310 lines
9.4 KiB
Go
310 lines
9.4 KiB
Go
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// Copyright 2016 The Go Authors. All rights reserved.
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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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import "math"
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// This encoding algorithm, which prioritizes speed over output size, is
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// based on Snappy's LZ77-style encoder: github.com/golang/snappy
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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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// Reset the buffer offset when reaching this.
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// Offsets are stored between blocks as int32 values.
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// Since the offset we are checking against is at the beginning
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// of the buffer, we need to subtract the current and input
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// buffer to not risk overflowing the int32.
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bufferReset = math.MaxInt32 - maxStoreBlockSize*2
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)
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func load32(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 load64(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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func hash(u uint32) uint32 {
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return (u * 0x1e35a7bd) >> tableShift
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}
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// These constants are defined by the Snappy implementation so that its
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// assembly implementation can fast-path some 16-bytes-at-a-time copies. They
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// aren't necessary in the pure Go implementation, as we don't use those same
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// optimizations, but using the same thresholds doesn't really hurt.
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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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type tableEntry struct {
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val uint32 // Value at destination
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offset int32
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}
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// deflateFast maintains the table for matches,
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// and the previous byte block for cross block matching.
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type deflateFast struct {
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table [tableSize]tableEntry
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prev []byte // Previous block, zero length if unknown.
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cur int32 // Current match offset.
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}
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func newDeflateFast() *deflateFast {
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return &deflateFast{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}
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}
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// encode encodes a block given in src and appends tokens
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// to dst and returns the result.
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func (e *deflateFast) encode(dst []token, src []byte) []token {
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// Ensure that e.cur doesn't wrap.
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if e.cur >= bufferReset {
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e.shiftOffsets()
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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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e.cur += maxStoreBlockSize
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e.prev = e.prev[:0]
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return emitLiteral(dst, src)
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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 := load32(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 := load32(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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dst = 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 = append(dst, matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset)))
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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 := load64(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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dst = 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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return dst
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}
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func emitLiteral(dst []token, lit []byte) []token {
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for _, v := range lit {
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dst = append(dst, literalToken(uint32(v)))
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}
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return dst
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}
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// matchLen returns the match length between src[s:] and src[t:].
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// t can be negative to indicate the match is starting in e.prev.
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// We assume that src[s-4:s] and src[t-4:t] already match.
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func (e *deflateFast) matchLen(s, t int32, src []byte) int32 {
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s1 := int(s) + maxMatchLength - 4
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if s1 > len(src) {
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s1 = len(src)
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}
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// If we are inside the current block
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if t >= 0 {
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b := src[t:]
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a := src[s:s1]
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b = b[:len(a)]
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// Extend the match to be as long as possible.
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for i := range a {
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if a[i] != b[i] {
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return int32(i)
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}
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}
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return int32(len(a))
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}
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// We found a match in the previous block.
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tp := int32(len(e.prev)) + t
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if tp < 0 {
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return 0
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}
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// Extend the match to be as long as possible.
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a := src[s:s1]
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b := e.prev[tp:]
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if len(b) > len(a) {
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b = b[:len(a)]
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}
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a = a[:len(b)]
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for i := range b {
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if a[i] != b[i] {
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return int32(i)
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}
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}
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// If we reached our limit, we matched everything we are
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// allowed to in the previous block and we return.
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n := int32(len(b))
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if int(s+n) == s1 {
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return n
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}
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// Continue looking for more matches in the current block.
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a = src[s+n : s1]
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b = src[:len(a)]
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for i := range a {
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if a[i] != b[i] {
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return int32(i) + n
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}
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}
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return int32(len(a)) + n
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}
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// Reset resets the encoding history.
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// This ensures that no matches are made to the previous block.
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func (e *deflateFast) reset() {
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e.prev = e.prev[:0]
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// Bump the offset, so all matches will fail distance check.
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// Nothing should be >= e.cur in the table.
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e.cur += maxMatchOffset
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// Protect against e.cur wraparound.
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if e.cur >= bufferReset {
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e.shiftOffsets()
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}
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}
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// shiftOffsets will shift down all match offset.
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// This is only called in rare situations to prevent integer overflow.
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//
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// See https://golang.org/issue/18636 and https://github.com/golang/go/issues/34121.
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func (e *deflateFast) shiftOffsets() {
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if len(e.prev) == 0 {
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// We have no history; just clear the table.
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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 + 1
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return
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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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for i := range e.table[:] {
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v := e.table[i].offset - e.cur + maxMatchOffset + 1
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if v < 0 {
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// We want to reset e.cur to maxMatchOffset + 1, so we need to shift
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// all table entries down by (e.cur - (maxMatchOffset + 1)).
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// Because we ignore matches > maxMatchOffset, we can cap
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// any negative offsets at 0.
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v = 0
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}
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e.table[i].offset = v
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}
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e.cur = maxMatchOffset + 1
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}
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