106 lines
4.2 KiB
Go
106 lines
4.2 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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// using an io.Reader (decompression) and io.Writer (compression).
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// It is designed to minimize memory usage while maximizing throughput by being able to
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// [de]compress data concurrently.
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//
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// The Reader and the Writer support concurrent processing provided the supplied buffers are
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// large enough (in multiples of BlockMaxSize) and there is no block dependency.
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// Reader.WriteTo and Writer.ReadFrom do leverage the concurrency transparently.
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// The runtime.GOMAXPROCS() value is used to apply concurrency or not.
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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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package lz4
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import (
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"hash"
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"sync"
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"github.com/pierrec/xxHash/xxHash32"
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)
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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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// 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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hasher = uint32(2654435761) // prime number used to hash minMatch
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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 bsMapID = map[byte]int{4: 64 << 10, 5: 256 << 10, 6: 1 << 20, 7: 4 << 20}
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var bsMapValue = map[int]byte{}
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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 (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 (typically when using the Reader concurrency).
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type Header struct {
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BlockDependency bool // compressed blocks are dependent (one block depends on the last 64Kb of the previous one)
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BlockChecksum bool // compressed blocks are checksumed
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NoChecksum bool // frame checksum
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BlockMaxSize int // the size of the decompressed data block (one of [64KB, 256KB, 1MB, 4MB]). Default=4MB.
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Size uint64 // the frame total size. It is _not_ computed by the Writer.
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HighCompression bool // use high compression (only for the Writer)
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done bool // whether the descriptor was processed (Read or Write and checked)
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// Removed as not supported
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// Dict bool // a dictionary id is to be used
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// DictID uint32 // the dictionary id read from the frame, if any.
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}
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// xxhPool wraps the standard pool for xxHash items.
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// Putting items back in the pool automatically resets them.
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type xxhPool struct {
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sync.Pool
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}
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func (p *xxhPool) Get() hash.Hash32 {
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return p.Pool.Get().(hash.Hash32)
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}
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func (p *xxhPool) Put(h hash.Hash32) {
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h.Reset()
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p.Pool.Put(h)
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}
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// hashPool is used by readers and writers and contains xxHash items.
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var hashPool = xxhPool{
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Pool: sync.Pool{
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New: func() interface{} { return xxHash32.New(0) },
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},
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}
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