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gitea/vendor/github.com/pierrec/lz4/v4/internal/lz4block/block.go

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package lz4block
import (
"encoding/binary"
"math/bits"
"sync"
"github.com/pierrec/lz4/v4/internal/lz4errors"
)
const (
// The following constants are used to setup the compression algorithm.
minMatch = 4 // the minimum size of the match sequence size (4 bytes)
winSizeLog = 16 // LZ4 64Kb window size limit
winSize = 1 << winSizeLog
winMask = winSize - 1 // 64Kb window of previous data for dependent blocks
// hashLog determines the size of the hash table used to quickly find a previous match position.
// Its value influences the compression speed and memory usage, the lower the faster,
// but at the expense of the compression ratio.
// 16 seems to be the best compromise for fast compression.
hashLog = 16
htSize = 1 << hashLog
mfLimit = 10 + minMatch // The last match cannot start within the last 14 bytes.
)
func recoverBlock(e *error) {
if r := recover(); r != nil && *e == nil {
*e = lz4errors.ErrInvalidSourceShortBuffer
}
}
// blockHash hashes the lower 6 bytes into a value < htSize.
func blockHash(x uint64) uint32 {
const prime6bytes = 227718039650203
return uint32(((x << (64 - 48)) * prime6bytes) >> (64 - hashLog))
}
func CompressBlockBound(n int) int {
return n + n/255 + 16
}
func UncompressBlock(src, dst []byte) (int, error) {
if len(src) == 0 {
return 0, nil
}
if di := decodeBlock(dst, src); di >= 0 {
return di, nil
}
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
type Compressor struct {
// Offsets are at most 64kiB, so we can store only the lower 16 bits of
// match positions: effectively, an offset from some 64kiB block boundary.
//
// When we retrieve such an offset, we interpret it as relative to the last
// block boundary si &^ 0xffff, or the one before, (si &^ 0xffff) - 0x10000,
// depending on which of these is inside the current window. If a table
// entry was generated more than 64kiB back in the input, we find out by
// inspecting the input stream.
table [htSize]uint16
needsReset bool
}
// Get returns the position of a presumptive match for the hash h.
// The match may be a false positive due to a hash collision or an old entry.
// If si < winSize, the return value may be negative.
func (c *Compressor) get(h uint32, si int) int {
h &= htSize - 1
i := int(c.table[h])
i += si &^ winMask
if i >= si {
// Try previous 64kiB block (negative when in first block).
i -= winSize
}
return i
}
func (c *Compressor) put(h uint32, si int) {
h &= htSize - 1
c.table[h] = uint16(si)
}
var compressorPool = sync.Pool{New: func() interface{} { return new(Compressor) }}
func CompressBlock(src, dst []byte) (int, error) {
c := compressorPool.Get().(*Compressor)
n, err := c.CompressBlock(src, dst)
compressorPool.Put(c)
return n, err
}
func (c *Compressor) CompressBlock(src, dst []byte) (int, error) {
if c.needsReset {
// Zero out reused table to avoid non-deterministic output (issue #65).
c.table = [htSize]uint16{}
}
c.needsReset = true // Only false on first call.
// Return 0, nil only if the destination buffer size is < CompressBlockBound.
isNotCompressible := len(dst) < CompressBlockBound(len(src))
// adaptSkipLog sets how quickly the compressor begins skipping blocks when data is incompressible.
// This significantly speeds up incompressible data and usually has very small impact on compression.
// bytes to skip = 1 + (bytes since last match >> adaptSkipLog)
const adaptSkipLog = 7
// si: Current position of the search.
// anchor: Position of the current literals.
var si, di, anchor int
sn := len(src) - mfLimit
if sn <= 0 {
goto lastLiterals
}
// Fast scan strategy: the hash table only stores the last 4 bytes sequences.
for si < sn {
// Hash the next 6 bytes (sequence)...
match := binary.LittleEndian.Uint64(src[si:])
h := blockHash(match)
h2 := blockHash(match >> 8)
// We check a match at s, s+1 and s+2 and pick the first one we get.
// Checking 3 only requires us to load the source one.
ref := c.get(h, si)
ref2 := c.get(h2, si)
c.put(h, si)
c.put(h2, si+1)
offset := si - ref
if offset <= 0 || offset >= winSize || uint32(match) != binary.LittleEndian.Uint32(src[ref:]) {
// No match. Start calculating another hash.
// The processor can usually do this out-of-order.
h = blockHash(match >> 16)
ref3 := c.get(h, si+2)
// Check the second match at si+1
si += 1
offset = si - ref2
if offset <= 0 || offset >= winSize || uint32(match>>8) != binary.LittleEndian.Uint32(src[ref2:]) {
// No match. Check the third match at si+2
si += 1
offset = si - ref3
c.put(h, si)
if offset <= 0 || offset >= winSize || uint32(match>>16) != binary.LittleEndian.Uint32(src[ref3:]) {
// Skip one extra byte (at si+3) before we check 3 matches again.
si += 2 + (si-anchor)>>adaptSkipLog
continue
}
}
}
// Match found.
lLen := si - anchor // Literal length.
// We already matched 4 bytes.
mLen := 4
// Extend backwards if we can, reducing literals.
tOff := si - offset - 1
for lLen > 0 && tOff >= 0 && src[si-1] == src[tOff] {
si--
tOff--
lLen--
mLen++
}
// Add the match length, so we continue search at the end.
// Use mLen to store the offset base.
si, mLen = si+mLen, si+minMatch
// Find the longest match by looking by batches of 8 bytes.
for si+8 < sn {
x := binary.LittleEndian.Uint64(src[si:]) ^ binary.LittleEndian.Uint64(src[si-offset:])
if x == 0 {
si += 8
} else {
// Stop is first non-zero byte.
si += bits.TrailingZeros64(x) >> 3
break
}
}
mLen = si - mLen
if mLen < 0xF {
dst[di] = byte(mLen)
} else {
dst[di] = 0xF
}
// Encode literals length.
if lLen < 0xF {
dst[di] |= byte(lLen << 4)
} else {
dst[di] |= 0xF0
di++
l := lLen - 0xF
for ; l >= 0xFF; l -= 0xFF {
dst[di] = 0xFF
di++
}
dst[di] = byte(l)
}
di++
// Literals.
if di+lLen > len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
copy(dst[di:di+lLen], src[anchor:anchor+lLen])
di += lLen + 2
anchor = si
// Encode offset.
if di > len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
dst[di-2], dst[di-1] = byte(offset), byte(offset>>8)
// Encode match length part 2.
if mLen >= 0xF {
for mLen -= 0xF; mLen >= 0xFF && di < len(dst); mLen -= 0xFF {
dst[di] = 0xFF
di++
}
if di >= len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
dst[di] = byte(mLen)
di++
}
// Check if we can load next values.
if si >= sn {
break
}
// Hash match end-2
h = blockHash(binary.LittleEndian.Uint64(src[si-2:]))
c.put(h, si-2)
}
lastLiterals:
if isNotCompressible && anchor == 0 {
// Incompressible.
return 0, nil
}
// Last literals.
if di >= len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
lLen := len(src) - anchor
if lLen < 0xF {
dst[di] = byte(lLen << 4)
} else {
dst[di] = 0xF0
di++
for lLen -= 0xF; lLen >= 0xFF && di < len(dst); lLen -= 0xFF {
dst[di] = 0xFF
di++
}
if di >= len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
dst[di] = byte(lLen)
}
di++
// Write the last literals.
if isNotCompressible && di >= anchor {
// Incompressible.
return 0, nil
}
if di+len(src)-anchor > len(dst) {
return 0, lz4errors.ErrInvalidSourceShortBuffer
}
di += copy(dst[di:di+len(src)-anchor], src[anchor:])
return di, nil
}
// blockHash hashes 4 bytes into a value < winSize.
func blockHashHC(x uint32) uint32 {
const hasher uint32 = 2654435761 // Knuth multiplicative hash.
return x * hasher >> (32 - winSizeLog)
}
type CompressorHC struct {
// hashTable: stores the last position found for a given hash
// chainTable: stores previous positions for a given hash
hashTable, chainTable [htSize]int
needsReset bool
}
var compressorHCPool = sync.Pool{New: func() interface{} { return new(CompressorHC) }}
func CompressBlockHC(src, dst []byte, depth CompressionLevel) (int, error) {
c := compressorHCPool.Get().(*CompressorHC)
n, err := c.CompressBlock(src, dst, depth)
compressorHCPool.Put(c)
return n, err
}
func (c *CompressorHC) CompressBlock(src, dst []byte, depth CompressionLevel) (_ int, err error) {
if c.needsReset {
// Zero out reused table to avoid non-deterministic output (issue #65).
c.hashTable = [htSize]int{}
c.chainTable = [htSize]int{}
}
c.needsReset = true // Only false on first call.
defer recoverBlock(&err)
// Return 0, nil only if the destination buffer size is < CompressBlockBound.
isNotCompressible := len(dst) < CompressBlockBound(len(src))
// adaptSkipLog sets how quickly the compressor begins skipping blocks when data is incompressible.
// This significantly speeds up incompressible data and usually has very small impact on compression.
// bytes to skip = 1 + (bytes since last match >> adaptSkipLog)
const adaptSkipLog = 7
var si, di, anchor int
sn := len(src) - mfLimit
if sn <= 0 {
goto lastLiterals
}
if depth == 0 {
depth = winSize
}
for si < sn {
// Hash the next 4 bytes (sequence).
match := binary.LittleEndian.Uint32(src[si:])
h := blockHashHC(match)
// Follow the chain until out of window and give the longest match.
mLen := 0
offset := 0
for next, try := c.hashTable[h], depth; try > 0 && next > 0 && si-next < winSize; next, try = c.chainTable[next&winMask], try-1 {
// The first (mLen==0) or next byte (mLen>=minMatch) at current match length
// must match to improve on the match length.
if src[next+mLen] != src[si+mLen] {
continue
}
ml := 0
// Compare the current position with a previous with the same hash.
for ml < sn-si {
x := binary.LittleEndian.Uint64(src[next+ml:]) ^ binary.LittleEndian.Uint64(src[si+ml:])
if x == 0 {
ml += 8
} else {
// Stop is first non-zero byte.
ml += bits.TrailingZeros64(x) >> 3
break
}
}
if ml < minMatch || ml <= mLen {
// Match too small (<minMath) or smaller than the current match.
continue
}
// Found a longer match, keep its position and length.
mLen = ml
offset = si - next
// Try another previous position with the same hash.
}
c.chainTable[si&winMask] = c.hashTable[h]
c.hashTable[h] = si
// No match found.
if mLen == 0 {
si += 1 + (si-anchor)>>adaptSkipLog
continue
}
// Match found.
// Update hash/chain tables with overlapping bytes:
// si already hashed, add everything from si+1 up to the match length.
winStart := si + 1
if ws := si + mLen - winSize; ws > winStart {
winStart = ws
}
for si, ml := winStart, si+mLen; si < ml; {
match >>= 8
match |= uint32(src[si+3]) << 24
h := blockHashHC(match)
c.chainTable[si&winMask] = c.hashTable[h]
c.hashTable[h] = si
si++
}
lLen := si - anchor
si += mLen
mLen -= minMatch // Match length does not include minMatch.
if mLen < 0xF {
dst[di] = byte(mLen)
} else {
dst[di] = 0xF
}
// Encode literals length.
if lLen < 0xF {
dst[di] |= byte(lLen << 4)
} else {
dst[di] |= 0xF0
di++
l := lLen - 0xF
for ; l >= 0xFF; l -= 0xFF {
dst[di] = 0xFF
di++
}
dst[di] = byte(l)
}
di++
// Literals.
copy(dst[di:di+lLen], src[anchor:anchor+lLen])
di += lLen
anchor = si
// Encode offset.
di += 2
dst[di-2], dst[di-1] = byte(offset), byte(offset>>8)
// Encode match length part 2.
if mLen >= 0xF {
for mLen -= 0xF; mLen >= 0xFF; mLen -= 0xFF {
dst[di] = 0xFF
di++
}
dst[di] = byte(mLen)
di++
}
}
if isNotCompressible && anchor == 0 {
// Incompressible.
return 0, nil
}
// Last literals.
lastLiterals:
lLen := len(src) - anchor
if lLen < 0xF {
dst[di] = byte(lLen << 4)
} else {
dst[di] = 0xF0
di++
lLen -= 0xF
for ; lLen >= 0xFF; lLen -= 0xFF {
dst[di] = 0xFF
di++
}
dst[di] = byte(lLen)
}
di++
// Write the last literals.
if isNotCompressible && di >= anchor {
// Incompressible.
return 0, nil
}
di += copy(dst[di:di+len(src)-anchor], src[anchor:])
return di, nil
}