src/compress/flate/huffman_code.go - go - Git at Google

 // 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"
 	"math/bits"
 	"sort"
 )

 // hcode is a huffman code with a bit code and bit length.
 type hcode struct {
 	code, len uint16
 }

 type huffmanEncoder struct {
 	codes     []hcode
 	freqcache []literalNode
 	bitCount  [17]int32
 	lns       byLiteral // stored to avoid repeated allocation in generate
 	lfs       byFreq    // stored to avoid repeated allocation in generate
 }

 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
 }

 // set sets the code and length of an hcode.
 func (h *hcode) set(code uint16, length uint16) {
 	h.len = length
 	h.code = code
 }

 func maxNode() literalNode { return literalNode{math.MaxUint16, math.MaxInt32} }

 func newHuffmanEncoder(size int) *huffmanEncoder {
 	return &huffmanEncoder{codes: make([]hcode, size)}
 }

 // Generates a HuffmanCode corresponding to the fixed literal table
 func generateFixedLiteralEncoding() *huffmanEncoder {
 	h := newHuffmanEncoder(maxNumLit)
 	codes := h.codes
 	var ch uint16
 	for ch = 0; ch < maxNumLit; ch++ {
 		var bits uint16
 		var size uint16
 		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
 		}
 		codes[ch] = hcode{code: reverseBits(bits, byte(size)), len: size}
 	}
 	return h
 }

 func generateFixedOffsetEncoding() *huffmanEncoder {
 	h := newHuffmanEncoder(30)
 	codes := h.codes
 	for ch := range codes {
 		codes[ch] = hcode{code: reverseBits(uint16(ch), 5), len: 5}
 	}
 	return h
 }

 var fixedLiteralEncoding *huffmanEncoder = generateFixedLiteralEncoding()
 var fixedOffsetEncoding *huffmanEncoder = generateFixedOffsetEncoding()

 func (h *huffmanEncoder) bitLength(freq []int32) int {
 	var total int
 	for i, f := range freq {
 		if f != 0 {
 			total += int(f) * int(h.codes[i].len)
 		}
 	}
 	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 := h.bitCount[: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):]

 		h.lns.sort(chunk)
 		for _, node := range chunk {
 			h.codes[node.literal] = hcode{code: reverseBits(code, uint8(n)), len: uint16(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) {
 	if h.freqcache == nil {
 		// Allocate a reusable buffer with the longest possible frequency table.
 		// Possible lengths are codegenCodeCount, offsetCodeCount and maxNumLit.
 		// The largest of these is maxNumLit, so we allocate for that case.
 		h.freqcache = make([]literalNode, maxNumLit+1)
 	}
 	list := h.freqcache[: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 {
 			list[count] = literalNode{}
 			h.codes[i].len = 0
 		}
 	}
 	list[len(freq)] = literalNode{}

 	list = list[: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.codes[node.literal].set(uint16(i), 1)
 		}
 		return
 	}
 	h.lfs.sort(list)

 	// Get the number of literals for each bit count
 	bitCount := h.bitCounts(list, maxBits)
 	// And do the assignment
 	h.assignEncodingAndSize(bitCount, list)
 }

 type byLiteral []literalNode

 func (s *byLiteral) sort(a []literalNode) {
 	*s = byLiteral(a)
 	sort.Sort(s)
 }

 func (s byLiteral) Len() int { return len(s) }

 func (s byLiteral) Less(i, j int) bool {
 	return s[i].literal < s[j].literal
 }

 func (s byLiteral) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

 type byFreq []literalNode

 func (s *byFreq) sort(a []literalNode) {
 	*s = byFreq(a)
 	sort.Sort(s)
 }

 func (s byFreq) Len() int { return len(s) }

 func (s byFreq) Less(i, j int) bool {
 	if s[i].freq == s[j].freq {
 		return s[i].literal < s[j].literal
 	}
 	return s[i].freq < s[j].freq
 }

 func (s byFreq) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

 func reverseBits(number uint16, bitLength byte) uint16 {
 	return bits.Reverse16(number << (16 - bitLength))
 }
	// 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"
	"math/bits"
	"sort"
	)

	// hcode is a huffman code with a bit code and bit length.
	type hcode struct {
	code, len uint16
	}

	type huffmanEncoder struct {
	codes []hcode
	freqcache []literalNode
	bitCount [17]int32
	lns byLiteral // stored to avoid repeated allocation in generate
	lfs byFreq // stored to avoid repeated allocation in generate
	}

	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
	}

	// set sets the code and length of an hcode.
	func (h *hcode) set(code uint16, length uint16) {
	h.len = length
	h.code = code
	}

	func maxNode() literalNode { return literalNode{math.MaxUint16, math.MaxInt32} }

	func newHuffmanEncoder(size int) *huffmanEncoder {
	return &huffmanEncoder{codes: make([]hcode, size)}
	}

	// Generates a HuffmanCode corresponding to the fixed literal table
	func generateFixedLiteralEncoding() *huffmanEncoder {
	h := newHuffmanEncoder(maxNumLit)
	codes := h.codes
	var ch uint16
	for ch = 0; ch < maxNumLit; ch++ {
	var bits uint16
	var size uint16
	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
	}
	codes[ch] = hcode{code: reverseBits(bits, byte(size)), len: size}
	}
	return h
	}

	func generateFixedOffsetEncoding() *huffmanEncoder {
	h := newHuffmanEncoder(30)
	codes := h.codes
	for ch := range codes {
	codes[ch] = hcode{code: reverseBits(uint16(ch), 5), len: 5}
	}
	return h
	}

	var fixedLiteralEncoding *huffmanEncoder = generateFixedLiteralEncoding()
	var fixedOffsetEncoding *huffmanEncoder = generateFixedOffsetEncoding()

	func (h *huffmanEncoder) bitLength(freq []int32) int {
	var total int
	for i, f := range freq {
	if f != 0 {
	total += int(f) * int(h.codes[i].len)
	}
	}
	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 := h.bitCount[: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):]

	h.lns.sort(chunk)
	for _, node := range chunk {
	h.codes[node.literal] = hcode{code: reverseBits(code, uint8(n)), len: uint16(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) {
	if h.freqcache == nil {
	// Allocate a reusable buffer with the longest possible frequency table.
	// Possible lengths are codegenCodeCount, offsetCodeCount and maxNumLit.
	// The largest of these is maxNumLit, so we allocate for that case.
	h.freqcache = make([]literalNode, maxNumLit+1)
	}
	list := h.freqcache[: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 {
	list[count] = literalNode{}
	h.codes[i].len = 0
	}
	}
	list[len(freq)] = literalNode{}

	list = list[: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.codes[node.literal].set(uint16(i), 1)
	}
	return
	}
	h.lfs.sort(list)

	// Get the number of literals for each bit count
	bitCount := h.bitCounts(list, maxBits)
	// And do the assignment
	h.assignEncodingAndSize(bitCount, list)
	}

	type byLiteral []literalNode

	func (s *byLiteral) sort(a []literalNode) {
	*s = byLiteral(a)
	sort.Sort(s)
	}

	func (s byLiteral) Len() int { return len(s) }

	func (s byLiteral) Less(i, j int) bool {
	return s[i].literal < s[j].literal
	}

	func (s byLiteral) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

	type byFreq []literalNode

	func (s *byFreq) sort(a []literalNode) {
	*s = byFreq(a)
	sort.Sort(s)
	}

	func (s byFreq) Len() int { return len(s) }

	func (s byFreq) Less(i, j int) bool {
	if s[i].freq == s[j].freq {
	return s[i].literal < s[j].literal
	}
	return s[i].freq < s[j].freq
	}

	func (s byFreq) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

	func reverseBits(number uint16, bitLength byte) uint16 {
	return bits.Reverse16(number << (16 - bitLength))
	}