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fse_decoder.go
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/
fse_decoder.go
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// Copyright 2019+ Klaus Post. All rights reserved.
// License information can be found in the LICENSE file.
// Based on work by Yann Collet, released under BSD License.
package zstd
import (
"errors"
"fmt"
)
const (
tablelogAbsoluteMax = 9
)
const (
/*!MEMORY_USAGE :
* Memory usage formula : N->2^N Bytes (examples : 10 -> 1KB; 12 -> 4KB ; 16 -> 64KB; 20 -> 1MB; etc.)
* Increasing memory usage improves compression ratio
* Reduced memory usage can improve speed, due to cache effect
* Recommended max value is 14, for 16KB, which nicely fits into Intel x86 L1 cache */
maxMemoryUsage = tablelogAbsoluteMax + 2
maxTableLog = maxMemoryUsage - 2
maxTablesize = 1 << maxTableLog
maxTableMask = (1 << maxTableLog) - 1
minTablelog = 5
maxSymbolValue = 255
)
// fseDecoder provides temporary storage for compression and decompression.
type fseDecoder struct {
dt [maxTablesize]decSymbol // Decompression table.
symbolLen uint16 // Length of active part of the symbol table.
actualTableLog uint8 // Selected tablelog.
maxBits uint8 // Maximum number of additional bits
// used for table creation to avoid allocations.
stateTable [256]uint16
norm [maxSymbolValue + 1]int16
preDefined bool
}
// tableStep returns the next table index.
func tableStep(tableSize uint32) uint32 {
return (tableSize >> 1) + (tableSize >> 3) + 3
}
// readNCount will read the symbol distribution so decoding tables can be constructed.
func (s *fseDecoder) readNCount(b *byteReader, maxSymbol uint16) error {
var (
charnum uint16
previous0 bool
)
if b.remain() < 4 {
return errors.New("input too small")
}
bitStream := b.Uint32NC()
nbBits := uint((bitStream & 0xF) + minTablelog) // extract tableLog
if nbBits > tablelogAbsoluteMax {
println("Invalid tablelog:", nbBits)
return errors.New("tableLog too large")
}
bitStream >>= 4
bitCount := uint(4)
s.actualTableLog = uint8(nbBits)
remaining := int32((1 << nbBits) + 1)
threshold := int32(1 << nbBits)
gotTotal := int32(0)
nbBits++
for remaining > 1 && charnum <= maxSymbol {
if previous0 {
//println("prev0")
n0 := charnum
for (bitStream & 0xFFFF) == 0xFFFF {
//println("24 x 0")
n0 += 24
if r := b.remain(); r > 5 {
b.advance(2)
// The check above should make sure we can read 32 bits
bitStream = b.Uint32NC() >> bitCount
} else {
// end of bit stream
bitStream >>= 16
bitCount += 16
}
}
//printf("bitstream: %d, 0b%b", bitStream&3, bitStream)
for (bitStream & 3) == 3 {
n0 += 3
bitStream >>= 2
bitCount += 2
}
n0 += uint16(bitStream & 3)
bitCount += 2
if n0 > maxSymbolValue {
return errors.New("maxSymbolValue too small")
}
//println("inserting ", n0-charnum, "zeroes from idx", charnum, "ending before", n0)
for charnum < n0 {
s.norm[uint8(charnum)] = 0
charnum++
}
if r := b.remain(); r >= 7 || r-int(bitCount>>3) >= 4 {
b.advance(bitCount >> 3)
bitCount &= 7
// The check above should make sure we can read 32 bits
bitStream = b.Uint32NC() >> bitCount
} else {
bitStream >>= 2
}
}
max := (2*threshold - 1) - remaining
var count int32
if int32(bitStream)&(threshold-1) < max {
count = int32(bitStream) & (threshold - 1)
if debugAsserts && nbBits < 1 {
panic("nbBits underflow")
}
bitCount += nbBits - 1
} else {
count = int32(bitStream) & (2*threshold - 1)
if count >= threshold {
count -= max
}
bitCount += nbBits
}
// extra accuracy
count--
if count < 0 {
// -1 means +1
remaining += count
gotTotal -= count
} else {
remaining -= count
gotTotal += count
}
s.norm[charnum&0xff] = int16(count)
charnum++
previous0 = count == 0
for remaining < threshold {
nbBits--
threshold >>= 1
}
if r := b.remain(); r >= 7 || r-int(bitCount>>3) >= 4 {
b.advance(bitCount >> 3)
bitCount &= 7
// The check above should make sure we can read 32 bits
bitStream = b.Uint32NC() >> (bitCount & 31)
} else {
bitCount -= (uint)(8 * (len(b.b) - 4 - b.off))
b.off = len(b.b) - 4
bitStream = b.Uint32() >> (bitCount & 31)
}
}
s.symbolLen = charnum
if s.symbolLen <= 1 {
return fmt.Errorf("symbolLen (%d) too small", s.symbolLen)
}
if s.symbolLen > maxSymbolValue+1 {
return fmt.Errorf("symbolLen (%d) too big", s.symbolLen)
}
if remaining != 1 {
return fmt.Errorf("corruption detected (remaining %d != 1)", remaining)
}
if bitCount > 32 {
return fmt.Errorf("corruption detected (bitCount %d > 32)", bitCount)
}
if gotTotal != 1<<s.actualTableLog {
return fmt.Errorf("corruption detected (total %d != %d)", gotTotal, 1<<s.actualTableLog)
}
b.advance((bitCount + 7) >> 3)
// println(s.norm[:s.symbolLen], s.symbolLen)
return s.buildDtable()
}
// decSymbol contains information about a state entry,
// Including the state offset base, the output symbol and
// the number of bits to read for the low part of the destination state.
// Using a composite uint64 is faster than a struct with separate members.
type decSymbol uint64
func newDecSymbol(nbits, addBits uint8, newState uint16, baseline uint32) decSymbol {
return decSymbol(nbits) | (decSymbol(addBits) << 8) | (decSymbol(newState) << 16) | (decSymbol(baseline) << 32)
}
func (d decSymbol) nbBits() uint8 {
return uint8(d)
}
func (d decSymbol) addBits() uint8 {
return uint8(d >> 8)
}
func (d decSymbol) newState() uint16 {
return uint16(d >> 16)
}
func (d decSymbol) baseline() uint32 {
return uint32(d >> 32)
}
func (d decSymbol) baselineInt() int {
return int(d >> 32)
}
func (d *decSymbol) set(nbits, addBits uint8, newState uint16, baseline uint32) {
*d = decSymbol(nbits) | (decSymbol(addBits) << 8) | (decSymbol(newState) << 16) | (decSymbol(baseline) << 32)
}
func (d *decSymbol) setNBits(nBits uint8) {
const mask = 0xffffffffffffff00
*d = (*d & mask) | decSymbol(nBits)
}
func (d *decSymbol) setAddBits(addBits uint8) {
const mask = 0xffffffffffff00ff
*d = (*d & mask) | (decSymbol(addBits) << 8)
}
func (d *decSymbol) setNewState(state uint16) {
const mask = 0xffffffff0000ffff
*d = (*d & mask) | decSymbol(state)<<16
}
func (d *decSymbol) setBaseline(baseline uint32) {
const mask = 0xffffffff
*d = (*d & mask) | decSymbol(baseline)<<32
}
func (d *decSymbol) setExt(addBits uint8, baseline uint32) {
const mask = 0xffff00ff
*d = (*d & mask) | (decSymbol(addBits) << 8) | (decSymbol(baseline) << 32)
}
// decSymbolValue returns the transformed decSymbol for the given symbol.
func decSymbolValue(symb uint8, t []baseOffset) (decSymbol, error) {
if int(symb) >= len(t) {
return 0, fmt.Errorf("rle symbol %d >= max %d", symb, len(t))
}
lu := t[symb]
return newDecSymbol(0, lu.addBits, 0, lu.baseLine), nil
}
// setRLE will set the decoder til RLE mode.
func (s *fseDecoder) setRLE(symbol decSymbol) {
s.actualTableLog = 0
s.maxBits = symbol.addBits()
s.dt[0] = symbol
}
// buildDtable will build the decoding table.
func (s *fseDecoder) buildDtable() error {
tableSize := uint32(1 << s.actualTableLog)
highThreshold := tableSize - 1
symbolNext := s.stateTable[:256]
// Init, lay down lowprob symbols
{
for i, v := range s.norm[:s.symbolLen] {
if v == -1 {
s.dt[highThreshold].setAddBits(uint8(i))
highThreshold--
symbolNext[i] = 1
} else {
symbolNext[i] = uint16(v)
}
}
}
// Spread symbols
{
tableMask := tableSize - 1
step := tableStep(tableSize)
position := uint32(0)
for ss, v := range s.norm[:s.symbolLen] {
for i := 0; i < int(v); i++ {
s.dt[position].setAddBits(uint8(ss))
position = (position + step) & tableMask
for position > highThreshold {
// lowprob area
position = (position + step) & tableMask
}
}
}
if position != 0 {
// position must reach all cells once, otherwise normalizedCounter is incorrect
return errors.New("corrupted input (position != 0)")
}
}
// Build Decoding table
{
tableSize := uint16(1 << s.actualTableLog)
for u, v := range s.dt[:tableSize] {
symbol := v.addBits()
nextState := symbolNext[symbol]
symbolNext[symbol] = nextState + 1
nBits := s.actualTableLog - byte(highBits(uint32(nextState)))
s.dt[u&maxTableMask].setNBits(nBits)
newState := (nextState << nBits) - tableSize
if newState > tableSize {
return fmt.Errorf("newState (%d) outside table size (%d)", newState, tableSize)
}
if newState == uint16(u) && nBits == 0 {
// Seems weird that this is possible with nbits > 0.
return fmt.Errorf("newState (%d) == oldState (%d) and no bits", newState, u)
}
s.dt[u&maxTableMask].setNewState(newState)
}
}
return nil
}
// transform will transform the decoder table into a table usable for
// decoding without having to apply the transformation while decoding.
// The state will contain the base value and the number of bits to read.
func (s *fseDecoder) transform(t []baseOffset) error {
tableSize := uint16(1 << s.actualTableLog)
s.maxBits = 0
for i, v := range s.dt[:tableSize] {
add := v.addBits()
if int(add) >= len(t) {
return fmt.Errorf("invalid decoding table entry %d, symbol %d >= max (%d)", i, v.addBits(), len(t))
}
lu := t[add]
if lu.addBits > s.maxBits {
s.maxBits = lu.addBits
}
v.setExt(lu.addBits, lu.baseLine)
s.dt[i] = v
}
return nil
}
type fseState struct {
dt []decSymbol
state decSymbol
}
// Initialize and decodeAsync first state and symbol.
func (s *fseState) init(br *bitReader, tableLog uint8, dt []decSymbol) {
s.dt = dt
br.fill()
s.state = dt[br.getBits(tableLog)]
}
// next returns the current symbol and sets the next state.
// At least tablelog bits must be available in the bit reader.
func (s *fseState) next(br *bitReader) {
lowBits := uint16(br.getBits(s.state.nbBits()))
s.state = s.dt[s.state.newState()+lowBits]
}
// finished returns true if all bits have been read from the bitstream
// and the next state would require reading bits from the input.
func (s *fseState) finished(br *bitReader) bool {
return br.finished() && s.state.nbBits() > 0
}
// final returns the current state symbol without decoding the next.
func (s *fseState) final() (int, uint8) {
return s.state.baselineInt(), s.state.addBits()
}
// final returns the current state symbol without decoding the next.
func (s decSymbol) final() (int, uint8) {
return s.baselineInt(), s.addBits()
}
// nextFast returns the next symbol and sets the next state.
// This can only be used if no symbols are 0 bits.
// At least tablelog bits must be available in the bit reader.
func (s *fseState) nextFast(br *bitReader) (uint32, uint8) {
lowBits := uint16(br.getBitsFast(s.state.nbBits()))
s.state = s.dt[s.state.newState()+lowBits]
return s.state.baseline(), s.state.addBits()
}