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69 lines
2.9 KiB
Go
69 lines
2.9 KiB
Go
// Package lz4 implements reading and writing lz4 compressed data (a frame),
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// as specified in http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html.
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//
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// Although the block level compression and decompression functions are exposed and are fully compatible
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// with the lz4 block format definition, they are low level and should not be used directly.
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// For a complete description of an lz4 compressed block, see:
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// http://fastcompression.blogspot.fr/2011/05/lz4-explained.html
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//
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// See https://github.com/Cyan4973/lz4 for the reference C implementation.
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//
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package lz4
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const (
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// Extension is the LZ4 frame file name extension
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Extension = ".lz4"
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// Version is the LZ4 frame format version
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Version = 1
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frameMagic uint32 = 0x184D2204
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frameSkipMagic uint32 = 0x184D2A50
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// The following constants are used to setup the compression algorithm.
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minMatch = 4 // the minimum size of the match sequence size (4 bytes)
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winSizeLog = 16 // LZ4 64Kb window size limit
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winSize = 1 << winSizeLog
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winMask = winSize - 1 // 64Kb window of previous data for dependent blocks
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compressedBlockFlag = 1 << 31
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compressedBlockMask = compressedBlockFlag - 1
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// hashLog determines the size of the hash table used to quickly find a previous match position.
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// Its value influences the compression speed and memory usage, the lower the faster,
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// but at the expense of the compression ratio.
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// 16 seems to be the best compromise.
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hashLog = 16
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hashTableSize = 1 << hashLog
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hashShift = uint((minMatch * 8) - hashLog)
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mfLimit = 8 + minMatch // The last match cannot start within the last 12 bytes.
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skipStrength = 6 // variable step for fast scan
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)
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// map the block max size id with its value in bytes: 64Kb, 256Kb, 1Mb and 4Mb.
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var (
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bsMapID = map[byte]int{4: 64 << 10, 5: 256 << 10, 6: 1 << 20, 7: 4 << 20}
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bsMapValue = make(map[int]byte, len(bsMapID))
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)
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// Reversed.
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func init() {
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for i, v := range bsMapID {
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bsMapValue[v] = i
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}
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}
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// Header describes the various flags that can be set on a Writer or obtained from a Reader.
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// The default values match those of the LZ4 frame format definition
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// (http://fastcompression.blogspot.com/2013/04/lz4-streaming-format-final.html).
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//
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// NB. in a Reader, in case of concatenated frames, the Header values may change between Read() calls.
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// It is the caller responsibility to check them if necessary.
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type Header struct {
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BlockChecksum bool // Compressed blocks checksum flag.
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NoChecksum bool // Frame checksum flag.
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BlockMaxSize int // Size of the uncompressed data block (one of [64KB, 256KB, 1MB, 4MB]). Default=4MB.
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Size uint64 // Frame total size. It is _not_ computed by the Writer.
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CompressionLevel int // Compression level (higher is better, use 0 for fastest compression).
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done bool // Header processed flag (Read or Write and checked).
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}
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