mirror of
https://github.com/go-gitea/gitea.git
synced 2025-10-31 21:28:11 +09:00
Added all required dependencies
This commit is contained in:
Thomas Boerger
856
vendor/github.com/klauspost/compress/flate/snappy.go
generated
vendored
Normal file
856
vendor/github.com/klauspost/compress/flate/snappy.go
generated
vendored
Normal file
@@ -0,0 +1,856 @@
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// Copyright 2011 The Snappy-Go Authors. All rights reserved.
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// Modified for deflate by Klaus Post (c) 2015.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package flate
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// emitLiteral writes a literal chunk and returns the number of bytes written.
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func emitLiteral(dst *tokens, lit []byte) {
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ol := int(dst.n)
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for i, v := range lit {
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dst.tokens[(i+ol)&maxStoreBlockSize] = token(v)
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}
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dst.n += uint16(len(lit))
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}
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// emitCopy writes a copy chunk and returns the number of bytes written.
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func emitCopy(dst *tokens, offset, length int) {
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dst.tokens[dst.n] = matchToken(uint32(length-3), uint32(offset-minOffsetSize))
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dst.n++
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}
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type snappyEnc interface {
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Encode(dst *tokens, src []byte)
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Reset()
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}
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func newSnappy(level int) snappyEnc {
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switch level {
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case 1:
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return &snappyL1{}
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case 2:
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return &snappyL2{snappyGen: snappyGen{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}}
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case 3:
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return &snappyL3{snappyGen: snappyGen{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}}
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case 4:
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return &snappyL4{snappyL3{snappyGen: snappyGen{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}}}
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default:
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panic("invalid level specified")
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}
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}
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const (
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tableBits = 14 // Bits used in the table
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tableSize = 1 << tableBits // Size of the table
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tableMask = tableSize - 1 // Mask for table indices. Redundant, but can eliminate bounds checks.
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tableShift = 32 - tableBits // Right-shift to get the tableBits most significant bits of a uint32.
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baseMatchOffset = 1 // The smallest match offset
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baseMatchLength = 3 // The smallest match length per the RFC section 3.2.5
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maxMatchOffset = 1 << 15 // The largest match offset
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)
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func load32(b []byte, i int) uint32 {
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b = b[i : i+4 : len(b)] // Help the compiler eliminate bounds checks on the next line.
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return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
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}
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func load64(b []byte, i int) uint64 {
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b = b[i : i+8 : len(b)] // Help the compiler eliminate bounds checks on the next line.
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return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
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uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
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}
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func hash(u uint32) uint32 {
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return (u * 0x1e35a7bd) >> tableShift
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}
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// snappyL1 encapsulates level 1 compression
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type snappyL1 struct{}
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func (e *snappyL1) Reset() {}
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func (e *snappyL1) Encode(dst *tokens, src []byte) {
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const (
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inputMargin = 16 - 1
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minNonLiteralBlockSize = 1 + 1 + inputMargin
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)
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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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// Initialize the hash table.
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//
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// The table element type is uint16, as s < sLimit and sLimit < len(src)
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// and len(src) <= maxStoreBlockSize and maxStoreBlockSize == 65535.
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var table [tableSize]uint16
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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 := len(src) - inputMargin
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// nextEmit is where in src the next emitLiteral should start from.
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nextEmit := 0
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// The encoded form must start with a literal, as there are no previous
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// bytes to copy, so we start looking for hash matches at s == 1.
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s := 1
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nextHash := hash(load32(src, s))
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for {
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// Copied from the C++ snappy implementation:
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//
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// Heuristic match skipping: If 32 bytes are scanned with no matches
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// found, start looking only at every other byte. If 32 more bytes are
|
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// scanned (or skipped), look at every third byte, etc.. When a match
|
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// is found, immediately go back to looking at every byte. This is a
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// small loss (~5% performance, ~0.1% density) for compressible data
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// due to more bookkeeping, but for non-compressible data (such as
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// JPEG) it's a huge win since the compressor quickly "realizes" the
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// data is incompressible and doesn't bother looking for matches
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// everywhere.
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//
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// The "skip" variable keeps track of how many bytes there are since
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// the last match; dividing it by 32 (ie. right-shifting by five) gives
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// the number of bytes to move ahead for each iteration.
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skip := 32
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nextS := s
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candidate := 0
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for {
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s = nextS
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bytesBetweenHashLookups := skip >> 5
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nextS = s + bytesBetweenHashLookups
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skip += bytesBetweenHashLookups
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if nextS > sLimit {
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goto emitRemainder
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}
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candidate = int(table[nextHash&tableMask])
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table[nextHash&tableMask] = uint16(s)
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nextHash = hash(load32(src, nextS))
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// TODO: < should be <=, and add a test for that.
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if s-candidate < maxMatchOffset && load32(src, s) == load32(src, candidate) {
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break
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}
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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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emitLiteral(dst, src[nextEmit:s])
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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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base := s
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// Extend the 4-byte match as long as possible.
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//
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// This is an inlined version of Snappy's:
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// s = extendMatch(src, candidate+4, s+4)
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s += 4
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s1 := base + maxMatchLength
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if s1 > len(src) {
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s1 = len(src)
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}
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a := src[s:s1]
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b := src[candidate+4:]
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b = b[:len(a)]
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l := len(a)
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for i := range a {
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if a[i] != b[i] {
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l = i
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break
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}
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}
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s += l
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// matchToken is flate's equivalent of Snappy's emitCopy.
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dst.tokens[dst.n] = matchToken(uint32(s-base-baseMatchLength), uint32(base-candidate-baseMatchOffset))
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dst.n++
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nextEmit = s
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if s >= sLimit {
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goto emitRemainder
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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-1 and at 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 := load64(src, s-1)
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prevHash := hash(uint32(x >> 0))
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table[prevHash&tableMask] = uint16(s - 1)
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currHash := hash(uint32(x >> 8))
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candidate = int(table[currHash&tableMask])
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table[currHash&tableMask] = uint16(s)
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// TODO: >= should be >, and add a test for that.
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if s-candidate >= maxMatchOffset || uint32(x>>8) != load32(src, candidate) {
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nextHash = hash(uint32(x >> 16))
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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 nextEmit < len(src) {
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emitLiteral(dst, src[nextEmit:])
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}
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}
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type tableEntry struct {
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val uint32
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offset int32
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}
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func load3232(b []byte, i int32) uint32 {
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b = b[i : i+4 : len(b)] // Help the compiler eliminate bounds checks on the next line.
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return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
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}
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func load6432(b []byte, i int32) uint64 {
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b = b[i : i+8 : len(b)] // Help the compiler eliminate bounds checks on the next line.
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return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
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uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
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}
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// snappyGen 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 snappyGen struct {
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prev []byte
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cur int32
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}
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// snappyGen 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 snappyL2 struct {
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snappyGen
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table [tableSize]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 *snappyL2) Encode(dst *tokens, src []byte) {
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const (
|
||||
inputMargin = 16 - 1
|
||||
minNonLiteralBlockSize = 1 + 1 + inputMargin
|
||||
)
|
||||
|
||||
// Ensure that e.cur doesn't wrap, mainly an issue on 32 bits.
|
||||
if e.cur > 1<<30 {
|
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for i := range e.table {
|
||||
e.table[i] = tableEntry{}
|
||||
}
|
||||
e.cur = maxStoreBlockSize
|
||||
}
|
||||
|
||||
// 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))
|
||||
e.cur += maxStoreBlockSize
|
||||
e.prev = e.prev[:0]
|
||||
return
|
||||
}
|
||||
|
||||
// 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.
|
||||
nextEmit := int32(0)
|
||||
s := int32(0)
|
||||
cv := load3232(src, s)
|
||||
nextHash := hash(cv)
|
||||
|
||||
for {
|
||||
// Copied from the C++ snappy implementation:
|
||||
//
|
||||
// Heuristic match skipping: If 32 bytes are scanned with no matches
|
||||
// found, start looking only at every other byte. If 32 more bytes are
|
||||
// scanned (or skipped), look at every third byte, etc.. When a match
|
||||
// is found, immediately go back to looking at every byte. This is a
|
||||
// small loss (~5% performance, ~0.1% density) for compressible data
|
||||
// due to more bookkeeping, but for non-compressible data (such as
|
||||
// JPEG) it's a huge win since the compressor quickly "realizes" the
|
||||
// data is incompressible and doesn't bother looking for matches
|
||||
// everywhere.
|
||||
//
|
||||
// The "skip" variable keeps track of how many bytes there are since
|
||||
// the last match; dividing it by 32 (ie. right-shifting by five) gives
|
||||
// the number of bytes to move ahead for each iteration.
|
||||
skip := int32(32)
|
||||
|
||||
nextS := s
|
||||
var candidate tableEntry
|
||||
for {
|
||||
s = nextS
|
||||
bytesBetweenHashLookups := skip >> 5
|
||||
nextS = s + bytesBetweenHashLookups
|
||||
skip += bytesBetweenHashLookups
|
||||
if nextS > sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
candidate = e.table[nextHash&tableMask]
|
||||
now := load3232(src, nextS)
|
||||
e.table[nextHash&tableMask] = tableEntry{offset: s + e.cur, val: cv}
|
||||
nextHash = hash(now)
|
||||
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset >= maxMatchOffset || cv != candidate.val {
|
||||
// Out of range or not matched.
|
||||
cv = now
|
||||
continue
|
||||
}
|
||||
break
|
||||
}
|
||||
|
||||
// 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.
|
||||
emitLiteral(dst, src[nextEmit:s])
|
||||
|
||||
// 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.
|
||||
//
|
||||
s += 4
|
||||
t := candidate.offset - e.cur + 4
|
||||
l := e.matchlen(s, t, src)
|
||||
|
||||
// matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
|
||||
dst.tokens[dst.n] = matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset))
|
||||
dst.n++
|
||||
s += l
|
||||
nextEmit = s
|
||||
if s >= sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
|
||||
// We could immediately start working at s now, but to improve
|
||||
// compression we first update the hash table at s-1 and at 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-1)
|
||||
prevHash := hash(uint32(x))
|
||||
e.table[prevHash&tableMask] = tableEntry{offset: e.cur + s - 1, val: uint32(x)}
|
||||
x >>= 8
|
||||
currHash := hash(uint32(x))
|
||||
candidate = e.table[currHash&tableMask]
|
||||
e.table[currHash&tableMask] = tableEntry{offset: e.cur + s, val: uint32(x)}
|
||||
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset >= maxMatchOffset || uint32(x) != candidate.val {
|
||||
cv = uint32(x >> 8)
|
||||
nextHash = hash(cv)
|
||||
s++
|
||||
break
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
emitRemainder:
|
||||
if int(nextEmit) < len(src) {
|
||||
emitLiteral(dst, src[nextEmit:])
|
||||
}
|
||||
e.cur += int32(len(src))
|
||||
e.prev = e.prev[:len(src)]
|
||||
copy(e.prev, src)
|
||||
}
|
||||
|
||||
type tableEntryPrev struct {
|
||||
Cur tableEntry
|
||||
Prev tableEntry
|
||||
}
|
||||
|
||||
// snappyL3
|
||||
type snappyL3 struct {
|
||||
snappyGen
|
||||
table [tableSize]tableEntryPrev
|
||||
}
|
||||
|
||||
// Encode uses a similar algorithm to level 2, will check up to two candidates.
|
||||
func (e *snappyL3) Encode(dst *tokens, src []byte) {
|
||||
const (
|
||||
inputMargin = 16 - 1
|
||||
minNonLiteralBlockSize = 1 + 1 + inputMargin
|
||||
)
|
||||
|
||||
// Ensure that e.cur doesn't wrap, mainly an issue on 32 bits.
|
||||
if e.cur > 1<<30 {
|
||||
for i := range e.table {
|
||||
e.table[i] = tableEntryPrev{}
|
||||
}
|
||||
e.cur = maxStoreBlockSize
|
||||
}
|
||||
|
||||
// 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))
|
||||
e.cur += maxStoreBlockSize
|
||||
e.prev = e.prev[:0]
|
||||
return
|
||||
}
|
||||
|
||||
// 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.
|
||||
nextEmit := int32(0)
|
||||
s := int32(0)
|
||||
cv := load3232(src, s)
|
||||
nextHash := hash(cv)
|
||||
|
||||
for {
|
||||
// Copied from the C++ snappy implementation:
|
||||
//
|
||||
// Heuristic match skipping: If 32 bytes are scanned with no matches
|
||||
// found, start looking only at every other byte. If 32 more bytes are
|
||||
// scanned (or skipped), look at every third byte, etc.. When a match
|
||||
// is found, immediately go back to looking at every byte. This is a
|
||||
// small loss (~5% performance, ~0.1% density) for compressible data
|
||||
// due to more bookkeeping, but for non-compressible data (such as
|
||||
// JPEG) it's a huge win since the compressor quickly "realizes" the
|
||||
// data is incompressible and doesn't bother looking for matches
|
||||
// everywhere.
|
||||
//
|
||||
// The "skip" variable keeps track of how many bytes there are since
|
||||
// the last match; dividing it by 32 (ie. right-shifting by five) gives
|
||||
// the number of bytes to move ahead for each iteration.
|
||||
skip := int32(32)
|
||||
|
||||
nextS := s
|
||||
var candidate tableEntry
|
||||
for {
|
||||
s = nextS
|
||||
bytesBetweenHashLookups := skip >> 5
|
||||
nextS = s + bytesBetweenHashLookups
|
||||
skip += bytesBetweenHashLookups
|
||||
if nextS > sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
candidates := e.table[nextHash&tableMask]
|
||||
now := load3232(src, nextS)
|
||||
e.table[nextHash&tableMask] = tableEntryPrev{Prev: candidates.Cur, Cur: tableEntry{offset: s + e.cur, val: cv}}
|
||||
nextHash = hash(now)
|
||||
|
||||
// Check both candidates
|
||||
candidate = candidates.Cur
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
break
|
||||
}
|
||||
} else {
|
||||
// We only check if value mismatches.
|
||||
// Offset will always be invalid in other cases.
|
||||
candidate = candidates.Prev
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
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.
|
||||
emitLiteral(dst, src[nextEmit:s])
|
||||
|
||||
// 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.
|
||||
//
|
||||
s += 4
|
||||
t := candidate.offset - e.cur + 4
|
||||
l := e.matchlen(s, t, src)
|
||||
|
||||
// matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
|
||||
dst.tokens[dst.n] = matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset))
|
||||
dst.n++
|
||||
s += l
|
||||
nextEmit = s
|
||||
if s >= sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
|
||||
// We could immediately start working at s now, but to improve
|
||||
// compression we first update the hash table at s-2, s-1 and at 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)
|
||||
prevHash := hash(uint32(x))
|
||||
|
||||
e.table[prevHash&tableMask] = tableEntryPrev{
|
||||
Prev: e.table[prevHash&tableMask].Cur,
|
||||
Cur: tableEntry{offset: e.cur + s - 2, val: uint32(x)},
|
||||
}
|
||||
x >>= 8
|
||||
prevHash = hash(uint32(x))
|
||||
|
||||
e.table[prevHash&tableMask] = tableEntryPrev{
|
||||
Prev: e.table[prevHash&tableMask].Cur,
|
||||
Cur: tableEntry{offset: e.cur + s - 1, val: uint32(x)},
|
||||
}
|
||||
x >>= 8
|
||||
currHash := hash(uint32(x))
|
||||
candidates := e.table[currHash&tableMask]
|
||||
cv = uint32(x)
|
||||
e.table[currHash&tableMask] = tableEntryPrev{
|
||||
Prev: candidates.Cur,
|
||||
Cur: tableEntry{offset: s + e.cur, val: cv},
|
||||
}
|
||||
|
||||
// Check both candidates
|
||||
candidate = candidates.Cur
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
continue
|
||||
}
|
||||
} else {
|
||||
// We only check if value mismatches.
|
||||
// Offset will always be invalid in other cases.
|
||||
candidate = candidates.Prev
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
continue
|
||||
}
|
||||
}
|
||||
}
|
||||
cv = uint32(x >> 8)
|
||||
nextHash = hash(cv)
|
||||
s++
|
||||
break
|
||||
}
|
||||
}
|
||||
|
||||
emitRemainder:
|
||||
if int(nextEmit) < len(src) {
|
||||
emitLiteral(dst, src[nextEmit:])
|
||||
}
|
||||
e.cur += int32(len(src))
|
||||
e.prev = e.prev[:len(src)]
|
||||
copy(e.prev, src)
|
||||
}
|
||||
|
||||
// snappyL4
|
||||
type snappyL4 struct {
|
||||
snappyL3
|
||||
}
|
||||
|
||||
// Encode uses a similar algorithm to level 3,
|
||||
// but will check up to two candidates if first isn't long enough.
|
||||
func (e *snappyL4) Encode(dst *tokens, src []byte) {
|
||||
const (
|
||||
inputMargin = 16 - 1
|
||||
minNonLiteralBlockSize = 1 + 1 + inputMargin
|
||||
matchLenGood = 12
|
||||
)
|
||||
|
||||
// Ensure that e.cur doesn't wrap, mainly an issue on 32 bits.
|
||||
if e.cur > 1<<30 {
|
||||
for i := range e.table {
|
||||
e.table[i] = tableEntryPrev{}
|
||||
}
|
||||
e.cur = maxStoreBlockSize
|
||||
}
|
||||
|
||||
// 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))
|
||||
e.cur += maxStoreBlockSize
|
||||
e.prev = e.prev[:0]
|
||||
return
|
||||
}
|
||||
|
||||
// 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.
|
||||
nextEmit := int32(0)
|
||||
s := int32(0)
|
||||
cv := load3232(src, s)
|
||||
nextHash := hash(cv)
|
||||
|
||||
for {
|
||||
// Copied from the C++ snappy implementation:
|
||||
//
|
||||
// Heuristic match skipping: If 32 bytes are scanned with no matches
|
||||
// found, start looking only at every other byte. If 32 more bytes are
|
||||
// scanned (or skipped), look at every third byte, etc.. When a match
|
||||
// is found, immediately go back to looking at every byte. This is a
|
||||
// small loss (~5% performance, ~0.1% density) for compressible data
|
||||
// due to more bookkeeping, but for non-compressible data (such as
|
||||
// JPEG) it's a huge win since the compressor quickly "realizes" the
|
||||
// data is incompressible and doesn't bother looking for matches
|
||||
// everywhere.
|
||||
//
|
||||
// The "skip" variable keeps track of how many bytes there are since
|
||||
// the last match; dividing it by 32 (ie. right-shifting by five) gives
|
||||
// the number of bytes to move ahead for each iteration.
|
||||
skip := int32(32)
|
||||
|
||||
nextS := s
|
||||
var candidate tableEntry
|
||||
var candidateAlt tableEntry
|
||||
for {
|
||||
s = nextS
|
||||
bytesBetweenHashLookups := skip >> 5
|
||||
nextS = s + bytesBetweenHashLookups
|
||||
skip += bytesBetweenHashLookups
|
||||
if nextS > sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
candidates := e.table[nextHash&tableMask]
|
||||
now := load3232(src, nextS)
|
||||
e.table[nextHash&tableMask] = tableEntryPrev{Prev: candidates.Cur, Cur: tableEntry{offset: s + e.cur, val: cv}}
|
||||
nextHash = hash(now)
|
||||
|
||||
// Check both candidates
|
||||
candidate = candidates.Cur
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
offset = s - (candidates.Prev.offset - e.cur)
|
||||
if cv == candidates.Prev.val && offset < maxMatchOffset {
|
||||
candidateAlt = candidates.Prev
|
||||
}
|
||||
break
|
||||
}
|
||||
} else {
|
||||
// We only check if value mismatches.
|
||||
// Offset will always be invalid in other cases.
|
||||
candidate = candidates.Prev
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
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.
|
||||
emitLiteral(dst, src[nextEmit:s])
|
||||
|
||||
// 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.
|
||||
//
|
||||
s += 4
|
||||
t := candidate.offset - e.cur + 4
|
||||
l := e.matchlen(s, t, src)
|
||||
// Try alternative candidate if match length < matchLenGood.
|
||||
if l < matchLenGood-4 && candidateAlt.offset != 0 {
|
||||
t2 := candidateAlt.offset - e.cur + 4
|
||||
l2 := e.matchlen(s, t2, src)
|
||||
if l2 > l {
|
||||
l = l2
|
||||
t = t2
|
||||
}
|
||||
}
|
||||
// matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
|
||||
dst.tokens[dst.n] = matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset))
|
||||
dst.n++
|
||||
s += l
|
||||
nextEmit = s
|
||||
if s >= sLimit {
|
||||
goto emitRemainder
|
||||
}
|
||||
|
||||
// We could immediately start working at s now, but to improve
|
||||
// compression we first update the hash table at s-2, s-1 and at 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)
|
||||
prevHash := hash(uint32(x))
|
||||
|
||||
e.table[prevHash&tableMask] = tableEntryPrev{
|
||||
Prev: e.table[prevHash&tableMask].Cur,
|
||||
Cur: tableEntry{offset: e.cur + s - 2, val: uint32(x)},
|
||||
}
|
||||
x >>= 8
|
||||
prevHash = hash(uint32(x))
|
||||
|
||||
e.table[prevHash&tableMask] = tableEntryPrev{
|
||||
Prev: e.table[prevHash&tableMask].Cur,
|
||||
Cur: tableEntry{offset: e.cur + s - 1, val: uint32(x)},
|
||||
}
|
||||
x >>= 8
|
||||
currHash := hash(uint32(x))
|
||||
candidates := e.table[currHash&tableMask]
|
||||
cv = uint32(x)
|
||||
e.table[currHash&tableMask] = tableEntryPrev{
|
||||
Prev: candidates.Cur,
|
||||
Cur: tableEntry{offset: s + e.cur, val: cv},
|
||||
}
|
||||
|
||||
// Check both candidates
|
||||
candidate = candidates.Cur
|
||||
candidateAlt = tableEntry{}
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
offset = s - (candidates.Prev.offset - e.cur)
|
||||
if cv == candidates.Prev.val && offset < maxMatchOffset {
|
||||
candidateAlt = candidates.Prev
|
||||
}
|
||||
continue
|
||||
}
|
||||
} else {
|
||||
// We only check if value mismatches.
|
||||
// Offset will always be invalid in other cases.
|
||||
candidate = candidates.Prev
|
||||
if cv == candidate.val {
|
||||
offset := s - (candidate.offset - e.cur)
|
||||
if offset < maxMatchOffset {
|
||||
continue
|
||||
}
|
||||
}
|
||||
}
|
||||
cv = uint32(x >> 8)
|
||||
nextHash = hash(cv)
|
||||
s++
|
||||
break
|
||||
}
|
||||
}
|
||||
|
||||
emitRemainder:
|
||||
if int(nextEmit) < len(src) {
|
||||
emitLiteral(dst, src[nextEmit:])
|
||||
}
|
||||
e.cur += int32(len(src))
|
||||
e.prev = e.prev[:len(src)]
|
||||
copy(e.prev, src)
|
||||
}
|
||||
|
||||
func (e *snappyGen) matchlen(s, t int32, src []byte) int32 {
|
||||
s1 := int(s) + maxMatchLength - 4
|
||||
if s1 > len(src) {
|
||||
s1 = len(src)
|
||||
}
|
||||
|
||||
// If we are inside the current block
|
||||
if t >= 0 {
|
||||
b := src[t:]
|
||||
a := src[s:s1]
|
||||
b = b[:len(a)]
|
||||
// Extend the match to be as long as possible.
|
||||
for i := range a {
|
||||
if a[i] != b[i] {
|
||||
return int32(i)
|
||||
}
|
||||
}
|
||||
return int32(len(a))
|
||||
}
|
||||
|
||||
// We found a match in the previous block.
|
||||
tp := int32(len(e.prev)) + t
|
||||
if tp < 0 {
|
||||
return 0
|
||||
}
|
||||
|
||||
// Extend the match to be as long as possible.
|
||||
a := src[s:s1]
|
||||
b := e.prev[tp:]
|
||||
if len(b) > len(a) {
|
||||
b = b[:len(a)]
|
||||
}
|
||||
a = a[:len(b)]
|
||||
for i := range b {
|
||||
if a[i] != b[i] {
|
||||
return int32(i)
|
||||
}
|
||||
}
|
||||
n := int32(len(b))
|
||||
a = src[s+n : s1]
|
||||
b = src[:len(a)]
|
||||
for i := range a {
|
||||
if a[i] != b[i] {
|
||||
return int32(i) + n
|
||||
}
|
||||
}
|
||||
return int32(len(a)) + n
|
||||
}
|
||||
|
||||
// Reset the encoding table.
|
||||
func (e *snappyGen) Reset() {
|
||||
e.prev = e.prev[:0]
|
||||
e.cur += maxMatchOffset + 1
|
||||
}
|
||||
Reference in New Issue
Block a user