|
|
// 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}
|
