format_description.txt (4388B)
1 Snappy compressed format description 2 Last revised: 2011-10-05 3 4 5 This is not a formal specification, but should suffice to explain most 6 relevant parts of how the Snappy format works. It is originally based on 7 text by Zeev Tarantov. 8 9 Snappy is a LZ77-type compressor with a fixed, byte-oriented encoding. 10 There is no entropy encoder backend nor framing layer -- the latter is 11 assumed to be handled by other parts of the system. 12 13 This document only describes the format, not how the Snappy compressor nor 14 decompressor actually works. The correctness of the decompressor should not 15 depend on implementation details of the compressor, and vice versa. 16 17 18 1. Preamble 19 20 The stream starts with the uncompressed length (up to a maximum of 2^32 - 1), 21 stored as a little-endian varint. Varints consist of a series of bytes, 22 where the lower 7 bits are data and the upper bit is set iff there are 23 more bytes to be read. In other words, an uncompressed length of 64 would 24 be stored as 0x40, and an uncompressed length of 2097150 (0x1FFFFE) 25 would be stored as 0xFE 0xFF 0x7F. 26 27 28 2. The compressed stream itself 29 30 There are two types of elements in a Snappy stream: Literals and 31 copies (backreferences). There is no restriction on the order of elements, 32 except that the stream naturally cannot start with a copy. (Having 33 two literals in a row is never optimal from a compression point of 34 view, but nevertheless fully permitted.) Each element starts with a tag byte, 35 and the lower two bits of this tag byte signal what type of element will 36 follow: 37 38 00: Literal 39 01: Copy with 1-byte offset 40 10: Copy with 2-byte offset 41 11: Copy with 4-byte offset 42 43 The interpretation of the upper six bits are element-dependent. 44 45 46 2.1. Literals (00) 47 48 Literals are uncompressed data stored directly in the byte stream. 49 The literal length is stored differently depending on the length 50 of the literal: 51 52 - For literals up to and including 60 bytes in length, the upper 53 six bits of the tag byte contain (len-1). The literal follows 54 immediately thereafter in the bytestream. 55 - For longer literals, the (len-1) value is stored after the tag byte, 56 little-endian. The upper six bits of the tag byte describe how 57 many bytes are used for the length; 60, 61, 62 or 63 for 58 1-4 bytes, respectively. The literal itself follows after the 59 length. 60 61 62 2.2. Copies 63 64 Copies are references back into previous decompressed data, telling 65 the decompressor to reuse data it has previously decoded. 66 They encode two values: The _offset_, saying how many bytes back 67 from the current position to read, and the _length_, how many bytes 68 to copy. Offsets of zero can be encoded, but are not legal; 69 similarly, it is possible to encode backreferences that would 70 go past the end of the block (offset > current decompressed position), 71 which is also nonsensical and thus not allowed. 72 73 As in most LZ77-based compressors, the length can be larger than the offset, 74 yielding a form of run-length encoding (RLE). For instance, 75 "xababab" could be encoded as 76 77 <literal: "xab"> <copy: offset=2 length=4> 78 79 Note that since the current Snappy compressor works in 32 kB 80 blocks and does not do matching across blocks, it will never produce 81 a bitstream with offsets larger than about 32768. However, the 82 decompressor should not rely on this, as it may change in the future. 83 84 There are several different kinds of copy elements, depending on 85 the amount of bytes to be copied (length), and how far back the 86 data to be copied is (offset). 87 88 89 2.2.1. Copy with 1-byte offset (01) 90 91 These elements can encode lengths between [4..11] bytes and offsets 92 between [0..2047] bytes. (len-4) occupies three bits and is stored 93 in bits [2..4] of the tag byte. The offset occupies 11 bits, of which the 94 upper three are stored in the upper three bits ([5..7]) of the tag byte, 95 and the lower eight are stored in a byte following the tag byte. 96 97 98 2.2.2. Copy with 2-byte offset (10) 99 100 These elements can encode lengths between [1..64] and offsets from 101 [0..65535]. (len-1) occupies six bits and is stored in the upper 102 six bits ([2..7]) of the tag byte. The offset is stored as a 103 little-endian 16-bit integer in the two bytes following the tag byte. 104 105 106 2.2.3. Copy with 4-byte offset (11) 107 108 These are like the copies with 2-byte offsets (see previous subsection), 109 except that the offset is stored as a 32-bit integer instead of a 110 16-bit integer (and thus will occupy four bytes).