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package brotli
import "github.com/andybalholm/brotli/matchfinder"
// An Encoder implements the matchfinder.Encoder interface, writing in Brotli format.
type Encoder struct {
wroteHeader bool
bw bitWriter
distCache []distanceCode
}
func (e *Encoder) Reset() {
e.wroteHeader = false
e.bw = bitWriter{}
}
func (e *Encoder) Encode(dst []byte, src []byte, matches []matchfinder.Match, lastBlock bool) []byte {
e.bw.dst = dst
if !e.wroteHeader {
e.bw.writeBits(4, 15)
e.wroteHeader = true
}
if len(src) == 0 {
if lastBlock {
e.bw.writeBits(2, 3) // islast + isempty
e.bw.jumpToByteBoundary()
return e.bw.dst
}
return dst
}
var literalHisto [256]uint32
var commandHisto [704]uint32
var distanceHisto [64]uint32
literalCount := 0
commandCount := 0
distanceCount := 0
if len(e.distCache) < len(matches) {
e.distCache = make([]distanceCode, len(matches))
}
// first pass: build the histograms
pos := 0
// d is the ring buffer of the last 4 distances.
d := [4]int{-10, -10, -10, -10}
for i, m := range matches {
if m.Unmatched > 0 {
for _, c := range src[pos : pos+m.Unmatched] {
literalHisto[c]++
}
literalCount += m.Unmatched
}
insertCode := getInsertLengthCode(uint(m.Unmatched))
copyCode := getCopyLengthCode(uint(m.Length))
if m.Length == 0 {
// If the stream ends with unmatched bytes, we need a dummy copy length.
copyCode = 2
}
command := combineLengthCodes(insertCode, copyCode, false)
commandHisto[command]++
commandCount++
if command >= 128 && m.Length != 0 {
var distCode distanceCode
switch m.Distance {
case d[3]:
distCode.code = 0
case d[2]:
distCode.code = 1
case d[1]:
distCode.code = 2
case d[0]:
distCode.code = 3
case d[3] - 1:
distCode.code = 4
case d[3] + 1:
distCode.code = 5
case d[3] - 2:
distCode.code = 6
case d[3] + 2:
distCode.code = 7
case d[3] - 3:
distCode.code = 8
case d[3] + 3:
distCode.code = 9
// In my testing, codes 10–15 actually reduced the compression ratio.
default:
distCode = getDistanceCode(m.Distance)
}
e.distCache[i] = distCode
distanceHisto[distCode.code]++
distanceCount++
if distCode.code != 0 {
d[0], d[1], d[2], d[3] = d[1], d[2], d[3], m.Distance
}
}
pos += m.Unmatched + m.Length
}
storeMetaBlockHeaderBW(uint(len(src)), false, &e.bw)
e.bw.writeBits(13, 0)
var literalDepths [256]byte
var literalBits [256]uint16
buildAndStoreHuffmanTreeFastBW(literalHisto[:], uint(literalCount), 8, literalDepths[:], literalBits[:], &e.bw)
var commandDepths [704]byte
var commandBits [704]uint16
buildAndStoreHuffmanTreeFastBW(commandHisto[:], uint(commandCount), 10, commandDepths[:], commandBits[:], &e.bw)
var distanceDepths [64]byte
var distanceBits [64]uint16
buildAndStoreHuffmanTreeFastBW(distanceHisto[:], uint(distanceCount), 6, distanceDepths[:], distanceBits[:], &e.bw)
pos = 0
for i, m := range matches {
insertCode := getInsertLengthCode(uint(m.Unmatched))
copyCode := getCopyLengthCode(uint(m.Length))
if m.Length == 0 {
// If the stream ends with unmatched bytes, we need a dummy copy length.
copyCode = 2
}
command := combineLengthCodes(insertCode, copyCode, false)
e.bw.writeBits(uint(commandDepths[command]), uint64(commandBits[command]))
if kInsExtra[insertCode] > 0 {
e.bw.writeBits(uint(kInsExtra[insertCode]), uint64(m.Unmatched)-uint64(kInsBase[insertCode]))
}
if kCopyExtra[copyCode] > 0 {
e.bw.writeBits(uint(kCopyExtra[copyCode]), uint64(m.Length)-uint64(kCopyBase[copyCode]))
}
if m.Unmatched > 0 {
for _, c := range src[pos : pos+m.Unmatched] {
e.bw.writeBits(uint(literalDepths[c]), uint64(literalBits[c]))
}
}
if command >= 128 && m.Length != 0 {
distCode := e.distCache[i]
e.bw.writeBits(uint(distanceDepths[distCode.code]), uint64(distanceBits[distCode.code]))
if distCode.nExtra > 0 {
e.bw.writeBits(distCode.nExtra, distCode.extraBits)
}
}
pos += m.Unmatched + m.Length
}
if lastBlock {
e.bw.writeBits(2, 3) // islast + isempty
e.bw.jumpToByteBoundary()
}
return e.bw.dst
}
type distanceCode struct {
code int
nExtra uint
extraBits uint64
}
func getDistanceCode(distance int) distanceCode {
d := distance + 3
nbits := log2FloorNonZero(uint(d)) - 1
prefix := (d >> nbits) & 1
offset := (2 + prefix) << nbits
distcode := int(2*(nbits-1)) + prefix + 16
extra := d - offset
return distanceCode{distcode, uint(nbits), uint64(extra)}
}
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