mirror of
https://github.com/go-gitea/gitea.git
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200 lines
5.6 KiB
Go
200 lines
5.6 KiB
Go
package flate
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// fastGen maintains the table for matches,
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// and the previous byte block for level 2.
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// This is the generic implementation.
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type fastEncL2 struct {
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fastGen
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table [bTableSize]tableEntry
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}
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// EncodeL2 uses a similar algorithm to level 1, but is capable
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// of matching across blocks giving better compression at a small slowdown.
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func (e *fastEncL2) Encode(dst *tokens, src []byte) {
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const (
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inputMargin = 12 - 1
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minNonLiteralBlockSize = 1 + 1 + inputMargin
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)
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// Protect against e.cur wraparound.
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for e.cur >= bufferReset {
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if len(e.hist) == 0 {
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for i := range e.table[:] {
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e.table[i] = tableEntry{}
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}
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e.cur = maxMatchOffset
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break
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}
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// Shift down everything in the table that isn't already too far away.
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minOff := e.cur + int32(len(e.hist)) - maxMatchOffset
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for i := range e.table[:] {
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v := e.table[i].offset
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if v <= minOff {
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v = 0
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} else {
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v = v - e.cur + maxMatchOffset
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}
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e.table[i].offset = v
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}
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e.cur = maxMatchOffset
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}
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s := e.addBlock(src)
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// This check isn't in the Snappy implementation, but there, the caller
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// instead of the callee handles this case.
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if len(src) < minNonLiteralBlockSize {
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// We do not fill the token table.
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// This will be picked up by caller.
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dst.n = uint16(len(src))
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return
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}
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// Override src
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src = e.hist
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nextEmit := s
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// sLimit is when to stop looking for offset/length copies. The inputMargin
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// lets us use a fast path for emitLiteral in the main loop, while we are
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// looking for copies.
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sLimit := int32(len(src) - inputMargin)
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// nextEmit is where in src the next emitLiteral should start from.
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cv := load3232(src, s)
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for {
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// When should we start skipping if we haven't found matches in a long while.
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const skipLog = 5
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const doEvery = 2
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nextS := s
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var candidate tableEntry
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for {
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nextHash := hash4u(cv, bTableBits)
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s = nextS
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nextS = s + doEvery + (s-nextEmit)>>skipLog
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if nextS > sLimit {
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goto emitRemainder
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}
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candidate = e.table[nextHash]
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now := load6432(src, nextS)
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e.table[nextHash] = tableEntry{offset: s + e.cur, val: cv}
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nextHash = hash4u(uint32(now), bTableBits)
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offset := s - (candidate.offset - e.cur)
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if offset < maxMatchOffset && cv == candidate.val {
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e.table[nextHash] = tableEntry{offset: nextS + e.cur, val: uint32(now)}
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break
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}
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// Do one right away...
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cv = uint32(now)
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s = nextS
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nextS++
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candidate = e.table[nextHash]
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now >>= 8
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e.table[nextHash] = tableEntry{offset: s + e.cur, val: cv}
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offset = s - (candidate.offset - e.cur)
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if offset < maxMatchOffset && cv == candidate.val {
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break
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}
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cv = uint32(now)
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}
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// A 4-byte match has been found. We'll later see if more than 4 bytes
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// match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
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// them as literal bytes.
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// Call emitCopy, and then see if another emitCopy could be our next
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// move. Repeat until we find no match for the input immediately after
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// what was consumed by the last emitCopy call.
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//
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// If we exit this loop normally then we need to call emitLiteral next,
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// though we don't yet know how big the literal will be. We handle that
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// by proceeding to the next iteration of the main loop. We also can
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// exit this loop via goto if we get close to exhausting the input.
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for {
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// Invariant: we have a 4-byte match at s, and no need to emit any
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// literal bytes prior to s.
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// Extend the 4-byte match as long as possible.
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t := candidate.offset - e.cur
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l := e.matchlenLong(s+4, t+4, src) + 4
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// Extend backwards
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for t > 0 && s > nextEmit && src[t-1] == src[s-1] {
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s--
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t--
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l++
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}
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if nextEmit < s {
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emitLiteral(dst, src[nextEmit:s])
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}
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dst.AddMatchLong(l, uint32(s-t-baseMatchOffset))
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s += l
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nextEmit = s
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if nextS >= s {
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s = nextS + 1
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}
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if s >= sLimit {
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// Index first pair after match end.
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if int(s+l+4) < len(src) {
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cv := load3232(src, s)
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e.table[hash4u(cv, bTableBits)] = tableEntry{offset: s + e.cur, val: cv}
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}
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goto emitRemainder
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}
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// Store every second hash in-between, but offset by 1.
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for i := s - l + 2; i < s-5; i += 7 {
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x := load6432(src, int32(i))
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nextHash := hash4u(uint32(x), bTableBits)
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e.table[nextHash] = tableEntry{offset: e.cur + i, val: uint32(x)}
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// Skip one
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x >>= 16
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nextHash = hash4u(uint32(x), bTableBits)
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e.table[nextHash] = tableEntry{offset: e.cur + i + 2, val: uint32(x)}
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// Skip one
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x >>= 16
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nextHash = hash4u(uint32(x), bTableBits)
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e.table[nextHash] = tableEntry{offset: e.cur + i + 4, val: uint32(x)}
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}
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// We could immediately start working at s now, but to improve
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// compression we first update the hash table at s-2 to s. If
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// another emitCopy is not our next move, also calculate nextHash
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// at s+1. At least on GOARCH=amd64, these three hash calculations
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// are faster as one load64 call (with some shifts) instead of
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// three load32 calls.
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x := load6432(src, s-2)
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o := e.cur + s - 2
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prevHash := hash4u(uint32(x), bTableBits)
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prevHash2 := hash4u(uint32(x>>8), bTableBits)
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e.table[prevHash] = tableEntry{offset: o, val: uint32(x)}
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e.table[prevHash2] = tableEntry{offset: o + 1, val: uint32(x >> 8)}
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currHash := hash4u(uint32(x>>16), bTableBits)
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candidate = e.table[currHash]
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e.table[currHash] = tableEntry{offset: o + 2, val: uint32(x >> 16)}
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offset := s - (candidate.offset - e.cur)
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if offset > maxMatchOffset || uint32(x>>16) != candidate.val {
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cv = uint32(x >> 24)
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s++
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break
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}
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}
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}
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emitRemainder:
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if int(nextEmit) < len(src) {
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// If nothing was added, don't encode literals.
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if dst.n == 0 {
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return
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}
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emitLiteral(dst, src[nextEmit:])
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}
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}
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