239 lines
8.0 KiB
Go
239 lines
8.0 KiB
Go
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// Copyright 2016 The Snappy-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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// +build !amd64,!arm64 appengine !gc noasm
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package snappy
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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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// emitLiteral writes a literal chunk and returns the number of bytes written.
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//
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// It assumes that:
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// dst is long enough to hold the encoded bytes
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// 1 <= len(lit) && len(lit) <= 65536
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func emitLiteral(dst, lit []byte) int {
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i, n := 0, uint(len(lit)-1)
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switch {
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case n < 60:
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dst[0] = uint8(n)<<2 | tagLiteral
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i = 1
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case n < 1<<8:
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dst[0] = 60<<2 | tagLiteral
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dst[1] = uint8(n)
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i = 2
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default:
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dst[0] = 61<<2 | tagLiteral
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dst[1] = uint8(n)
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dst[2] = uint8(n >> 8)
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i = 3
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}
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return i + copy(dst[i:], 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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//
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// It assumes that:
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// dst is long enough to hold the encoded bytes
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// 1 <= offset && offset <= 65535
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// 4 <= length && length <= 65535
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func emitCopy(dst []byte, offset, length int) int {
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i := 0
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// The maximum length for a single tagCopy1 or tagCopy2 op is 64 bytes. The
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// threshold for this loop is a little higher (at 68 = 64 + 4), and the
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// length emitted down below is is a little lower (at 60 = 64 - 4), because
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// it's shorter to encode a length 67 copy as a length 60 tagCopy2 followed
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// by a length 7 tagCopy1 (which encodes as 3+2 bytes) than to encode it as
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// a length 64 tagCopy2 followed by a length 3 tagCopy2 (which encodes as
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// 3+3 bytes). The magic 4 in the 64±4 is because the minimum length for a
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// tagCopy1 op is 4 bytes, which is why a length 3 copy has to be an
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// encodes-as-3-bytes tagCopy2 instead of an encodes-as-2-bytes tagCopy1.
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for length >= 68 {
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// Emit a length 64 copy, encoded as 3 bytes.
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dst[i+0] = 63<<2 | tagCopy2
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dst[i+1] = uint8(offset)
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dst[i+2] = uint8(offset >> 8)
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i += 3
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length -= 64
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}
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if length > 64 {
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// Emit a length 60 copy, encoded as 3 bytes.
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dst[i+0] = 59<<2 | tagCopy2
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dst[i+1] = uint8(offset)
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dst[i+2] = uint8(offset >> 8)
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i += 3
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length -= 60
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}
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if length >= 12 || offset >= 2048 {
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// Emit the remaining copy, encoded as 3 bytes.
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dst[i+0] = uint8(length-1)<<2 | tagCopy2
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dst[i+1] = uint8(offset)
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dst[i+2] = uint8(offset >> 8)
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return i + 3
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}
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// Emit the remaining copy, encoded as 2 bytes.
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dst[i+0] = uint8(offset>>8)<<5 | uint8(length-4)<<2 | tagCopy1
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dst[i+1] = uint8(offset)
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return i + 2
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}
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// extendMatch returns the largest k such that k <= len(src) and that
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// src[i:i+k-j] and src[j:k] have the same contents.
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//
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// It assumes that:
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// 0 <= i && i < j && j <= len(src)
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func extendMatch(src []byte, i, j int) int {
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for ; j < len(src) && src[i] == src[j]; i, j = i+1, j+1 {
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}
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return j
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}
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func hash(u, shift uint32) uint32 {
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return (u * 0x1e35a7bd) >> shift
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}
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// encodeBlock encodes a non-empty src to a guaranteed-large-enough dst. It
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// assumes that the varint-encoded length of the decompressed bytes has already
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// been written.
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//
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// It also assumes that:
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// len(dst) >= MaxEncodedLen(len(src)) &&
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// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
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func encodeBlock(dst, src []byte) (d int) {
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// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
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// The table element type is uint16, as s < sLimit and sLimit < len(src)
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// and len(src) <= maxBlockSize and maxBlockSize == 65536.
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const (
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maxTableSize = 1 << 14
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// tableMask is redundant, but helps the compiler eliminate bounds
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// checks.
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tableMask = maxTableSize - 1
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)
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shift := uint32(32 - 8)
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for tableSize := 1 << 8; tableSize < maxTableSize && tableSize < len(src); tableSize *= 2 {
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shift--
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}
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// In Go, all array elements are zero-initialized, so there is no advantage
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// to a smaller tableSize per se. However, it matches the C++ algorithm,
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// and in the asm versions of this code, we can get away with zeroing only
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// the first tableSize elements.
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var table [maxTableSize]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), shift)
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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), shift)
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if 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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d += emitLiteral(dst[d:], 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:
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// s = extendMatch(src, candidate+4, s+4)
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s += 4
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for i := candidate + 4; s < len(src) && src[i] == src[s]; i, s = i+1, s+1 {
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}
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d += emitCopy(dst[d:], base-candidate, s-base)
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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), shift)
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table[prevHash&tableMask] = uint16(s - 1)
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currHash := hash(uint32(x>>8), shift)
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candidate = int(table[currHash&tableMask])
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table[currHash&tableMask] = uint16(s)
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if uint32(x>>8) != load32(src, candidate) {
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nextHash = hash(uint32(x>>16), shift)
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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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d += emitLiteral(dst[d:], src[nextEmit:])
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}
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return d
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}
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