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596 lines
20 KiB
596 lines
20 KiB
package brotli
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import "encoding/binary"
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/* Copyright 2015 Google Inc. All Rights Reserved.
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Distributed under MIT license.
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See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
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*/
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/* Function for fast encoding of an input fragment, independently from the input
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history. This function uses two-pass processing: in the first pass we save
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the found backward matches and literal bytes into a buffer, and in the
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second pass we emit them into the bit stream using prefix codes built based
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on the actual command and literal byte histograms. */
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const kCompressFragmentTwoPassBlockSize uint = 1 << 17
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func hash1(p []byte, shift uint, length uint) uint32 {
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var h uint64 = (binary.LittleEndian.Uint64(p) << ((8 - length) * 8)) * uint64(kHashMul32)
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return uint32(h >> shift)
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}
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func hashBytesAtOffset(v uint64, offset uint, shift uint, length uint) uint32 {
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assert(offset <= 8-length)
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{
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var h uint64 = ((v >> (8 * offset)) << ((8 - length) * 8)) * uint64(kHashMul32)
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return uint32(h >> shift)
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}
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}
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func isMatch1(p1 []byte, p2 []byte, length uint) bool {
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if binary.LittleEndian.Uint32(p1) != binary.LittleEndian.Uint32(p2) {
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return false
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}
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if length == 4 {
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return true
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}
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return p1[4] == p2[4] && p1[5] == p2[5]
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}
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/* Builds a command and distance prefix code (each 64 symbols) into "depth" and
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"bits" based on "histogram" and stores it into the bit stream. */
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func buildAndStoreCommandPrefixCode(histogram []uint32, depth []byte, bits []uint16, bw *bitWriter) {
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var tree [129]huffmanTree
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var cmd_depth = [numCommandSymbols]byte{0}
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/* Tree size for building a tree over 64 symbols is 2 * 64 + 1. */
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var cmd_bits [64]uint16
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createHuffmanTree(histogram, 64, 15, tree[:], depth)
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createHuffmanTree(histogram[64:], 64, 14, tree[:], depth[64:])
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/* We have to jump through a few hoops here in order to compute
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the command bits because the symbols are in a different order than in
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the full alphabet. This looks complicated, but having the symbols
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in this order in the command bits saves a few branches in the Emit*
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functions. */
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copy(cmd_depth[:], depth[24:][:24])
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copy(cmd_depth[24:][:], depth[:8])
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copy(cmd_depth[32:][:], depth[48:][:8])
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copy(cmd_depth[40:][:], depth[8:][:8])
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copy(cmd_depth[48:][:], depth[56:][:8])
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copy(cmd_depth[56:][:], depth[16:][:8])
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convertBitDepthsToSymbols(cmd_depth[:], 64, cmd_bits[:])
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copy(bits, cmd_bits[24:][:8])
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copy(bits[8:], cmd_bits[40:][:8])
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copy(bits[16:], cmd_bits[56:][:8])
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copy(bits[24:], cmd_bits[:24])
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copy(bits[48:], cmd_bits[32:][:8])
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copy(bits[56:], cmd_bits[48:][:8])
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convertBitDepthsToSymbols(depth[64:], 64, bits[64:])
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{
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/* Create the bit length array for the full command alphabet. */
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var i uint
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for i := 0; i < int(64); i++ {
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cmd_depth[i] = 0
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} /* only 64 first values were used */
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copy(cmd_depth[:], depth[24:][:8])
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copy(cmd_depth[64:][:], depth[32:][:8])
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copy(cmd_depth[128:][:], depth[40:][:8])
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copy(cmd_depth[192:][:], depth[48:][:8])
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copy(cmd_depth[384:][:], depth[56:][:8])
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for i = 0; i < 8; i++ {
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cmd_depth[128+8*i] = depth[i]
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cmd_depth[256+8*i] = depth[8+i]
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cmd_depth[448+8*i] = depth[16+i]
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}
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storeHuffmanTree(cmd_depth[:], numCommandSymbols, tree[:], bw)
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}
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storeHuffmanTree(depth[64:], 64, tree[:], bw)
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}
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func emitInsertLen(insertlen uint32, commands *[]uint32) {
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if insertlen < 6 {
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(*commands)[0] = insertlen
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} else if insertlen < 130 {
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var tail uint32 = insertlen - 2
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var nbits uint32 = log2FloorNonZero(uint(tail)) - 1
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var prefix uint32 = tail >> nbits
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var inscode uint32 = (nbits << 1) + prefix + 2
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var extra uint32 = tail - (prefix << nbits)
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(*commands)[0] = inscode | extra<<8
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} else if insertlen < 2114 {
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var tail uint32 = insertlen - 66
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var nbits uint32 = log2FloorNonZero(uint(tail))
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var code uint32 = nbits + 10
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var extra uint32 = tail - (1 << nbits)
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(*commands)[0] = code | extra<<8
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} else if insertlen < 6210 {
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var extra uint32 = insertlen - 2114
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(*commands)[0] = 21 | extra<<8
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} else if insertlen < 22594 {
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var extra uint32 = insertlen - 6210
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(*commands)[0] = 22 | extra<<8
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} else {
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var extra uint32 = insertlen - 22594
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(*commands)[0] = 23 | extra<<8
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}
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*commands = (*commands)[1:]
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}
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func emitCopyLen(copylen uint, commands *[]uint32) {
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if copylen < 10 {
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(*commands)[0] = uint32(copylen + 38)
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} else if copylen < 134 {
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var tail uint = copylen - 6
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var nbits uint = uint(log2FloorNonZero(tail) - 1)
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var prefix uint = tail >> nbits
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var code uint = (nbits << 1) + prefix + 44
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var extra uint = tail - (prefix << nbits)
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(*commands)[0] = uint32(code | extra<<8)
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} else if copylen < 2118 {
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var tail uint = copylen - 70
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var nbits uint = uint(log2FloorNonZero(tail))
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var code uint = nbits + 52
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var extra uint = tail - (uint(1) << nbits)
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(*commands)[0] = uint32(code | extra<<8)
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} else {
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var extra uint = copylen - 2118
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(*commands)[0] = uint32(63 | extra<<8)
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}
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*commands = (*commands)[1:]
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}
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func emitCopyLenLastDistance(copylen uint, commands *[]uint32) {
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if copylen < 12 {
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(*commands)[0] = uint32(copylen + 20)
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*commands = (*commands)[1:]
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} else if copylen < 72 {
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var tail uint = copylen - 8
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var nbits uint = uint(log2FloorNonZero(tail) - 1)
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var prefix uint = tail >> nbits
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var code uint = (nbits << 1) + prefix + 28
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var extra uint = tail - (prefix << nbits)
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(*commands)[0] = uint32(code | extra<<8)
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*commands = (*commands)[1:]
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} else if copylen < 136 {
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var tail uint = copylen - 8
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var code uint = (tail >> 5) + 54
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var extra uint = tail & 31
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(*commands)[0] = uint32(code | extra<<8)
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*commands = (*commands)[1:]
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(*commands)[0] = 64
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*commands = (*commands)[1:]
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} else if copylen < 2120 {
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var tail uint = copylen - 72
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var nbits uint = uint(log2FloorNonZero(tail))
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var code uint = nbits + 52
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var extra uint = tail - (uint(1) << nbits)
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(*commands)[0] = uint32(code | extra<<8)
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*commands = (*commands)[1:]
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(*commands)[0] = 64
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*commands = (*commands)[1:]
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} else {
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var extra uint = copylen - 2120
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(*commands)[0] = uint32(63 | extra<<8)
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*commands = (*commands)[1:]
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(*commands)[0] = 64
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*commands = (*commands)[1:]
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}
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}
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func emitDistance(distance uint32, commands *[]uint32) {
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var d uint32 = distance + 3
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var nbits uint32 = log2FloorNonZero(uint(d)) - 1
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var prefix uint32 = (d >> nbits) & 1
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var offset uint32 = (2 + prefix) << nbits
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var distcode uint32 = 2*(nbits-1) + prefix + 80
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var extra uint32 = d - offset
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(*commands)[0] = distcode | extra<<8
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*commands = (*commands)[1:]
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}
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/* REQUIRES: len <= 1 << 24. */
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func storeMetaBlockHeader(len uint, is_uncompressed bool, bw *bitWriter) {
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var nibbles uint = 6
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/* ISLAST */
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bw.writeBits(1, 0)
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if len <= 1<<16 {
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nibbles = 4
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} else if len <= 1<<20 {
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nibbles = 5
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}
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bw.writeBits(2, uint64(nibbles)-4)
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bw.writeBits(nibbles*4, uint64(len)-1)
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/* ISUNCOMPRESSED */
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bw.writeSingleBit(is_uncompressed)
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}
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func createCommands(input []byte, block_size uint, input_size uint, base_ip_ptr []byte, table []int, table_bits uint, min_match uint, literals *[]byte, commands *[]uint32) {
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var ip int = 0
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var shift uint = 64 - table_bits
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var ip_end int = int(block_size)
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var base_ip int = -cap(base_ip_ptr) + cap(input)
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var next_emit int = 0
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var last_distance int = -1
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/* "ip" is the input pointer. */
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const kInputMarginBytes uint = windowGap
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/* "next_emit" is a pointer to the first byte that is not covered by a
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previous copy. Bytes between "next_emit" and the start of the next copy or
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the end of the input will be emitted as literal bytes. */
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if block_size >= kInputMarginBytes {
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var len_limit uint = brotli_min_size_t(block_size-min_match, input_size-kInputMarginBytes)
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var ip_limit int = int(len_limit)
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/* For the last block, we need to keep a 16 bytes margin so that we can be
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sure that all distances are at most window size - 16.
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For all other blocks, we only need to keep a margin of 5 bytes so that
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we don't go over the block size with a copy. */
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var next_hash uint32
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ip++
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for next_hash = hash1(input[ip:], shift, min_match); ; {
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var skip uint32 = 32
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var next_ip int = ip
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/* Step 1: Scan forward in the input looking for a 6-byte-long match.
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If we get close to exhausting the input then goto emit_remainder.
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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, look at every third byte, etc.. When a match is found,
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immediately go back to looking at every byte. This is a small loss
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(~5% performance, ~0.1% density) for compressible data due to more
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bookkeeping, but for non-compressible data (such as JPEG) it's a huge
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win since the compressor quickly "realizes" the data is incompressible
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and doesn't bother looking for matches everywhere.
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The "skip" variable keeps track of how many bytes there are since the
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last match; dividing it by 32 (ie. right-shifting by five) gives the
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number of bytes to move ahead for each iteration. */
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var candidate int
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assert(next_emit < ip)
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trawl:
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for {
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var hash uint32 = next_hash
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var bytes_between_hash_lookups uint32 = skip >> 5
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skip++
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ip = next_ip
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assert(hash == hash1(input[ip:], shift, min_match))
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next_ip = int(uint32(ip) + bytes_between_hash_lookups)
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if next_ip > ip_limit {
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goto emit_remainder
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}
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next_hash = hash1(input[next_ip:], shift, min_match)
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candidate = ip - last_distance
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if isMatch1(input[ip:], base_ip_ptr[candidate-base_ip:], min_match) {
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if candidate < ip {
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table[hash] = int(ip - base_ip)
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break
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}
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}
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candidate = base_ip + table[hash]
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assert(candidate >= base_ip)
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assert(candidate < ip)
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table[hash] = int(ip - base_ip)
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if isMatch1(input[ip:], base_ip_ptr[candidate-base_ip:], min_match) {
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break
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}
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}
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/* Check copy distance. If candidate is not feasible, continue search.
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Checking is done outside of hot loop to reduce overhead. */
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if ip-candidate > maxDistance_compress_fragment {
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goto trawl
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}
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/* Step 2: Emit the found match together with the literal bytes from
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"next_emit", and then see if we can find a next match immediately
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afterwards. Repeat until we find no match for the input
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without emitting some literal bytes. */
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{
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var base int = ip
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/* > 0 */
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var matched uint = min_match + findMatchLengthWithLimit(base_ip_ptr[uint(candidate-base_ip)+min_match:], input[uint(ip)+min_match:], uint(ip_end-ip)-min_match)
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var distance int = int(base - candidate)
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/* We have a 6-byte match at ip, and we need to emit bytes in
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[next_emit, ip). */
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var insert int = int(base - next_emit)
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ip += int(matched)
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emitInsertLen(uint32(insert), commands)
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copy(*literals, input[next_emit:][:uint(insert)])
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*literals = (*literals)[insert:]
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if distance == last_distance {
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(*commands)[0] = 64
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*commands = (*commands)[1:]
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} else {
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emitDistance(uint32(distance), commands)
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last_distance = distance
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}
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emitCopyLenLastDistance(matched, commands)
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next_emit = ip
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if ip >= ip_limit {
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goto emit_remainder
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}
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{
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var input_bytes uint64
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var cur_hash uint32
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/* We could immediately start working at ip now, but to improve
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compression we first update "table" with the hashes of some
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positions within the last copy. */
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var prev_hash uint32
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if min_match == 4 {
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input_bytes = binary.LittleEndian.Uint64(input[ip-3:])
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cur_hash = hashBytesAtOffset(input_bytes, 3, shift, min_match)
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 3)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 2)
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 1)
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} else {
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input_bytes = binary.LittleEndian.Uint64(input[ip-5:])
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 5)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 4)
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prev_hash = hashBytesAtOffset(input_bytes, 2, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 3)
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input_bytes = binary.LittleEndian.Uint64(input[ip-2:])
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cur_hash = hashBytesAtOffset(input_bytes, 2, shift, min_match)
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 2)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 1)
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}
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candidate = base_ip + table[cur_hash]
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table[cur_hash] = int(ip - base_ip)
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}
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}
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for ip-candidate <= maxDistance_compress_fragment && isMatch1(input[ip:], base_ip_ptr[candidate-base_ip:], min_match) {
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var base int = ip
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/* We have a 6-byte match at ip, and no need to emit any
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literal bytes prior to ip. */
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var matched uint = min_match + findMatchLengthWithLimit(base_ip_ptr[uint(candidate-base_ip)+min_match:], input[uint(ip)+min_match:], uint(ip_end-ip)-min_match)
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ip += int(matched)
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last_distance = int(base - candidate) /* > 0 */
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emitCopyLen(matched, commands)
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emitDistance(uint32(last_distance), commands)
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next_emit = ip
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if ip >= ip_limit {
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goto emit_remainder
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}
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{
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var input_bytes uint64
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var cur_hash uint32
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/* We could immediately start working at ip now, but to improve
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compression we first update "table" with the hashes of some
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positions within the last copy. */
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var prev_hash uint32
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if min_match == 4 {
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input_bytes = binary.LittleEndian.Uint64(input[ip-3:])
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cur_hash = hashBytesAtOffset(input_bytes, 3, shift, min_match)
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 3)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 2)
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prev_hash = hashBytesAtOffset(input_bytes, 2, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 1)
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} else {
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input_bytes = binary.LittleEndian.Uint64(input[ip-5:])
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 5)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 4)
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prev_hash = hashBytesAtOffset(input_bytes, 2, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 3)
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input_bytes = binary.LittleEndian.Uint64(input[ip-2:])
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cur_hash = hashBytesAtOffset(input_bytes, 2, shift, min_match)
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prev_hash = hashBytesAtOffset(input_bytes, 0, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 2)
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prev_hash = hashBytesAtOffset(input_bytes, 1, shift, min_match)
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table[prev_hash] = int(ip - base_ip - 1)
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}
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candidate = base_ip + table[cur_hash]
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table[cur_hash] = int(ip - base_ip)
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}
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}
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ip++
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next_hash = hash1(input[ip:], shift, min_match)
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}
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}
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emit_remainder:
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assert(next_emit <= ip_end)
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/* Emit the remaining bytes as literals. */
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if next_emit < ip_end {
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var insert uint32 = uint32(ip_end - next_emit)
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emitInsertLen(insert, commands)
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copy(*literals, input[next_emit:][:insert])
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*literals = (*literals)[insert:]
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}
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}
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var storeCommands_kNumExtraBits = [128]uint32{
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0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9, 10, 12, 14, 24,
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0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4,
|
|
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9, 10, 24,
|
|
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
|
|
1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
|
|
9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16,
|
|
17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, 23, 23, 24, 24,
|
|
}
|
|
var storeCommands_kInsertOffset = [24]uint32{
|
|
0, 1, 2, 3, 4, 5, 6, 8, 10, 14, 18, 26, 34, 50, 66, 98, 130, 194, 322, 578,
|
|
1090, 2114, 6210, 22594,
|
|
}
|
|
|
|
func storeCommands(literals []byte, num_literals uint, commands []uint32, num_commands uint, bw *bitWriter) {
|
|
var lit_depths [256]byte
|
|
var lit_bits [256]uint16
|
|
var lit_histo = [256]uint32{0}
|
|
var cmd_depths = [128]byte{0}
|
|
var cmd_bits = [128]uint16{0}
|
|
var cmd_histo = [128]uint32{0}
|
|
var i uint
|
|
for i = 0; i < num_literals; i++ {
|
|
lit_histo[literals[i]]++
|
|
}
|
|
|
|
buildAndStoreHuffmanTreeFast(lit_histo[:], num_literals, /* max_bits = */
|
|
8, lit_depths[:], lit_bits[:], bw)
|
|
|
|
for i = 0; i < num_commands; i++ {
|
|
var code uint32 = commands[i] & 0xFF
|
|
assert(code < 128)
|
|
cmd_histo[code]++
|
|
}
|
|
|
|
cmd_histo[1] += 1
|
|
cmd_histo[2] += 1
|
|
cmd_histo[64] += 1
|
|
cmd_histo[84] += 1
|
|
buildAndStoreCommandPrefixCode(cmd_histo[:], cmd_depths[:], cmd_bits[:], bw)
|
|
|
|
for i = 0; i < num_commands; i++ {
|
|
var cmd uint32 = commands[i]
|
|
var code uint32 = cmd & 0xFF
|
|
var extra uint32 = cmd >> 8
|
|
assert(code < 128)
|
|
bw.writeBits(uint(cmd_depths[code]), uint64(cmd_bits[code]))
|
|
bw.writeBits(uint(storeCommands_kNumExtraBits[code]), uint64(extra))
|
|
if code < 24 {
|
|
var insert uint32 = storeCommands_kInsertOffset[code] + extra
|
|
var j uint32
|
|
for j = 0; j < insert; j++ {
|
|
var lit byte = literals[0]
|
|
bw.writeBits(uint(lit_depths[lit]), uint64(lit_bits[lit]))
|
|
literals = literals[1:]
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
/* Acceptable loss for uncompressible speedup is 2% */
|
|
const minRatio = 0.98
|
|
|
|
const sampleRate = 43
|
|
|
|
func shouldCompress(input []byte, input_size uint, num_literals uint) bool {
|
|
var corpus_size float64 = float64(input_size)
|
|
if float64(num_literals) < minRatio*corpus_size {
|
|
return true
|
|
} else {
|
|
var literal_histo = [256]uint32{0}
|
|
var max_total_bit_cost float64 = corpus_size * 8 * minRatio / sampleRate
|
|
var i uint
|
|
for i = 0; i < input_size; i += sampleRate {
|
|
literal_histo[input[i]]++
|
|
}
|
|
|
|
return bitsEntropy(literal_histo[:], 256) < max_total_bit_cost
|
|
}
|
|
}
|
|
|
|
func emitUncompressedMetaBlock(input []byte, input_size uint, bw *bitWriter) {
|
|
storeMetaBlockHeader(input_size, true, bw)
|
|
bw.jumpToByteBoundary()
|
|
bw.writeBytes(input[:input_size])
|
|
}
|
|
|
|
func compressFragmentTwoPassImpl(input []byte, input_size uint, is_last bool, command_buf []uint32, literal_buf []byte, table []int, table_bits uint, min_match uint, bw *bitWriter) {
|
|
/* Save the start of the first block for position and distance computations.
|
|
*/
|
|
var base_ip []byte = input
|
|
|
|
for input_size > 0 {
|
|
var block_size uint = brotli_min_size_t(input_size, kCompressFragmentTwoPassBlockSize)
|
|
var commands []uint32 = command_buf
|
|
var literals []byte = literal_buf
|
|
var num_literals uint
|
|
createCommands(input, block_size, input_size, base_ip, table, table_bits, min_match, &literals, &commands)
|
|
num_literals = uint(-cap(literals) + cap(literal_buf))
|
|
if shouldCompress(input, block_size, num_literals) {
|
|
var num_commands uint = uint(-cap(commands) + cap(command_buf))
|
|
storeMetaBlockHeader(block_size, false, bw)
|
|
|
|
/* No block splits, no contexts. */
|
|
bw.writeBits(13, 0)
|
|
|
|
storeCommands(literal_buf, num_literals, command_buf, num_commands, bw)
|
|
} else {
|
|
/* Since we did not find many backward references and the entropy of
|
|
the data is close to 8 bits, we can simply emit an uncompressed block.
|
|
This makes compression speed of uncompressible data about 3x faster. */
|
|
emitUncompressedMetaBlock(input, block_size, bw)
|
|
}
|
|
|
|
input = input[block_size:]
|
|
input_size -= block_size
|
|
}
|
|
}
|
|
|
|
/* Compresses "input" string to bw as one or more complete meta-blocks.
|
|
|
|
If "is_last" is 1, emits an additional empty last meta-block.
|
|
|
|
REQUIRES: "input_size" is greater than zero, or "is_last" is 1.
|
|
REQUIRES: "input_size" is less or equal to maximal metablock size (1 << 24).
|
|
REQUIRES: "command_buf" and "literal_buf" point to at least
|
|
kCompressFragmentTwoPassBlockSize long arrays.
|
|
REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
|
|
REQUIRES: "table_size" is a power of two
|
|
OUTPUT: maximal copy distance <= |input_size|
|
|
OUTPUT: maximal copy distance <= BROTLI_MAX_BACKWARD_LIMIT(18) */
|
|
func compressFragmentTwoPass(input []byte, input_size uint, is_last bool, command_buf []uint32, literal_buf []byte, table []int, table_size uint, bw *bitWriter) {
|
|
var initial_storage_ix uint = bw.getPos()
|
|
var table_bits uint = uint(log2FloorNonZero(table_size))
|
|
var min_match uint
|
|
if table_bits <= 15 {
|
|
min_match = 4
|
|
} else {
|
|
min_match = 6
|
|
}
|
|
compressFragmentTwoPassImpl(input, input_size, is_last, command_buf, literal_buf, table, table_bits, min_match, bw)
|
|
|
|
/* If output is larger than single uncompressed block, rewrite it. */
|
|
if bw.getPos()-initial_storage_ix > 31+(input_size<<3) {
|
|
bw.rewind(initial_storage_ix)
|
|
emitUncompressedMetaBlock(input, input_size, bw)
|
|
}
|
|
|
|
if is_last {
|
|
bw.writeBits(1, 1) /* islast */
|
|
bw.writeBits(1, 1) /* isempty */
|
|
bw.jumpToByteBoundary()
|
|
}
|
|
}
|