435c0d9fc8
This PR switches the Nomad repository from using govendor to Go modules for managing dependencies. Aspects of the Nomad workflow remain pretty much the same. The usual Makefile targets should continue to work as they always did. The API submodule simply defers to the parent Nomad version on the repository, keeping the semantics of API versioning that currently exists.
67 lines
2.9 KiB
Go
67 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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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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)
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// map the block max size id with its value in bytes: 64Kb, 256Kb, 1Mb and 4Mb.
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const (
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blockSize64K = 64 << 10
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blockSize256K = 256 << 10
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blockSize1M = 1 << 20
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blockSize4M = 4 << 20
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)
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var (
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bsMapID = map[byte]int{4: blockSize64K, 5: blockSize256K, 6: blockSize1M, 7: blockSize4M}
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bsMapValue = map[int]byte{blockSize64K: 4, blockSize256K: 5, blockSize1M: 6, blockSize4M: 7}
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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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