

Bit 0 in the flags is only an optional indication, which can be set by a small lookahead in the input data. In case of doubt, the flag is cleared indicating binary data. For systems which have different file formats for ascii text and binary data, the decompressor can use the flag to choose the appropriate format.

The extra field, if present, must consist of one or more subfields, each with the following format:

subfield id 2 bytes
subfield size 2 bytes (little-endian format)
subfield data

The subfield id can consist of two letters with some mnemonic value. Please send any such id to <gzip@gnu.org>. Ids with a zero second byte are reserved for future use. The following ids are defined:

Ap (0x41, 0x70) Apollo file type information

The subfield size is the size of the subfield data and does not include the id and the size itself. The field 'extra field length' is the total size of the extra field, including subfield ids and sizes.

It must be possible to detect the end of the compressed data with any compression format, regardless of the actual size of the compressed data. If the compressed data cannot fit in one file (in particular for diskettes), each part starts with a header as described above, but only the last part has the crc32 and uncompressed size. A decompressor may prompt for additional data for multi-part compressed files. It is desirable but not mandatory that multiple parts be extractable independently so that partial data can be recovered if one of the parts is damaged. This is possible only if no compression state is kept from one part to the other. The compression-type dependent flags can indicate this.

If the file being compressed is on a file system with case insensitive names, the original name field must be forced to lower case. There is no original file name if the data was compressed from standard input.

Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the gzip file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. Note that the actual number of used disk blocks almost never increases.

Jean-loup Gailly


[LZ77] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data Compression", IEEE Transactions on Information Theory, Vol. 23, No. 3, May 1977, pp. 337-343.

[RFC1952] Deutsch P., "GZIP file format specification version 4.3", Internet RFC 1952, May 1996, <https://www.ietf.org/rfc/rfc1952.txt>.

APPNOTE.TXT documentation file in PKZIP 1.93a (October 1991). This version no longer seems to be available online; the latest version is in <https://www.pkware.com/documents/casestudies/APPNOTE.TXT>.