726 lines
19 KiB
Go
726 lines
19 KiB
Go
// Copyright 2019+ Klaus Post. All rights reserved.
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// License information can be found in the LICENSE file.
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// Based on work by Yann Collet, released under BSD License.
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package zstd
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import (
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"errors"
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"fmt"
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"math"
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)
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const (
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// For encoding we only support up to
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maxEncTableLog = 8
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maxEncTablesize = 1 << maxTableLog
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maxEncTableMask = (1 << maxTableLog) - 1
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minEncTablelog = 5
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maxEncSymbolValue = maxMatchLengthSymbol
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)
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// Scratch provides temporary storage for compression and decompression.
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type fseEncoder struct {
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symbolLen uint16 // Length of active part of the symbol table.
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actualTableLog uint8 // Selected tablelog.
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ct cTable // Compression tables.
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maxCount int // count of the most probable symbol
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zeroBits bool // no bits has prob > 50%.
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clearCount bool // clear count
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useRLE bool // This encoder is for RLE
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preDefined bool // This encoder is predefined.
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reUsed bool // Set to know when the encoder has been reused.
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rleVal uint8 // RLE Symbol
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maxBits uint8 // Maximum output bits after transform.
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// TODO: Technically zstd should be fine with 64 bytes.
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count [256]uint32
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norm [256]int16
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}
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// cTable contains tables used for compression.
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type cTable struct {
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tableSymbol []byte
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stateTable []uint16
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symbolTT []symbolTransform
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}
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// symbolTransform contains the state transform for a symbol.
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type symbolTransform struct {
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deltaNbBits uint32
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deltaFindState int16
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outBits uint8
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}
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// String prints values as a human readable string.
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func (s symbolTransform) String() string {
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return fmt.Sprintf("{deltabits: %08x, findstate:%d outbits:%d}", s.deltaNbBits, s.deltaFindState, s.outBits)
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}
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// Histogram allows to populate the histogram and skip that step in the compression,
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// It otherwise allows to inspect the histogram when compression is done.
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// To indicate that you have populated the histogram call HistogramFinished
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// with the value of the highest populated symbol, as well as the number of entries
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// in the most populated entry. These are accepted at face value.
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// The returned slice will always be length 256.
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func (s *fseEncoder) Histogram() []uint32 {
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return s.count[:]
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}
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// HistogramFinished can be called to indicate that the histogram has been populated.
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// maxSymbol is the index of the highest set symbol of the next data segment.
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// maxCount is the number of entries in the most populated entry.
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// These are accepted at face value.
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func (s *fseEncoder) HistogramFinished(maxSymbol uint8, maxCount int) {
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s.maxCount = maxCount
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s.symbolLen = uint16(maxSymbol) + 1
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s.clearCount = maxCount != 0
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}
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// prepare will prepare and allocate scratch tables used for both compression and decompression.
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func (s *fseEncoder) prepare() (*fseEncoder, error) {
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if s == nil {
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s = &fseEncoder{}
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}
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s.useRLE = false
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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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return s, nil
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}
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// allocCtable will allocate tables needed for compression.
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// If existing tables a re big enough, they are simply re-used.
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func (s *fseEncoder) allocCtable() {
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tableSize := 1 << s.actualTableLog
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// get tableSymbol that is big enough.
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if cap(s.ct.tableSymbol) < tableSize {
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s.ct.tableSymbol = make([]byte, tableSize)
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}
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s.ct.tableSymbol = s.ct.tableSymbol[:tableSize]
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ctSize := tableSize
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if cap(s.ct.stateTable) < ctSize {
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s.ct.stateTable = make([]uint16, ctSize)
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}
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s.ct.stateTable = s.ct.stateTable[:ctSize]
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if cap(s.ct.symbolTT) < 256 {
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s.ct.symbolTT = make([]symbolTransform, 256)
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}
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s.ct.symbolTT = s.ct.symbolTT[:256]
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}
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// buildCTable will populate the compression table so it is ready to be used.
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func (s *fseEncoder) buildCTable() error {
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tableSize := uint32(1 << s.actualTableLog)
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highThreshold := tableSize - 1
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var cumul [256]int16
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s.allocCtable()
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tableSymbol := s.ct.tableSymbol[:tableSize]
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// symbol start positions
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{
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cumul[0] = 0
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for ui, v := range s.norm[:s.symbolLen-1] {
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u := byte(ui) // one less than reference
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if v == -1 {
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// Low proba symbol
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cumul[u+1] = cumul[u] + 1
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tableSymbol[highThreshold] = u
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highThreshold--
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} else {
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cumul[u+1] = cumul[u] + v
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}
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}
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// Encode last symbol separately to avoid overflowing u
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u := int(s.symbolLen - 1)
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v := s.norm[s.symbolLen-1]
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if v == -1 {
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// Low proba symbol
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cumul[u+1] = cumul[u] + 1
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tableSymbol[highThreshold] = byte(u)
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highThreshold--
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} else {
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cumul[u+1] = cumul[u] + v
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}
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if uint32(cumul[s.symbolLen]) != tableSize {
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return fmt.Errorf("internal error: expected cumul[s.symbolLen] (%d) == tableSize (%d)", cumul[s.symbolLen], tableSize)
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}
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cumul[s.symbolLen] = int16(tableSize) + 1
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}
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// Spread symbols
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s.zeroBits = false
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{
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step := tableStep(tableSize)
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tableMask := tableSize - 1
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var position uint32
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// if any symbol > largeLimit, we may have 0 bits output.
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largeLimit := int16(1 << (s.actualTableLog - 1))
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for ui, v := range s.norm[:s.symbolLen] {
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symbol := byte(ui)
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if v > largeLimit {
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s.zeroBits = true
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}
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for nbOccurrences := int16(0); nbOccurrences < v; nbOccurrences++ {
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tableSymbol[position] = symbol
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position = (position + step) & tableMask
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for position > highThreshold {
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position = (position + step) & tableMask
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} /* Low proba area */
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}
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}
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// Check if we have gone through all positions
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if position != 0 {
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return errors.New("position!=0")
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}
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}
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// Build table
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table := s.ct.stateTable
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{
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tsi := int(tableSize)
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for u, v := range tableSymbol {
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// TableU16 : sorted by symbol order; gives next state value
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table[cumul[v]] = uint16(tsi + u)
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cumul[v]++
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}
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}
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// Build Symbol Transformation Table
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{
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total := int16(0)
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symbolTT := s.ct.symbolTT[:s.symbolLen]
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tableLog := s.actualTableLog
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tl := (uint32(tableLog) << 16) - (1 << tableLog)
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for i, v := range s.norm[:s.symbolLen] {
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switch v {
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case 0:
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case -1, 1:
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symbolTT[i].deltaNbBits = tl
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symbolTT[i].deltaFindState = total - 1
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total++
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default:
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maxBitsOut := uint32(tableLog) - highBit(uint32(v-1))
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minStatePlus := uint32(v) << maxBitsOut
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symbolTT[i].deltaNbBits = (maxBitsOut << 16) - minStatePlus
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symbolTT[i].deltaFindState = total - v
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total += v
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}
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}
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if total != int16(tableSize) {
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return fmt.Errorf("total mismatch %d (got) != %d (want)", total, tableSize)
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}
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}
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return nil
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}
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var rtbTable = [...]uint32{0, 473195, 504333, 520860, 550000, 700000, 750000, 830000}
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func (s *fseEncoder) setRLE(val byte) {
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s.allocCtable()
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s.actualTableLog = 0
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s.ct.stateTable = s.ct.stateTable[:1]
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s.ct.symbolTT[val] = symbolTransform{
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deltaFindState: 0,
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deltaNbBits: 0,
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}
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if debugEncoder {
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println("setRLE: val", val, "symbolTT", s.ct.symbolTT[val])
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}
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s.rleVal = val
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s.useRLE = true
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}
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// setBits will set output bits for the transform.
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// if nil is provided, the number of bits is equal to the index.
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func (s *fseEncoder) setBits(transform []byte) {
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if s.reUsed || s.preDefined {
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return
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}
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if s.useRLE {
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if transform == nil {
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s.ct.symbolTT[s.rleVal].outBits = s.rleVal
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s.maxBits = s.rleVal
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return
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}
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s.maxBits = transform[s.rleVal]
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s.ct.symbolTT[s.rleVal].outBits = s.maxBits
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return
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}
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if transform == nil {
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for i := range s.ct.symbolTT[:s.symbolLen] {
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s.ct.symbolTT[i].outBits = uint8(i)
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}
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s.maxBits = uint8(s.symbolLen - 1)
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return
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}
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s.maxBits = 0
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for i, v := range transform[:s.symbolLen] {
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s.ct.symbolTT[i].outBits = v
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if v > s.maxBits {
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// We could assume bits always going up, but we play safe.
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s.maxBits = v
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}
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}
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}
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// normalizeCount will normalize the count of the symbols so
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// the total is equal to the table size.
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// If successful, compression tables will also be made ready.
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func (s *fseEncoder) normalizeCount(length int) error {
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if s.reUsed {
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return nil
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}
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s.optimalTableLog(length)
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var (
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tableLog = s.actualTableLog
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scale = 62 - uint64(tableLog)
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step = (1 << 62) / uint64(length)
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vStep = uint64(1) << (scale - 20)
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stillToDistribute = int16(1 << tableLog)
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largest int
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largestP int16
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lowThreshold = (uint32)(length >> tableLog)
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)
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if s.maxCount == length {
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s.useRLE = true
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return nil
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}
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s.useRLE = false
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for i, cnt := range s.count[:s.symbolLen] {
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// already handled
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// if (count[s] == s.length) return 0; /* rle special case */
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if cnt == 0 {
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s.norm[i] = 0
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continue
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}
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if cnt <= lowThreshold {
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s.norm[i] = -1
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stillToDistribute--
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} else {
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proba := (int16)((uint64(cnt) * step) >> scale)
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if proba < 8 {
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restToBeat := vStep * uint64(rtbTable[proba])
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v := uint64(cnt)*step - (uint64(proba) << scale)
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if v > restToBeat {
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proba++
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}
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}
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if proba > largestP {
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largestP = proba
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largest = i
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}
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s.norm[i] = proba
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stillToDistribute -= proba
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}
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}
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if -stillToDistribute >= (s.norm[largest] >> 1) {
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// corner case, need another normalization method
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err := s.normalizeCount2(length)
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if err != nil {
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return err
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}
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if debugAsserts {
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err = s.validateNorm()
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if err != nil {
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return err
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}
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}
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return s.buildCTable()
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}
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s.norm[largest] += stillToDistribute
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if debugAsserts {
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err := s.validateNorm()
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if err != nil {
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return err
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}
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}
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return s.buildCTable()
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}
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// Secondary normalization method.
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// To be used when primary method fails.
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func (s *fseEncoder) normalizeCount2(length int) error {
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const notYetAssigned = -2
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var (
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distributed uint32
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total = uint32(length)
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tableLog = s.actualTableLog
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lowThreshold = total >> tableLog
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lowOne = (total * 3) >> (tableLog + 1)
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)
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for i, cnt := range s.count[:s.symbolLen] {
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if cnt == 0 {
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s.norm[i] = 0
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continue
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}
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if cnt <= lowThreshold {
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s.norm[i] = -1
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distributed++
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total -= cnt
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continue
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}
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if cnt <= lowOne {
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s.norm[i] = 1
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distributed++
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total -= cnt
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continue
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}
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s.norm[i] = notYetAssigned
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}
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toDistribute := (1 << tableLog) - distributed
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if (total / toDistribute) > lowOne {
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// risk of rounding to zero
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lowOne = (total * 3) / (toDistribute * 2)
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for i, cnt := range s.count[:s.symbolLen] {
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if (s.norm[i] == notYetAssigned) && (cnt <= lowOne) {
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s.norm[i] = 1
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distributed++
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total -= cnt
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continue
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}
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}
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toDistribute = (1 << tableLog) - distributed
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}
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if distributed == uint32(s.symbolLen)+1 {
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// all values are pretty poor;
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// probably incompressible data (should have already been detected);
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// find max, then give all remaining points to max
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var maxV int
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var maxC uint32
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for i, cnt := range s.count[:s.symbolLen] {
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if cnt > maxC {
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maxV = i
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maxC = cnt
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}
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}
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s.norm[maxV] += int16(toDistribute)
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return nil
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}
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if total == 0 {
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// all of the symbols were low enough for the lowOne or lowThreshold
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for i := uint32(0); toDistribute > 0; i = (i + 1) % (uint32(s.symbolLen)) {
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if s.norm[i] > 0 {
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toDistribute--
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s.norm[i]++
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}
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}
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return nil
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}
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var (
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vStepLog = 62 - uint64(tableLog)
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mid = uint64((1 << (vStepLog - 1)) - 1)
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rStep = (((1 << vStepLog) * uint64(toDistribute)) + mid) / uint64(total) // scale on remaining
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tmpTotal = mid
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)
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for i, cnt := range s.count[:s.symbolLen] {
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if s.norm[i] == notYetAssigned {
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var (
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end = tmpTotal + uint64(cnt)*rStep
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sStart = uint32(tmpTotal >> vStepLog)
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sEnd = uint32(end >> vStepLog)
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weight = sEnd - sStart
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)
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if weight < 1 {
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return errors.New("weight < 1")
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}
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s.norm[i] = int16(weight)
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tmpTotal = end
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}
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}
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return nil
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}
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// optimalTableLog calculates and sets the optimal tableLog in s.actualTableLog
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func (s *fseEncoder) optimalTableLog(length int) {
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tableLog := uint8(maxEncTableLog)
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minBitsSrc := highBit(uint32(length)) + 1
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minBitsSymbols := highBit(uint32(s.symbolLen-1)) + 2
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minBits := uint8(minBitsSymbols)
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if minBitsSrc < minBitsSymbols {
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minBits = uint8(minBitsSrc)
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}
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maxBitsSrc := uint8(highBit(uint32(length-1))) - 2
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if maxBitsSrc < tableLog {
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// Accuracy can be reduced
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tableLog = maxBitsSrc
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}
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if minBits > tableLog {
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tableLog = minBits
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}
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// Need a minimum to safely represent all symbol values
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if tableLog < minEncTablelog {
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tableLog = minEncTablelog
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}
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if tableLog > maxEncTableLog {
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tableLog = maxEncTableLog
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}
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s.actualTableLog = tableLog
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}
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// validateNorm validates the normalized histogram table.
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func (s *fseEncoder) validateNorm() (err error) {
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var total int
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for _, v := range s.norm[:s.symbolLen] {
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if v >= 0 {
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total += int(v)
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} else {
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total -= int(v)
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}
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}
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defer func() {
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if err == nil {
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return
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}
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fmt.Printf("selected TableLog: %d, Symbol length: %d\n", s.actualTableLog, s.symbolLen)
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for i, v := range s.norm[:s.symbolLen] {
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fmt.Printf("%3d: %5d -> %4d \n", i, s.count[i], v)
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}
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}()
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if total != (1 << s.actualTableLog) {
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return fmt.Errorf("warning: Total == %d != %d", total, 1<<s.actualTableLog)
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}
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for i, v := range s.count[s.symbolLen:] {
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if v != 0 {
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return fmt.Errorf("warning: Found symbol out of range, %d after cut", i)
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}
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}
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return nil
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}
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// writeCount will write the normalized histogram count to header.
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// This is read back by readNCount.
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func (s *fseEncoder) writeCount(out []byte) ([]byte, error) {
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if s.useRLE {
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return append(out, s.rleVal), nil
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}
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if s.preDefined || s.reUsed {
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// Never write predefined.
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return out, nil
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}
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var (
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tableLog = s.actualTableLog
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tableSize = 1 << tableLog
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previous0 bool
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charnum uint16
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// maximum header size plus 2 extra bytes for final output if bitCount == 0.
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maxHeaderSize = ((int(s.symbolLen) * int(tableLog)) >> 3) + 3 + 2
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// Write Table Size
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bitStream = uint32(tableLog - minEncTablelog)
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bitCount = uint(4)
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remaining = int16(tableSize + 1) /* +1 for extra accuracy */
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threshold = int16(tableSize)
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nbBits = uint(tableLog + 1)
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outP = len(out)
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)
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if cap(out) < outP+maxHeaderSize {
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out = append(out, make([]byte, maxHeaderSize*3)...)
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out = out[:len(out)-maxHeaderSize*3]
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|
}
|
|
out = out[:outP+maxHeaderSize]
|
|
|
|
// stops at 1
|
|
for remaining > 1 {
|
|
if previous0 {
|
|
start := charnum
|
|
for s.norm[charnum] == 0 {
|
|
charnum++
|
|
}
|
|
for charnum >= start+24 {
|
|
start += 24
|
|
bitStream += uint32(0xFFFF) << bitCount
|
|
out[outP] = byte(bitStream)
|
|
out[outP+1] = byte(bitStream >> 8)
|
|
outP += 2
|
|
bitStream >>= 16
|
|
}
|
|
for charnum >= start+3 {
|
|
start += 3
|
|
bitStream += 3 << bitCount
|
|
bitCount += 2
|
|
}
|
|
bitStream += uint32(charnum-start) << bitCount
|
|
bitCount += 2
|
|
if bitCount > 16 {
|
|
out[outP] = byte(bitStream)
|
|
out[outP+1] = byte(bitStream >> 8)
|
|
outP += 2
|
|
bitStream >>= 16
|
|
bitCount -= 16
|
|
}
|
|
}
|
|
|
|
count := s.norm[charnum]
|
|
charnum++
|
|
max := (2*threshold - 1) - remaining
|
|
if count < 0 {
|
|
remaining += count
|
|
} else {
|
|
remaining -= count
|
|
}
|
|
count++ // +1 for extra accuracy
|
|
if count >= threshold {
|
|
count += max // [0..max[ [max..threshold[ (...) [threshold+max 2*threshold[
|
|
}
|
|
bitStream += uint32(count) << bitCount
|
|
bitCount += nbBits
|
|
if count < max {
|
|
bitCount--
|
|
}
|
|
|
|
previous0 = count == 1
|
|
if remaining < 1 {
|
|
return nil, errors.New("internal error: remaining < 1")
|
|
}
|
|
for remaining < threshold {
|
|
nbBits--
|
|
threshold >>= 1
|
|
}
|
|
|
|
if bitCount > 16 {
|
|
out[outP] = byte(bitStream)
|
|
out[outP+1] = byte(bitStream >> 8)
|
|
outP += 2
|
|
bitStream >>= 16
|
|
bitCount -= 16
|
|
}
|
|
}
|
|
|
|
if outP+2 > len(out) {
|
|
return nil, fmt.Errorf("internal error: %d > %d, maxheader: %d, sl: %d, tl: %d, normcount: %v", outP+2, len(out), maxHeaderSize, s.symbolLen, int(tableLog), s.norm[:s.symbolLen])
|
|
}
|
|
out[outP] = byte(bitStream)
|
|
out[outP+1] = byte(bitStream >> 8)
|
|
outP += int((bitCount + 7) / 8)
|
|
|
|
if charnum > s.symbolLen {
|
|
return nil, errors.New("internal error: charnum > s.symbolLen")
|
|
}
|
|
return out[:outP], nil
|
|
}
|
|
|
|
// Approximate symbol cost, as fractional value, using fixed-point format (accuracyLog fractional bits)
|
|
// note 1 : assume symbolValue is valid (<= maxSymbolValue)
|
|
// note 2 : if freq[symbolValue]==0, @return a fake cost of tableLog+1 bits *
|
|
func (s *fseEncoder) bitCost(symbolValue uint8, accuracyLog uint32) uint32 {
|
|
minNbBits := s.ct.symbolTT[symbolValue].deltaNbBits >> 16
|
|
threshold := (minNbBits + 1) << 16
|
|
if debugAsserts {
|
|
if !(s.actualTableLog < 16) {
|
|
panic("!s.actualTableLog < 16")
|
|
}
|
|
// ensure enough room for renormalization double shift
|
|
if !(uint8(accuracyLog) < 31-s.actualTableLog) {
|
|
panic("!uint8(accuracyLog) < 31-s.actualTableLog")
|
|
}
|
|
}
|
|
tableSize := uint32(1) << s.actualTableLog
|
|
deltaFromThreshold := threshold - (s.ct.symbolTT[symbolValue].deltaNbBits + tableSize)
|
|
// linear interpolation (very approximate)
|
|
normalizedDeltaFromThreshold := (deltaFromThreshold << accuracyLog) >> s.actualTableLog
|
|
bitMultiplier := uint32(1) << accuracyLog
|
|
if debugAsserts {
|
|
if s.ct.symbolTT[symbolValue].deltaNbBits+tableSize > threshold {
|
|
panic("s.ct.symbolTT[symbolValue].deltaNbBits+tableSize > threshold")
|
|
}
|
|
if normalizedDeltaFromThreshold > bitMultiplier {
|
|
panic("normalizedDeltaFromThreshold > bitMultiplier")
|
|
}
|
|
}
|
|
return (minNbBits+1)*bitMultiplier - normalizedDeltaFromThreshold
|
|
}
|
|
|
|
// Returns the cost in bits of encoding the distribution in count using ctable.
|
|
// Histogram should only be up to the last non-zero symbol.
|
|
// Returns an -1 if ctable cannot represent all the symbols in count.
|
|
func (s *fseEncoder) approxSize(hist []uint32) uint32 {
|
|
if int(s.symbolLen) < len(hist) {
|
|
// More symbols than we have.
|
|
return math.MaxUint32
|
|
}
|
|
if s.useRLE {
|
|
// We will never reuse RLE encoders.
|
|
return math.MaxUint32
|
|
}
|
|
const kAccuracyLog = 8
|
|
badCost := (uint32(s.actualTableLog) + 1) << kAccuracyLog
|
|
var cost uint32
|
|
for i, v := range hist {
|
|
if v == 0 {
|
|
continue
|
|
}
|
|
if s.norm[i] == 0 {
|
|
return math.MaxUint32
|
|
}
|
|
bitCost := s.bitCost(uint8(i), kAccuracyLog)
|
|
if bitCost > badCost {
|
|
return math.MaxUint32
|
|
}
|
|
cost += v * bitCost
|
|
}
|
|
return cost >> kAccuracyLog
|
|
}
|
|
|
|
// maxHeaderSize returns the maximum header size in bits.
|
|
// This is not exact size, but we want a penalty for new tables anyway.
|
|
func (s *fseEncoder) maxHeaderSize() uint32 {
|
|
if s.preDefined {
|
|
return 0
|
|
}
|
|
if s.useRLE {
|
|
return 8
|
|
}
|
|
return (((uint32(s.symbolLen) * uint32(s.actualTableLog)) >> 3) + 3) * 8
|
|
}
|
|
|
|
// cState contains the compression state of a stream.
|
|
type cState struct {
|
|
bw *bitWriter
|
|
stateTable []uint16
|
|
state uint16
|
|
}
|
|
|
|
// init will initialize the compression state to the first symbol of the stream.
|
|
func (c *cState) init(bw *bitWriter, ct *cTable, first symbolTransform) {
|
|
c.bw = bw
|
|
c.stateTable = ct.stateTable
|
|
if len(c.stateTable) == 1 {
|
|
// RLE
|
|
c.stateTable[0] = uint16(0)
|
|
c.state = 0
|
|
return
|
|
}
|
|
nbBitsOut := (first.deltaNbBits + (1 << 15)) >> 16
|
|
im := int32((nbBitsOut << 16) - first.deltaNbBits)
|
|
lu := (im >> nbBitsOut) + int32(first.deltaFindState)
|
|
c.state = c.stateTable[lu]
|
|
}
|
|
|
|
// encode the output symbol provided and write it to the bitstream.
|
|
func (c *cState) encode(symbolTT symbolTransform) {
|
|
nbBitsOut := (uint32(c.state) + symbolTT.deltaNbBits) >> 16
|
|
dstState := int32(c.state>>(nbBitsOut&15)) + int32(symbolTT.deltaFindState)
|
|
c.bw.addBits16NC(c.state, uint8(nbBitsOut))
|
|
c.state = c.stateTable[dstState]
|
|
}
|
|
|
|
// flush will write the tablelog to the output and flush the remaining full bytes.
|
|
func (c *cState) flush(tableLog uint8) {
|
|
c.bw.flush32()
|
|
c.bw.addBits16NC(c.state, tableLog)
|
|
}
|