source: code/trunk/vendor/github.com/klauspost/compress/flate/level2.go@ 145

package flate

import "fmt"

// fastGen maintains the table for matches,
// and the previous byte block for level 2.
// This is the generic implementation.
type fastEncL2 struct {
        fastGen
        table [bTableSize]tableEntry
}

// EncodeL2 uses a similar algorithm to level 1, but is capable
// of matching across blocks giving better compression at a small slowdown.
func (e *fastEncL2) Encode(dst *tokens, src []byte) {
        const (
                inputMargin            = 12 - 1
                minNonLiteralBlockSize = 1 + 1 + inputMargin
        )

        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] = tableEntry{}
                        }
                        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].offset
                        if v <= minOff {
                                v = 0
                        } else {
                                v = v - e.cur + maxMatchOffset
                        }
                        e.table[i].offset = v
                }
                e.cur = maxMatchOffset
        }

        s := e.addBlock(src)

        // This check isn't in the Snappy implementation, but there, the caller
        // instead of the callee handles this case.
        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 {
                // When should we start skipping if we haven't found matches in a long while.
                const skipLog = 5
                const doEvery = 2

                nextS := s
                var candidate tableEntry
                for {
                        nextHash := hash4u(cv, bTableBits)
                        s = nextS
                        nextS = s + doEvery + (s-nextEmit)>>skipLog
                        if nextS > sLimit {
                                goto emitRemainder
                        }
                        candidate = e.table[nextHash]
                        now := load6432(src, nextS)
                        e.table[nextHash] = tableEntry{offset: s + e.cur}
                        nextHash = hash4u(uint32(now), bTableBits)

                        offset := s - (candidate.offset - e.cur)
                        if offset < maxMatchOffset && cv == load3232(src, candidate.offset-e.cur) {
                                e.table[nextHash] = tableEntry{offset: nextS + e.cur}
                                break
                        }

                        // Do one right away...
                        cv = uint32(now)
                        s = nextS
                        nextS++
                        candidate = e.table[nextHash]
                        now >>= 8
                        e.table[nextHash] = tableEntry{offset: s + e.cur}

                        offset = s - (candidate.offset - e.cur)
                        if offset < maxMatchOffset && cv == load3232(src, candidate.offset-e.cur) {
                                break
                        }
                        cv = uint32(now)
                }

                // A 4-byte match has been found. We'll later see if more than 4 bytes
                // match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
                // them as literal bytes.

                // 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 {
                                // Index first pair after match end.
                                if int(s+l+4) < len(src) {
                                        cv := load3232(src, s)
                                        e.table[hash4u(cv, bTableBits)] = tableEntry{offset: s + e.cur}
                                }
                                goto emitRemainder
                        }

                        // Store every second hash in-between, but offset by 1.
                        for i := s - l + 2; i < s-5; i += 7 {
                                x := load6432(src, i)
                                nextHash := hash4u(uint32(x), bTableBits)
                                e.table[nextHash] = tableEntry{offset: e.cur + i}
                                // Skip one
                                x >>= 16
                                nextHash = hash4u(uint32(x), bTableBits)
                                e.table[nextHash] = tableEntry{offset: e.cur + i + 2}
                                // Skip one
                                x >>= 16
                                nextHash = hash4u(uint32(x), bTableBits)
                                e.table[nextHash] = tableEntry{offset: e.cur + i + 4}
                        }

                        // We could immediately start working at s now, but to improve
                        // compression we first update the hash table at s-2 to s. If
                        // another emitCopy is not our next move, also calculate nextHash
                        // at s+1. At least on GOARCH=amd64, these three hash calculations
                        // are faster as one load64 call (with some shifts) instead of
                        // three load32 calls.
                        x := load6432(src, s-2)
                        o := e.cur + s - 2
                        prevHash := hash4u(uint32(x), bTableBits)
                        prevHash2 := hash4u(uint32(x>>8), bTableBits)
                        e.table[prevHash] = tableEntry{offset: o}
                        e.table[prevHash2] = tableEntry{offset: o + 1}
                        currHash := hash4u(uint32(x>>16), bTableBits)
                        candidate = e.table[currHash]
                        e.table[currHash] = tableEntry{offset: o + 2}

                        offset := s - (candidate.offset - e.cur)
                        if offset > maxMatchOffset || uint32(x>>16) != load3232(src, candidate.offset-e.cur) {
                                cv = uint32(x >> 24)
                                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:])
        }
}