+++ /dev/null
-// Copyright 2016 The Snappy-Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build !amd64 appengine !gc noasm
-
-package snappy
-
-func load32(b []byte, i int) uint32 {
- b = b[i : i+4 : len(b)] // Help the compiler eliminate bounds checks on the next line.
- return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
-}
-
-func load64(b []byte, i int) uint64 {
- b = b[i : i+8 : len(b)] // Help the compiler eliminate bounds checks on the next line.
- return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
- uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
-}
-
-// emitLiteral writes a literal chunk and returns the number of bytes written.
-//
-// It assumes that:
-// dst is long enough to hold the encoded bytes
-// 1 <= len(lit) && len(lit) <= 65536
-func emitLiteral(dst, lit []byte) int {
- i, n := 0, uint(len(lit)-1)
- switch {
- case n < 60:
- dst[0] = uint8(n)<<2 | tagLiteral
- i = 1
- case n < 1<<8:
- dst[0] = 60<<2 | tagLiteral
- dst[1] = uint8(n)
- i = 2
- default:
- dst[0] = 61<<2 | tagLiteral
- dst[1] = uint8(n)
- dst[2] = uint8(n >> 8)
- i = 3
- }
- return i + copy(dst[i:], lit)
-}
-
-// emitCopy writes a copy chunk and returns the number of bytes written.
-//
-// It assumes that:
-// dst is long enough to hold the encoded bytes
-// 1 <= offset && offset <= 65535
-// 4 <= length && length <= 65535
-func emitCopy(dst []byte, offset, length int) int {
- i := 0
- // The maximum length for a single tagCopy1 or tagCopy2 op is 64 bytes. The
- // threshold for this loop is a little higher (at 68 = 64 + 4), and the
- // length emitted down below is is a little lower (at 60 = 64 - 4), because
- // it's shorter to encode a length 67 copy as a length 60 tagCopy2 followed
- // by a length 7 tagCopy1 (which encodes as 3+2 bytes) than to encode it as
- // a length 64 tagCopy2 followed by a length 3 tagCopy2 (which encodes as
- // 3+3 bytes). The magic 4 in the 64±4 is because the minimum length for a
- // tagCopy1 op is 4 bytes, which is why a length 3 copy has to be an
- // encodes-as-3-bytes tagCopy2 instead of an encodes-as-2-bytes tagCopy1.
- for length >= 68 {
- // Emit a length 64 copy, encoded as 3 bytes.
- dst[i+0] = 63<<2 | tagCopy2
- dst[i+1] = uint8(offset)
- dst[i+2] = uint8(offset >> 8)
- i += 3
- length -= 64
- }
- if length > 64 {
- // Emit a length 60 copy, encoded as 3 bytes.
- dst[i+0] = 59<<2 | tagCopy2
- dst[i+1] = uint8(offset)
- dst[i+2] = uint8(offset >> 8)
- i += 3
- length -= 60
- }
- if length >= 12 || offset >= 2048 {
- // Emit the remaining copy, encoded as 3 bytes.
- dst[i+0] = uint8(length-1)<<2 | tagCopy2
- dst[i+1] = uint8(offset)
- dst[i+2] = uint8(offset >> 8)
- return i + 3
- }
- // Emit the remaining copy, encoded as 2 bytes.
- dst[i+0] = uint8(offset>>8)<<5 | uint8(length-4)<<2 | tagCopy1
- dst[i+1] = uint8(offset)
- return i + 2
-}
-
-// extendMatch returns the largest k such that k <= len(src) and that
-// src[i:i+k-j] and src[j:k] have the same contents.
-//
-// It assumes that:
-// 0 <= i && i < j && j <= len(src)
-func extendMatch(src []byte, i, j int) int {
- for ; j < len(src) && src[i] == src[j]; i, j = i+1, j+1 {
- }
- return j
-}
-
-func hash(u, shift uint32) uint32 {
- return (u * 0x1e35a7bd) >> shift
-}
-
-// encodeBlock encodes a non-empty src to a guaranteed-large-enough dst. It
-// assumes that the varint-encoded length of the decompressed bytes has already
-// been written.
-//
-// It also assumes that:
-// len(dst) >= MaxEncodedLen(len(src)) &&
-// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
-func encodeBlock(dst, src []byte) (d int) {
- // Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
- // The table element type is uint16, as s < sLimit and sLimit < len(src)
- // and len(src) <= maxBlockSize and maxBlockSize == 65536.
- const (
- maxTableSize = 1 << 14
- // tableMask is redundant, but helps the compiler eliminate bounds
- // checks.
- tableMask = maxTableSize - 1
- )
- shift := uint32(32 - 8)
- for tableSize := 1 << 8; tableSize < maxTableSize && tableSize < len(src); tableSize *= 2 {
- shift--
- }
- // In Go, all array elements are zero-initialized, so there is no advantage
- // to a smaller tableSize per se. However, it matches the C++ algorithm,
- // and in the asm versions of this code, we can get away with zeroing only
- // the first tableSize elements.
- var table [maxTableSize]uint16
-
- // 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 := len(src) - inputMargin
-
- // nextEmit is where in src the next emitLiteral should start from.
- nextEmit := 0
-
- // The encoded form must start with a literal, as there are no previous
- // bytes to copy, so we start looking for hash matches at s == 1.
- s := 1
- nextHash := hash(load32(src, s), shift)
-
- 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 := 32
-
- nextS := s
- candidate := 0
- for {
- s = nextS
- bytesBetweenHashLookups := skip >> 5
- nextS = s + bytesBetweenHashLookups
- skip += bytesBetweenHashLookups
- if nextS > sLimit {
- goto emitRemainder
- }
- candidate = int(table[nextHash&tableMask])
- table[nextHash&tableMask] = uint16(s)
- nextHash = hash(load32(src, nextS), shift)
- if load32(src, s) == load32(src, candidate) {
- 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.
- d += emitLiteral(dst[d:], 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.
- base := s
-
- // Extend the 4-byte match as long as possible.
- //
- // This is an inlined version of:
- // s = extendMatch(src, candidate+4, s+4)
- s += 4
- for i := candidate + 4; s < len(src) && src[i] == src[s]; i, s = i+1, s+1 {
- }
-
- d += emitCopy(dst[d:], base-candidate, s-base)
- 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 := load64(src, s-1)
- prevHash := hash(uint32(x>>0), shift)
- table[prevHash&tableMask] = uint16(s - 1)
- currHash := hash(uint32(x>>8), shift)
- candidate = int(table[currHash&tableMask])
- table[currHash&tableMask] = uint16(s)
- if uint32(x>>8) != load32(src, candidate) {
- nextHash = hash(uint32(x>>16), shift)
- s++
- break
- }
- }
- }
-
-emitRemainder:
- if nextEmit < len(src) {
- d += emitLiteral(dst[d:], src[nextEmit:])
- }
- return d
-}
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