85 lines
3.0 KiB
Go
85 lines
3.0 KiB
Go
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package snappy implements the snappy block-based compression format.
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// It aims for very high speeds and reasonable compression.
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//
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// The C++ snappy implementation is at https://github.com/google/snappy
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package snappy
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import (
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"hash/crc32"
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)
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/*
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Each encoded block begins with the varint-encoded length of the decoded data,
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followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
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first byte of each chunk is broken into its 2 least and 6 most significant bits
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called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
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Zero means a literal tag. All other values mean a copy tag.
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For literal tags:
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- If m < 60, the next 1 + m bytes are literal bytes.
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- Otherwise, let n be the little-endian unsigned integer denoted by the next
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m - 59 bytes. The next 1 + n bytes after that are literal bytes.
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For copy tags, length bytes are copied from offset bytes ago, in the style of
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Lempel-Ziv compression algorithms. In particular:
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- For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
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The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
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of the offset. The next byte is bits 0-7 of the offset.
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- For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
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The length is 1 + m. The offset is the little-endian unsigned integer
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denoted by the next 2 bytes.
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- For l == 3, this tag is a legacy format that is no longer supported.
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*/
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const (
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tagLiteral = 0x00
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tagCopy1 = 0x01
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tagCopy2 = 0x02
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tagCopy4 = 0x03
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)
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const (
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checksumSize = 4
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chunkHeaderSize = 4
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magicChunk = "\xff\x06\x00\x00" + magicBody
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magicBody = "sNaPpY"
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// maxBlockSize is the maximum size of the input to encodeBlock. It is not
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// part of the wire format per se, but some parts of the encoder assume
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// that an offset fits into a uint16.
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//
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// Also, for the framing format (Writer type instead of Encode function),
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// https://github.com/google/snappy/blob/master/framing_format.txt says
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// that "the uncompressed data in a chunk must be no longer than 65536
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// bytes".
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maxBlockSize = 65536
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// maxEncodedLenOfMaxBlockSize equals MaxEncodedLen(maxBlockSize), but is
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// hard coded to be a const instead of a variable, so that obufLen can also
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// be a const. Their equivalence is confirmed by
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// TestMaxEncodedLenOfMaxBlockSize.
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maxEncodedLenOfMaxBlockSize = 76490
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obufHeaderLen = len(magicChunk) + checksumSize + chunkHeaderSize
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obufLen = obufHeaderLen + maxEncodedLenOfMaxBlockSize
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)
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const (
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chunkTypeCompressedData = 0x00
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chunkTypeUncompressedData = 0x01
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chunkTypePadding = 0xfe
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chunkTypeStreamIdentifier = 0xff
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)
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var crcTable = crc32.MakeTable(crc32.Castagnoli)
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// crc implements the checksum specified in section 3 of
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// https://github.com/google/snappy/blob/master/framing_format.txt
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func crc(b []byte) uint32 {
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c := crc32.Update(0, crcTable, b)
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return uint32(c>>15|c<<17) + 0xa282ead8
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}
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