Snappy (compression)

Snappy encoding is not bit-oriented, but byte-oriented (only whole bytes are emitted or consumed from a stream). The format uses no entropy encoder, like Huffman tree or arithmetic encoder.

The first bytes of the stream are the length of uncompressed data, stored as a little-endian varint,[8] which allows for variable-length encoding. The lower seven bits of each byte are used for data and the high bit is a flag to indicate the end of the length field.

The remaining bytes in the stream are encoded using one of four element types. The element type is encoded in the lower two bits of the first byte (tag byte) of the element:[9]

• 00 – Literal – uncompressed data; upper 6 bits are used to store length (len-1) of data. Lengths larger than 60 are stored in a 1-4 byte integer indicated by a 6 bit length of 60 (1 byte) to 63 (4 bytes).
• 01 – Copy with length stored as 3 bits and offset stored as 11 bits; one byte after tag byte is used for part of offset;
• 10 – Copy with length stored as 6 bits of tag byte and offset stored as two-byte integer after the tag byte;
• 11 – Copy with length stored as 6 bits of tag byte and offset stored as four-byte little-endian integer after the tag byte;

The copy refers to the dictionary (just-decompressed data). The offset is the shift from the current position back to the already decompressed stream. The length is the number of bytes to copy from the dictionary. The size of the dictionary was limited by the 1.0 Snappy compressor to 32,768 bytes, and updated to 65,536 in version 1.1.

The text

Wikipedia is a free, web-based, collaborative, multilingual encyclopedia project supported by the non-profit Wikimedia Foundation. Its 19 million articles (over 3.6 million in English) have been written collaboratively by volunteers around the world, and almost all of its articles can be edited by anyone with access to the site.

may be compressed to this, shown as hex data with explanations:

0000000: ca02 f042 5769 6b69 7065 6469 6120 6973  ...BWikipedia is

The first 2 bytes, ca02 are the length, as a little-endian varint (see Protocol Buffers for the varint specification).[8] Thus the most-significant byte is '02' . 0x02ca(varint) = 0x014a = 330 bytes. The next two bytes, 0xf042, indicate that a literal of 66+1 bytes follows

0000010: 2061 2066 7265 652c 2077 6562 2d62 6173   a free, web-bas
0000020: 6564 2c20 636f 6c6c 6162 6f72 6174 6976  ed, collaborativ
0000030: 652c 206d 756c 7469 6c69 6e67 7561 6c20  e, multilingual
0000040: 656e 6379 636c 6f09 3ff0 8170 726f 6a65  encyclo.?..proje

0x09 is tag-byte of type 01 with length - 4 = 010₂ = 2₁₀ and offset = 0x03f = 63 or "pedia ";
0xf081 is a literal with length of 129+1 bytes

In this example, all common substrings with four or more characters were eliminated by the compression process. More common compressors can compress this better. Unlike compression methods such as gzip and bzip2, there is no entropy encoding used to pack alphabet into the bit stream.

Snappy distributions include C++ and C bindings. Third party-provided bindings and ports include[10] C#, Common Lisp, Erlang, Go, Haskell, Lua, Java, Node.js, Perl, PHP, Python, R, Ruby, Rust, Smalltalk, and OpenCL.[11][12]

• Zstandard

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