--- /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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