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336 lines
8.9 KiB
Go
336 lines
8.9 KiB
Go
// Package huff0 provides fast huffman encoding as used in zstd.
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//
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// See README.md at https://github.com/klauspost/compress/tree/master/huff0 for details.
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package huff0
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import (
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"errors"
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"fmt"
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"math"
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"math/bits"
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"github.com/klauspost/compress/fse"
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)
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const (
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maxSymbolValue = 255
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// zstandard limits tablelog to 11, see:
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// https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#huffman-tree-description
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tableLogMax = 11
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tableLogDefault = 11
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minTablelog = 5
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huffNodesLen = 512
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// BlockSizeMax is maximum input size for a single block uncompressed.
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BlockSizeMax = 1<<18 - 1
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)
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var (
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// ErrIncompressible is returned when input is judged to be too hard to compress.
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ErrIncompressible = errors.New("input is not compressible")
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// ErrUseRLE is returned from the compressor when the input is a single byte value repeated.
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ErrUseRLE = errors.New("input is single value repeated")
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// ErrTooBig is return if input is too large for a single block.
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ErrTooBig = errors.New("input too big")
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// ErrMaxDecodedSizeExceeded is return if input is too large for a single block.
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ErrMaxDecodedSizeExceeded = errors.New("maximum output size exceeded")
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)
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type ReusePolicy uint8
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const (
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// ReusePolicyAllow will allow reuse if it produces smaller output.
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ReusePolicyAllow ReusePolicy = iota
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// ReusePolicyPrefer will re-use aggressively if possible.
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// This will not check if a new table will produce smaller output,
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// except if the current table is impossible to use or
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// compressed output is bigger than input.
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ReusePolicyPrefer
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// ReusePolicyNone will disable re-use of tables.
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// This is slightly faster than ReusePolicyAllow but may produce larger output.
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ReusePolicyNone
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// ReusePolicyMust must allow reuse and produce smaller output.
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ReusePolicyMust
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)
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type Scratch struct {
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count [maxSymbolValue + 1]uint32
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// Per block parameters.
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// These can be used to override compression parameters of the block.
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// Do not touch, unless you know what you are doing.
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// Out is output buffer.
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// If the scratch is re-used before the caller is done processing the output,
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// set this field to nil.
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// Otherwise the output buffer will be re-used for next Compression/Decompression step
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// and allocation will be avoided.
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Out []byte
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// OutTable will contain the table data only, if a new table has been generated.
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// Slice of the returned data.
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OutTable []byte
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// OutData will contain the compressed data.
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// Slice of the returned data.
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OutData []byte
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// MaxDecodedSize will set the maximum allowed output size.
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// This value will automatically be set to BlockSizeMax if not set.
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// Decoders will return ErrMaxDecodedSizeExceeded is this limit is exceeded.
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MaxDecodedSize int
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br byteReader
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// MaxSymbolValue will override the maximum symbol value of the next block.
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MaxSymbolValue uint8
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// TableLog will attempt to override the tablelog for the next block.
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// Must be <= 11 and >= 5.
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TableLog uint8
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// Reuse will specify the reuse policy
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Reuse ReusePolicy
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// WantLogLess allows to specify a log 2 reduction that should at least be achieved,
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// otherwise the block will be returned as incompressible.
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// The reduction should then at least be (input size >> WantLogLess)
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// If WantLogLess == 0 any improvement will do.
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WantLogLess uint8
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symbolLen uint16 // Length of active part of the symbol table.
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maxCount int // count of the most probable symbol
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clearCount bool // clear count
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actualTableLog uint8 // Selected tablelog.
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prevTableLog uint8 // Tablelog for previous table
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prevTable cTable // Table used for previous compression.
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cTable cTable // compression table
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dt dTable // decompression table
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nodes []nodeElt
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tmpOut [4][]byte
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fse *fse.Scratch
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huffWeight [maxSymbolValue + 1]byte
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}
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// TransferCTable will transfer the previously used compression table.
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func (s *Scratch) TransferCTable(src *Scratch) {
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if cap(s.prevTable) < len(src.prevTable) {
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s.prevTable = make(cTable, 0, maxSymbolValue+1)
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}
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s.prevTable = s.prevTable[:len(src.prevTable)]
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copy(s.prevTable, src.prevTable)
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s.prevTableLog = src.prevTableLog
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}
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func (s *Scratch) prepare(in []byte) (*Scratch, error) {
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if len(in) > BlockSizeMax {
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return nil, ErrTooBig
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}
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if s == nil {
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s = &Scratch{}
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}
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if s.MaxSymbolValue == 0 {
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s.MaxSymbolValue = maxSymbolValue
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}
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if s.TableLog == 0 {
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s.TableLog = tableLogDefault
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}
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if s.TableLog > tableLogMax || s.TableLog < minTablelog {
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return nil, fmt.Errorf(" invalid tableLog %d (%d -> %d)", s.TableLog, minTablelog, tableLogMax)
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}
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if s.MaxDecodedSize <= 0 || s.MaxDecodedSize > BlockSizeMax {
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s.MaxDecodedSize = BlockSizeMax
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}
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if s.clearCount && s.maxCount == 0 {
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for i := range s.count {
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s.count[i] = 0
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}
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s.clearCount = false
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}
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if cap(s.Out) == 0 {
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s.Out = make([]byte, 0, len(in))
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}
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s.Out = s.Out[:0]
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s.OutTable = nil
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s.OutData = nil
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if cap(s.nodes) < huffNodesLen+1 {
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s.nodes = make([]nodeElt, 0, huffNodesLen+1)
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}
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s.nodes = s.nodes[:0]
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if s.fse == nil {
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s.fse = &fse.Scratch{}
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}
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s.br.init(in)
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return s, nil
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}
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type cTable []cTableEntry
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func (c cTable) write(s *Scratch) error {
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var (
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// precomputed conversion table
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bitsToWeight [tableLogMax + 1]byte
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huffLog = s.actualTableLog
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// last weight is not saved.
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maxSymbolValue = uint8(s.symbolLen - 1)
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huffWeight = s.huffWeight[:256]
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)
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const (
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maxFSETableLog = 6
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)
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// convert to weight
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bitsToWeight[0] = 0
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for n := uint8(1); n < huffLog+1; n++ {
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bitsToWeight[n] = huffLog + 1 - n
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}
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// Acquire histogram for FSE.
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hist := s.fse.Histogram()
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hist = hist[:256]
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for i := range hist[:16] {
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hist[i] = 0
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}
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for n := uint8(0); n < maxSymbolValue; n++ {
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v := bitsToWeight[c[n].nBits] & 15
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huffWeight[n] = v
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hist[v]++
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}
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// FSE compress if feasible.
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if maxSymbolValue >= 2 {
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huffMaxCnt := uint32(0)
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huffMax := uint8(0)
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for i, v := range hist[:16] {
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if v == 0 {
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continue
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}
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huffMax = byte(i)
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if v > huffMaxCnt {
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huffMaxCnt = v
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}
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}
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s.fse.HistogramFinished(huffMax, int(huffMaxCnt))
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s.fse.TableLog = maxFSETableLog
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b, err := fse.Compress(huffWeight[:maxSymbolValue], s.fse)
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if err == nil && len(b) < int(s.symbolLen>>1) {
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s.Out = append(s.Out, uint8(len(b)))
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s.Out = append(s.Out, b...)
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return nil
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}
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// Unable to compress (RLE/uncompressible)
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}
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// write raw values as 4-bits (max : 15)
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if maxSymbolValue > (256 - 128) {
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// should not happen : likely means source cannot be compressed
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return ErrIncompressible
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}
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op := s.Out
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// special case, pack weights 4 bits/weight.
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op = append(op, 128|(maxSymbolValue-1))
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// be sure it doesn't cause msan issue in final combination
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huffWeight[maxSymbolValue] = 0
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for n := uint16(0); n < uint16(maxSymbolValue); n += 2 {
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op = append(op, (huffWeight[n]<<4)|huffWeight[n+1])
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}
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s.Out = op
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return nil
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}
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func (c cTable) estTableSize(s *Scratch) (sz int, err error) {
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var (
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// precomputed conversion table
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bitsToWeight [tableLogMax + 1]byte
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huffLog = s.actualTableLog
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// last weight is not saved.
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maxSymbolValue = uint8(s.symbolLen - 1)
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huffWeight = s.huffWeight[:256]
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)
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const (
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maxFSETableLog = 6
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)
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// convert to weight
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bitsToWeight[0] = 0
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for n := uint8(1); n < huffLog+1; n++ {
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bitsToWeight[n] = huffLog + 1 - n
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}
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// Acquire histogram for FSE.
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hist := s.fse.Histogram()
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hist = hist[:256]
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for i := range hist[:16] {
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hist[i] = 0
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}
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for n := uint8(0); n < maxSymbolValue; n++ {
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v := bitsToWeight[c[n].nBits] & 15
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huffWeight[n] = v
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hist[v]++
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}
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// FSE compress if feasible.
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if maxSymbolValue >= 2 {
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huffMaxCnt := uint32(0)
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huffMax := uint8(0)
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for i, v := range hist[:16] {
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if v == 0 {
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continue
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}
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huffMax = byte(i)
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if v > huffMaxCnt {
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huffMaxCnt = v
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}
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}
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s.fse.HistogramFinished(huffMax, int(huffMaxCnt))
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s.fse.TableLog = maxFSETableLog
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b, err := fse.Compress(huffWeight[:maxSymbolValue], s.fse)
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if err == nil && len(b) < int(s.symbolLen>>1) {
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sz += 1 + len(b)
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return sz, nil
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}
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// Unable to compress (RLE/uncompressible)
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}
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// write raw values as 4-bits (max : 15)
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if maxSymbolValue > (256 - 128) {
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// should not happen : likely means source cannot be compressed
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return 0, ErrIncompressible
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}
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// special case, pack weights 4 bits/weight.
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sz += 1 + int(maxSymbolValue/2)
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return sz, nil
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}
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// estimateSize returns the estimated size in bytes of the input represented in the
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// histogram supplied.
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func (c cTable) estimateSize(hist []uint32) int {
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nbBits := uint32(7)
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for i, v := range c[:len(hist)] {
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nbBits += uint32(v.nBits) * hist[i]
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}
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return int(nbBits >> 3)
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}
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// minSize returns the minimum possible size considering the shannon limit.
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func (s *Scratch) minSize(total int) int {
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nbBits := float64(7)
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fTotal := float64(total)
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for _, v := range s.count[:s.symbolLen] {
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n := float64(v)
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if n > 0 {
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nbBits += math.Log2(fTotal/n) * n
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}
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}
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return int(nbBits) >> 3
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}
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func highBit32(val uint32) (n uint32) {
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return uint32(bits.Len32(val) - 1)
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}
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