gtsocial-umbx

level2.go (6015B)

---

 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
 		hashBytes              = 5
 	)
 
 	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 := load6432(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 := hashLen(cv, bTableBits, hashBytes)
 			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 = hashLen(now, bTableBits, hashBytes)
 
 			offset := s - (candidate.offset - e.cur)
 			if offset < maxMatchOffset && uint32(cv) == load3232(src, candidate.offset-e.cur) {
 				e.table[nextHash] = tableEntry{offset: nextS + e.cur}
 				break
 			}
 
 			// Do one right away...
 			cv = 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 && uint32(cv) == load3232(src, candidate.offset-e.cur) {
 				break
 			}
 			cv = 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+8) < len(src) {
 					cv := load6432(src, s)
 					e.table[hashLen(cv, bTableBits, hashBytes)] = 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 := hashLen(x, bTableBits, hashBytes)
 				e.table[nextHash] = tableEntry{offset: e.cur + i}
 				// Skip one
 				x >>= 16
 				nextHash = hashLen(x, bTableBits, hashBytes)
 				e.table[nextHash] = tableEntry{offset: e.cur + i + 2}
 				// Skip one
 				x >>= 16
 				nextHash = hashLen(x, bTableBits, hashBytes)
 				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 := hashLen(x, bTableBits, hashBytes)
 			prevHash2 := hashLen(x>>8, bTableBits, hashBytes)
 			e.table[prevHash] = tableEntry{offset: o}
 			e.table[prevHash2] = tableEntry{offset: o + 1}
 			currHash := hashLen(x>>16, bTableBits, hashBytes)
 			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 = 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:])
 	}
 }