source: code/trunk/vendor/github.com/klauspost/compress/flate/level3.go@ 145

package flate

import "fmt"

// fastEncL3
type fastEncL3 struct {
        fastGen
        table [1 << 16]tableEntryPrev
}

// Encode uses a similar algorithm to level 2, will check up to two candidates.
func (e *fastEncL3) Encode(dst *tokens, src []byte) {
        const (
                inputMargin            = 8 - 1
                minNonLiteralBlockSize = 1 + 1 + inputMargin
                tableBits              = 16
                tableSize              = 1 << tableBits
        )

        if debugDeflate && e.cur < 0 {
                panic(fmt.Sprint("e.cur < 0: ", e.cur))
        }

        // Protect against e.cur wraparound.
        for e.cur >= bufferReset {
                if len(e.hist) == 0 {
                        for i := range e.table[:] {
                                e.table[i] = tableEntryPrev{}
                        }
                        e.cur = maxMatchOffset
                        break
                }
                // Shift down everything in the table that isn't already too far away.
                minOff := e.cur + int32(len(e.hist)) - maxMatchOffset
                for i := range e.table[:] {
                        v := e.table[i]
                        if v.Cur.offset <= minOff {
                                v.Cur.offset = 0
                        } else {
                                v.Cur.offset = v.Cur.offset - e.cur + maxMatchOffset
                        }
                        if v.Prev.offset <= minOff {
                                v.Prev.offset = 0
                        } else {
                                v.Prev.offset = v.Prev.offset - e.cur + maxMatchOffset
                        }
                        e.table[i] = v
                }
                e.cur = maxMatchOffset
        }

        s := e.addBlock(src)

        // Skip if too small.
        if len(src) < minNonLiteralBlockSize {
                // We do not fill the token table.
                // This will be picked up by caller.
                dst.n = uint16(len(src))
                return
        }

        // Override src
        src = e.hist
        nextEmit := s

        // sLimit is when to stop looking for offset/length copies. The inputMargin
        // lets us use a fast path for emitLiteral in the main loop, while we are
        // looking for copies.
        sLimit := int32(len(src) - inputMargin)

        // nextEmit is where in src the next emitLiteral should start from.
        cv := load3232(src, s)
        for {
                const skipLog = 6
                nextS := s
                var candidate tableEntry
                for {
                        nextHash := hash4u(cv, tableBits)
                        s = nextS
                        nextS = s + 1 + (s-nextEmit)>>skipLog
                        if nextS > sLimit {
                                goto emitRemainder
                        }
                        candidates := e.table[nextHash]
                        now := load3232(src, nextS)

                        // Safe offset distance until s + 4...
                        minOffset := e.cur + s - (maxMatchOffset - 4)
                        e.table[nextHash] = tableEntryPrev{Prev: candidates.Cur, Cur: tableEntry{offset: s + e.cur}}

                        // Check both candidates
                        candidate = candidates.Cur
                        if candidate.offset < minOffset {
                                cv = now
                                // Previous will also be invalid, we have nothing.
                                continue
                        }

                        if cv == load3232(src, candidate.offset-e.cur) {
                                if candidates.Prev.offset < minOffset || cv != load3232(src, candidates.Prev.offset-e.cur) {
                                        break
                                }
                                // Both match and are valid, pick longest.
                                offset := s - (candidate.offset - e.cur)
                                o2 := s - (candidates.Prev.offset - e.cur)
                                l1, l2 := matchLen(src[s+4:], src[s-offset+4:]), matchLen(src[s+4:], src[s-o2+4:])
                                if l2 > l1 {
                                        candidate = candidates.Prev
                                }
                                break
                        } else {
                                // We only check if value mismatches.
                                // Offset will always be invalid in other cases.
                                candidate = candidates.Prev
                                if candidate.offset > minOffset && cv == load3232(src, candidate.offset-e.cur) {
                                        break
                                }
                        }
                        cv = now
                }

                // Call emitCopy, and then see if another emitCopy could be our next
                // move. Repeat until we find no match for the input immediately after
                // what was consumed by the last emitCopy call.
                //
                // If we exit this loop normally then we need to call emitLiteral next,
                // though we don't yet know how big the literal will be. We handle that
                // by proceeding to the next iteration of the main loop. We also can
                // exit this loop via goto if we get close to exhausting the input.
                for {
                        // Invariant: we have a 4-byte match at s, and no need to emit any
                        // literal bytes prior to s.

                        // Extend the 4-byte match as long as possible.
                        //
                        t := candidate.offset - e.cur
                        l := e.matchlenLong(s+4, t+4, src) + 4

                        // Extend backwards
                        for t > 0 && s > nextEmit && src[t-1] == src[s-1] {
                                s--
                                t--
                                l++
                        }
                        if nextEmit < s {
                                if false {
                                        emitLiteral(dst, src[nextEmit:s])
                                } else {
                                        for _, v := range src[nextEmit:s] {
                                                dst.tokens[dst.n] = token(v)
                                                dst.litHist[v]++
                                                dst.n++
                                        }
                                }
                        }

                        dst.AddMatchLong(l, uint32(s-t-baseMatchOffset))
                        s += l
                        nextEmit = s
                        if nextS >= s {
                                s = nextS + 1
                        }

                        if s >= sLimit {
                                t += l
                                // Index first pair after match end.
                                if int(t+4) < len(src) && t > 0 {
                                        cv := load3232(src, t)
                                        nextHash := hash4u(cv, tableBits)
                                        e.table[nextHash] = tableEntryPrev{
                                                Prev: e.table[nextHash].Cur,
                                                Cur:  tableEntry{offset: e.cur + t},
                                        }
                                }
                                goto emitRemainder
                        }

                        // Store every 5th hash in-between.
                        for i := s - l + 2; i < s-5; i += 5 {
                                nextHash := hash4u(load3232(src, i), tableBits)
                                e.table[nextHash] = tableEntryPrev{
                                        Prev: e.table[nextHash].Cur,
                                        Cur:  tableEntry{offset: e.cur + i}}
                        }
                        // We could immediately start working at s now, but to improve
                        // compression we first update the hash table at s-2 to s.
                        x := load6432(src, s-2)
                        prevHash := hash4u(uint32(x), tableBits)

                        e.table[prevHash] = tableEntryPrev{
                                Prev: e.table[prevHash].Cur,
                                Cur:  tableEntry{offset: e.cur + s - 2},
                        }
                        x >>= 8
                        prevHash = hash4u(uint32(x), tableBits)

                        e.table[prevHash] = tableEntryPrev{
                                Prev: e.table[prevHash].Cur,
                                Cur:  tableEntry{offset: e.cur + s - 1},
                        }
                        x >>= 8
                        currHash := hash4u(uint32(x), tableBits)
                        candidates := e.table[currHash]
                        cv = uint32(x)
                        e.table[currHash] = tableEntryPrev{
                                Prev: candidates.Cur,
                                Cur:  tableEntry{offset: s + e.cur},
                        }

                        // Check both candidates
                        candidate = candidates.Cur
                        minOffset := e.cur + s - (maxMatchOffset - 4)

                        if candidate.offset > minOffset {
                                if cv == load3232(src, candidate.offset-e.cur) {
                                        // Found a match...
                                        continue
                                }
                                candidate = candidates.Prev
                                if candidate.offset > minOffset && cv == load3232(src, candidate.offset-e.cur) {
                                        // Match at prev...
                                        continue
                                }
                        }
                        cv = uint32(x >> 8)
                        s++
                        break
                }
        }

emitRemainder:
        if int(nextEmit) < len(src) {
                // If nothing was added, don't encode literals.
                if dst.n == 0 {
                        return
                }

                emitLiteral(dst, src[nextEmit:])
        }
}