huffman.go

Documentation: golang.org/x/image/vp8l

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package vp8l
     6  
     7  import (
     8  	"io"
     9  )
    10  
    11  // reverseBits reverses the bits in a byte.
    12  var reverseBits = [256]uint8{
    13  	0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0, 0x60, 0xe0, 0x10, 0x90, 0x50, 0xd0, 0x30, 0xb0, 0x70, 0xf0,
    14  	0x08, 0x88, 0x48, 0xc8, 0x28, 0xa8, 0x68, 0xe8, 0x18, 0x98, 0x58, 0xd8, 0x38, 0xb8, 0x78, 0xf8,
    15  	0x04, 0x84, 0x44, 0xc4, 0x24, 0xa4, 0x64, 0xe4, 0x14, 0x94, 0x54, 0xd4, 0x34, 0xb4, 0x74, 0xf4,
    16  	0x0c, 0x8c, 0x4c, 0xcc, 0x2c, 0xac, 0x6c, 0xec, 0x1c, 0x9c, 0x5c, 0xdc, 0x3c, 0xbc, 0x7c, 0xfc,
    17  	0x02, 0x82, 0x42, 0xc2, 0x22, 0xa2, 0x62, 0xe2, 0x12, 0x92, 0x52, 0xd2, 0x32, 0xb2, 0x72, 0xf2,
    18  	0x0a, 0x8a, 0x4a, 0xca, 0x2a, 0xaa, 0x6a, 0xea, 0x1a, 0x9a, 0x5a, 0xda, 0x3a, 0xba, 0x7a, 0xfa,
    19  	0x06, 0x86, 0x46, 0xc6, 0x26, 0xa6, 0x66, 0xe6, 0x16, 0x96, 0x56, 0xd6, 0x36, 0xb6, 0x76, 0xf6,
    20  	0x0e, 0x8e, 0x4e, 0xce, 0x2e, 0xae, 0x6e, 0xee, 0x1e, 0x9e, 0x5e, 0xde, 0x3e, 0xbe, 0x7e, 0xfe,
    21  	0x01, 0x81, 0x41, 0xc1, 0x21, 0xa1, 0x61, 0xe1, 0x11, 0x91, 0x51, 0xd1, 0x31, 0xb1, 0x71, 0xf1,
    22  	0x09, 0x89, 0x49, 0xc9, 0x29, 0xa9, 0x69, 0xe9, 0x19, 0x99, 0x59, 0xd9, 0x39, 0xb9, 0x79, 0xf9,
    23  	0x05, 0x85, 0x45, 0xc5, 0x25, 0xa5, 0x65, 0xe5, 0x15, 0x95, 0x55, 0xd5, 0x35, 0xb5, 0x75, 0xf5,
    24  	0x0d, 0x8d, 0x4d, 0xcd, 0x2d, 0xad, 0x6d, 0xed, 0x1d, 0x9d, 0x5d, 0xdd, 0x3d, 0xbd, 0x7d, 0xfd,
    25  	0x03, 0x83, 0x43, 0xc3, 0x23, 0xa3, 0x63, 0xe3, 0x13, 0x93, 0x53, 0xd3, 0x33, 0xb3, 0x73, 0xf3,
    26  	0x0b, 0x8b, 0x4b, 0xcb, 0x2b, 0xab, 0x6b, 0xeb, 0x1b, 0x9b, 0x5b, 0xdb, 0x3b, 0xbb, 0x7b, 0xfb,
    27  	0x07, 0x87, 0x47, 0xc7, 0x27, 0xa7, 0x67, 0xe7, 0x17, 0x97, 0x57, 0xd7, 0x37, 0xb7, 0x77, 0xf7,
    28  	0x0f, 0x8f, 0x4f, 0xcf, 0x2f, 0xaf, 0x6f, 0xef, 0x1f, 0x9f, 0x5f, 0xdf, 0x3f, 0xbf, 0x7f, 0xff,
    29  }
    30  
    31  // hNode is a node in a Huffman tree.
    32  type hNode struct {
    33  	// symbol is the symbol held by this node.
    34  	symbol uint32
    35  	// children, if positive, is the hTree.nodes index of the first of
    36  	// this node's two children. Zero means an uninitialized node,
    37  	// and -1 means a leaf node.
    38  	children int32
    39  }
    40  
    41  const leafNode = -1
    42  
    43  // lutSize is the log-2 size of an hTree's look-up table.
    44  const lutSize, lutMask = 7, 1<<7 - 1
    45  
    46  // hTree is a Huffman tree.
    47  type hTree struct {
    48  	// nodes are the nodes of the Huffman tree. During construction,
    49  	// len(nodes) grows from 1 up to cap(nodes) by steps of two.
    50  	// After construction, len(nodes) == cap(nodes), and both equal
    51  	// 2*theNumberOfSymbols - 1.
    52  	nodes []hNode
    53  	// lut is a look-up table for walking the nodes. The x in lut[x] is
    54  	// the next lutSize bits in the bit-stream. The low 8 bits of lut[x]
    55  	// equals 1 plus the number of bits in the next code, or 0 if the
    56  	// next code requires more than lutSize bits. The high 24 bits are:
    57  	//   - the symbol, if the code requires lutSize or fewer bits, or
    58  	//   - the hTree.nodes index to start the tree traversal from, if
    59  	//     the next code requires more than lutSize bits.
    60  	lut [1 << lutSize]uint32
    61  }
    62  
    63  // insert inserts into the hTree a symbol whose encoding is the least
    64  // significant codeLength bits of code.
    65  func (h *hTree) insert(symbol uint32, code uint32, codeLength uint32) error {
    66  	if symbol > 0xffff || codeLength > 0xfe {
    67  		return errInvalidHuffmanTree
    68  	}
    69  	baseCode := uint32(0)
    70  	if codeLength > lutSize {
    71  		baseCode = uint32(reverseBits[(code>>(codeLength-lutSize))&0xff]) >> (8 - lutSize)
    72  	} else {
    73  		baseCode = uint32(reverseBits[code&0xff]) >> (8 - codeLength)
    74  		for i := 0; i < 1<<(lutSize-codeLength); i++ {
    75  			h.lut[baseCode|uint32(i)<<codeLength] = symbol<<8 | (codeLength + 1)
    76  		}
    77  	}
    78  
    79  	n := uint32(0)
    80  	for jump := lutSize; codeLength > 0; {
    81  		codeLength--
    82  		if int(n) > len(h.nodes) {
    83  			return errInvalidHuffmanTree
    84  		}
    85  		switch h.nodes[n].children {
    86  		case leafNode:
    87  			return errInvalidHuffmanTree
    88  		case 0:
    89  			if len(h.nodes) == cap(h.nodes) {
    90  				return errInvalidHuffmanTree
    91  			}
    92  			// Create two empty child nodes.
    93  			h.nodes[n].children = int32(len(h.nodes))
    94  			h.nodes = h.nodes[:len(h.nodes)+2]
    95  		}
    96  		n = uint32(h.nodes[n].children) + 1&(code>>codeLength)
    97  		jump--
    98  		if jump == 0 && h.lut[baseCode] == 0 {
    99  			h.lut[baseCode] = n << 8
   100  		}
   101  	}
   102  
   103  	switch h.nodes[n].children {
   104  	case leafNode:
   105  		// No-op.
   106  	case 0:
   107  		// Turn the uninitialized node into a leaf.
   108  		h.nodes[n].children = leafNode
   109  	default:
   110  		return errInvalidHuffmanTree
   111  	}
   112  	h.nodes[n].symbol = symbol
   113  	return nil
   114  }
   115  
   116  // codeLengthsToCodes returns the canonical Huffman codes implied by the
   117  // sequence of code lengths.
   118  func codeLengthsToCodes(codeLengths []uint32) ([]uint32, error) {
   119  	maxCodeLength := uint32(0)
   120  	for _, cl := range codeLengths {
   121  		if maxCodeLength < cl {
   122  			maxCodeLength = cl
   123  		}
   124  	}
   125  	const maxAllowedCodeLength = 15
   126  	if len(codeLengths) == 0 || maxCodeLength > maxAllowedCodeLength {
   127  		return nil, errInvalidHuffmanTree
   128  	}
   129  	histogram := [maxAllowedCodeLength + 1]uint32{}
   130  	for _, cl := range codeLengths {
   131  		histogram[cl]++
   132  	}
   133  	currCode, nextCodes := uint32(0), [maxAllowedCodeLength + 1]uint32{}
   134  	for cl := 1; cl < len(nextCodes); cl++ {
   135  		currCode = (currCode + histogram[cl-1]) << 1
   136  		nextCodes[cl] = currCode
   137  	}
   138  	codes := make([]uint32, len(codeLengths))
   139  	for symbol, cl := range codeLengths {
   140  		if cl > 0 {
   141  			codes[symbol] = nextCodes[cl]
   142  			nextCodes[cl]++
   143  		}
   144  	}
   145  	return codes, nil
   146  }
   147  
   148  // build builds a canonical Huffman tree from the given code lengths.
   149  func (h *hTree) build(codeLengths []uint32) error {
   150  	// Calculate the number of symbols.
   151  	var nSymbols, lastSymbol uint32
   152  	for symbol, cl := range codeLengths {
   153  		if cl != 0 {
   154  			nSymbols++
   155  			lastSymbol = uint32(symbol)
   156  		}
   157  	}
   158  	if nSymbols == 0 {
   159  		return errInvalidHuffmanTree
   160  	}
   161  	h.nodes = make([]hNode, 1, 2*nSymbols-1)
   162  	// Handle the trivial case.
   163  	if nSymbols == 1 {
   164  		if len(codeLengths) <= int(lastSymbol) {
   165  			return errInvalidHuffmanTree
   166  		}
   167  		return h.insert(lastSymbol, 0, 0)
   168  	}
   169  	// Handle the non-trivial case.
   170  	codes, err := codeLengthsToCodes(codeLengths)
   171  	if err != nil {
   172  		return err
   173  	}
   174  	for symbol, cl := range codeLengths {
   175  		if cl > 0 {
   176  			if err := h.insert(uint32(symbol), codes[symbol], cl); err != nil {
   177  				return err
   178  			}
   179  		}
   180  	}
   181  	return nil
   182  }
   183  
   184  // buildSimple builds a Huffman tree with 1 or 2 symbols.
   185  func (h *hTree) buildSimple(nSymbols uint32, symbols [2]uint32, alphabetSize uint32) error {
   186  	h.nodes = make([]hNode, 1, 2*nSymbols-1)
   187  	for i := uint32(0); i < nSymbols; i++ {
   188  		if symbols[i] >= alphabetSize {
   189  			return errInvalidHuffmanTree
   190  		}
   191  		if err := h.insert(symbols[i], i, nSymbols-1); err != nil {
   192  			return err
   193  		}
   194  	}
   195  	return nil
   196  }
   197  
   198  // next returns the next Huffman-encoded symbol from the bit-stream d.
   199  func (h *hTree) next(d *decoder) (uint32, error) {
   200  	var n uint32
   201  	// Read enough bits so that we can use the look-up table.
   202  	if d.nBits < lutSize {
   203  		c, err := d.r.ReadByte()
   204  		if err != nil {
   205  			if err == io.EOF {
   206  				// There are no more bytes of data, but we may still be able
   207  				// to read the next symbol out of the previously read bits.
   208  				goto slowPath
   209  			}
   210  			return 0, err
   211  		}
   212  		d.bits |= uint32(c) << d.nBits
   213  		d.nBits += 8
   214  	}
   215  	// Use the look-up table.
   216  	n = h.lut[d.bits&lutMask]
   217  	if b := n & 0xff; b != 0 {
   218  		b--
   219  		d.bits >>= b
   220  		d.nBits -= b
   221  		return n >> 8, nil
   222  	}
   223  	n >>= 8
   224  	d.bits >>= lutSize
   225  	d.nBits -= lutSize
   226  
   227  slowPath:
   228  	for h.nodes[n].children != leafNode {
   229  		if d.nBits == 0 {
   230  			c, err := d.r.ReadByte()
   231  			if err != nil {
   232  				if err == io.EOF {
   233  					err = io.ErrUnexpectedEOF
   234  				}
   235  				return 0, err
   236  			}
   237  			d.bits = uint32(c)
   238  			d.nBits = 8
   239  		}
   240  		n = uint32(h.nodes[n].children) + 1&d.bits
   241  		d.bits >>= 1
   242  		d.nBits--
   243  	}
   244  	return h.nodes[n].symbol, nil
   245  }
   246
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