857 lines
26 KiB
Go
857 lines
26 KiB
Go
// 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 (
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inputMargin = 16 - 1
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minNonLiteralBlockSize = 1 + 1 + inputMargin
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)
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// Ensure that e.cur doesn't wrap, mainly an issue on 32 bits.
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if e.cur > 1<<30 {
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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 = maxStoreBlockSize
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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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e.cur += maxStoreBlockSize
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e.prev = e.prev[:0]
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return
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}
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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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nextEmit := int32(0)
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s := int32(0)
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cv := load3232(src, s)
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nextHash := hash(cv)
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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 := int32(32)
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nextS := s
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var candidate tableEntry
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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 = e.table[nextHash&tableMask]
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now := load3232(src, nextS)
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e.table[nextHash&tableMask] = tableEntry{offset: s + e.cur, val: cv}
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nextHash = hash(now)
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offset := s - (candidate.offset - e.cur)
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if offset >= maxMatchOffset || cv != candidate.val {
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// Out of range or not matched.
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cv = now
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continue
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}
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break
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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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// Extend the 4-byte match as long as possible.
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//
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s += 4
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t := candidate.offset - e.cur + 4
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l := e.matchlen(s, t, src)
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// matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
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dst.tokens[dst.n] = matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset))
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dst.n++
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s += l
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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 := load6432(src, s-1)
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prevHash := hash(uint32(x))
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e.table[prevHash&tableMask] = tableEntry{offset: e.cur + s - 1, val: uint32(x)}
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x >>= 8
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currHash := hash(uint32(x))
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candidate = e.table[currHash&tableMask]
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e.table[currHash&tableMask] = tableEntry{offset: e.cur + s, val: uint32(x)}
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offset := s - (candidate.offset - e.cur)
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if offset >= maxMatchOffset || uint32(x) != candidate.val {
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cv = uint32(x >> 8)
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nextHash = hash(cv)
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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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emitLiteral(dst, src[nextEmit:])
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}
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e.cur += int32(len(src))
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e.prev = e.prev[:len(src)]
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copy(e.prev, src)
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}
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type tableEntryPrev struct {
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Cur tableEntry
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Prev tableEntry
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}
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// snappyL3
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type snappyL3 struct {
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snappyGen
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table [tableSize]tableEntryPrev
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}
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// Encode uses a similar algorithm to level 2, will check up to two candidates.
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func (e *snappyL3) 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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|
)
|
|
|
|
// 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 {
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e.table[i] = tableEntryPrev{}
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}
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e.cur = maxStoreBlockSize
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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.
|
|
if len(src) < minNonLiteralBlockSize {
|
|
// We do not fill the token table.
|
|
// This will be picked up by caller.
|
|
dst.n = uint16(len(src))
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e.cur += maxStoreBlockSize
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e.prev = e.prev[:0]
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return
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}
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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.
|
|
sLimit := int32(len(src) - inputMargin)
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// nextEmit is where in src the next emitLiteral should start from.
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nextEmit := int32(0)
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s := int32(0)
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cv := load3232(src, s)
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nextHash := hash(cv)
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for {
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|
// Copied from the C++ snappy implementation:
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|
//
|
|
// 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)
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nextS := s
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var candidate tableEntry
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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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candidates := e.table[nextHash&tableMask]
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now := load3232(src, nextS)
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e.table[nextHash&tableMask] = tableEntryPrev{Prev: candidates.Cur, Cur: tableEntry{offset: s + e.cur, val: cv}}
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nextHash = hash(now)
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// Check both candidates
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candidate = candidates.Cur
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if cv == candidate.val {
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offset := s - (candidate.offset - e.cur)
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if offset < maxMatchOffset {
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break
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}
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} else {
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// We only check if value mismatches.
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|
// Offset will always be invalid in other cases.
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|
candidate = candidates.Prev
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if cv == candidate.val {
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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
|
|
}
|