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Diffstat (limited to 'src/compress/flate/huffman_code.go')
| -rw-r--r-- | src/compress/flate/huffman_code.go | 323 |
1 files changed, 323 insertions, 0 deletions
diff --git a/src/compress/flate/huffman_code.go b/src/compress/flate/huffman_code.go new file mode 100644 index 000000000..3b9fce466 --- /dev/null +++ b/src/compress/flate/huffman_code.go @@ -0,0 +1,323 @@ +// Copyright 2009 The 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. + +package flate + +import ( + "math" + "sort" +) + +type huffmanEncoder struct { + codeBits []uint8 + code []uint16 +} + +type literalNode struct { + literal uint16 + freq int32 +} + +// A levelInfo describes the state of the constructed tree for a given depth. +type levelInfo struct { + // Our level. for better printing + level int32 + + // The frequency of the last node at this level + lastFreq int32 + + // The frequency of the next character to add to this level + nextCharFreq int32 + + // The frequency of the next pair (from level below) to add to this level. + // Only valid if the "needed" value of the next lower level is 0. + nextPairFreq int32 + + // The number of chains remaining to generate for this level before moving + // up to the next level + needed int32 +} + +func maxNode() literalNode { return literalNode{math.MaxUint16, math.MaxInt32} } + +func newHuffmanEncoder(size int) *huffmanEncoder { + return &huffmanEncoder{make([]uint8, size), make([]uint16, size)} +} + +// Generates a HuffmanCode corresponding to the fixed literal table +func generateFixedLiteralEncoding() *huffmanEncoder { + h := newHuffmanEncoder(maxLit) + codeBits := h.codeBits + code := h.code + var ch uint16 + for ch = 0; ch < maxLit; ch++ { + var bits uint16 + var size uint8 + switch { + case ch < 144: + // size 8, 000110000 .. 10111111 + bits = ch + 48 + size = 8 + break + case ch < 256: + // size 9, 110010000 .. 111111111 + bits = ch + 400 - 144 + size = 9 + break + case ch < 280: + // size 7, 0000000 .. 0010111 + bits = ch - 256 + size = 7 + break + default: + // size 8, 11000000 .. 11000111 + bits = ch + 192 - 280 + size = 8 + } + codeBits[ch] = size + code[ch] = reverseBits(bits, size) + } + return h +} + +func generateFixedOffsetEncoding() *huffmanEncoder { + h := newHuffmanEncoder(30) + codeBits := h.codeBits + code := h.code + for ch := uint16(0); ch < 30; ch++ { + codeBits[ch] = 5 + code[ch] = reverseBits(ch, 5) + } + return h +} + +var fixedLiteralEncoding *huffmanEncoder = generateFixedLiteralEncoding() +var fixedOffsetEncoding *huffmanEncoder = generateFixedOffsetEncoding() + +func (h *huffmanEncoder) bitLength(freq []int32) int64 { + var total int64 + for i, f := range freq { + if f != 0 { + total += int64(f) * int64(h.codeBits[i]) + } + } + return total +} + +const maxBitsLimit = 16 + +// Return the number of literals assigned to each bit size in the Huffman encoding +// +// This method is only called when list.length >= 3 +// The cases of 0, 1, and 2 literals are handled by special case code. +// +// list An array of the literals with non-zero frequencies +// and their associated frequencies. The array is in order of increasing +// frequency, and has as its last element a special element with frequency +// MaxInt32 +// maxBits The maximum number of bits that should be used to encode any literal. +// Must be less than 16. +// return An integer array in which array[i] indicates the number of literals +// that should be encoded in i bits. +func (h *huffmanEncoder) bitCounts(list []literalNode, maxBits int32) []int32 { + if maxBits >= maxBitsLimit { + panic("flate: maxBits too large") + } + n := int32(len(list)) + list = list[0 : n+1] + list[n] = maxNode() + + // The tree can't have greater depth than n - 1, no matter what. This + // saves a little bit of work in some small cases + if maxBits > n-1 { + maxBits = n - 1 + } + + // Create information about each of the levels. + // A bogus "Level 0" whose sole purpose is so that + // level1.prev.needed==0. This makes level1.nextPairFreq + // be a legitimate value that never gets chosen. + var levels [maxBitsLimit]levelInfo + // leafCounts[i] counts the number of literals at the left + // of ancestors of the rightmost node at level i. + // leafCounts[i][j] is the number of literals at the left + // of the level j ancestor. + var leafCounts [maxBitsLimit][maxBitsLimit]int32 + + for level := int32(1); level <= maxBits; level++ { + // For every level, the first two items are the first two characters. + // We initialize the levels as if we had already figured this out. + levels[level] = levelInfo{ + level: level, + lastFreq: list[1].freq, + nextCharFreq: list[2].freq, + nextPairFreq: list[0].freq + list[1].freq, + } + leafCounts[level][level] = 2 + if level == 1 { + levels[level].nextPairFreq = math.MaxInt32 + } + } + + // We need a total of 2*n - 2 items at top level and have already generated 2. + levels[maxBits].needed = 2*n - 4 + + level := maxBits + for { + l := &levels[level] + if l.nextPairFreq == math.MaxInt32 && l.nextCharFreq == math.MaxInt32 { + // We've run out of both leafs and pairs. + // End all calculations for this level. + // To make sure we never come back to this level or any lower level, + // set nextPairFreq impossibly large. + l.needed = 0 + levels[level+1].nextPairFreq = math.MaxInt32 + level++ + continue + } + + prevFreq := l.lastFreq + if l.nextCharFreq < l.nextPairFreq { + // The next item on this row is a leaf node. + n := leafCounts[level][level] + 1 + l.lastFreq = l.nextCharFreq + // Lower leafCounts are the same of the previous node. + leafCounts[level][level] = n + l.nextCharFreq = list[n].freq + } else { + // The next item on this row is a pair from the previous row. + // nextPairFreq isn't valid until we generate two + // more values in the level below + l.lastFreq = l.nextPairFreq + // Take leaf counts from the lower level, except counts[level] remains the same. + copy(leafCounts[level][:level], leafCounts[level-1][:level]) + levels[l.level-1].needed = 2 + } + + if l.needed--; l.needed == 0 { + // We've done everything we need to do for this level. + // Continue calculating one level up. Fill in nextPairFreq + // of that level with the sum of the two nodes we've just calculated on + // this level. + if l.level == maxBits { + // All done! + break + } + levels[l.level+1].nextPairFreq = prevFreq + l.lastFreq + level++ + } else { + // If we stole from below, move down temporarily to replenish it. + for levels[level-1].needed > 0 { + level-- + } + } + } + + // Somethings is wrong if at the end, the top level is null or hasn't used + // all of the leaves. + if leafCounts[maxBits][maxBits] != n { + panic("leafCounts[maxBits][maxBits] != n") + } + + bitCount := make([]int32, maxBits+1) + bits := 1 + counts := &leafCounts[maxBits] + for level := maxBits; level > 0; level-- { + // chain.leafCount gives the number of literals requiring at least "bits" + // bits to encode. + bitCount[bits] = counts[level] - counts[level-1] + bits++ + } + return bitCount +} + +// Look at the leaves and assign them a bit count and an encoding as specified +// in RFC 1951 3.2.2 +func (h *huffmanEncoder) assignEncodingAndSize(bitCount []int32, list []literalNode) { + code := uint16(0) + for n, bits := range bitCount { + code <<= 1 + if n == 0 || bits == 0 { + continue + } + // The literals list[len(list)-bits] .. list[len(list)-bits] + // are encoded using "bits" bits, and get the values + // code, code + 1, .... The code values are + // assigned in literal order (not frequency order). + chunk := list[len(list)-int(bits):] + sortByLiteral(chunk) + for _, node := range chunk { + h.codeBits[node.literal] = uint8(n) + h.code[node.literal] = reverseBits(code, uint8(n)) + code++ + } + list = list[0 : len(list)-int(bits)] + } +} + +// Update this Huffman Code object to be the minimum code for the specified frequency count. +// +// freq An array of frequencies, in which frequency[i] gives the frequency of literal i. +// maxBits The maximum number of bits to use for any literal. +func (h *huffmanEncoder) generate(freq []int32, maxBits int32) { + list := make([]literalNode, len(freq)+1) + // Number of non-zero literals + count := 0 + // Set list to be the set of all non-zero literals and their frequencies + for i, f := range freq { + if f != 0 { + list[count] = literalNode{uint16(i), f} + count++ + } else { + h.codeBits[i] = 0 + } + } + // If freq[] is shorter than codeBits[], fill rest of codeBits[] with zeros + h.codeBits = h.codeBits[0:len(freq)] + list = list[0:count] + if count <= 2 { + // Handle the small cases here, because they are awkward for the general case code. With + // two or fewer literals, everything has bit length 1. + for i, node := range list { + // "list" is in order of increasing literal value. + h.codeBits[node.literal] = 1 + h.code[node.literal] = uint16(i) + } + return + } + sortByFreq(list) + + // Get the number of literals for each bit count + bitCount := h.bitCounts(list, maxBits) + // And do the assignment + h.assignEncodingAndSize(bitCount, list) +} + +type literalNodeSorter struct { + a []literalNode + less func(i, j int) bool +} + +func (s literalNodeSorter) Len() int { return len(s.a) } + +func (s literalNodeSorter) Less(i, j int) bool { + return s.less(i, j) +} + +func (s literalNodeSorter) Swap(i, j int) { s.a[i], s.a[j] = s.a[j], s.a[i] } + +func sortByFreq(a []literalNode) { + s := &literalNodeSorter{a, func(i, j int) bool { + if a[i].freq == a[j].freq { + return a[i].literal < a[j].literal + } + return a[i].freq < a[j].freq + }} + sort.Sort(s) +} + +func sortByLiteral(a []literalNode) { + s := &literalNodeSorter{a, func(i, j int) bool { return a[i].literal < a[j].literal }} + sort.Sort(s) +} |