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Java > Open Source Codes > org > eclipse > swt > internal > image > PngHuffmanTable


1 /*******************************************************************************
2  * Copyright (c) 2000, 2006 IBM Corporation and others.
3  * All rights reserved. This program and the accompanying materials
4  * are made available under the terms of the Eclipse Public License v1.0
5  * which accompanies this distribution, and is available at
6  * http://www.eclipse.org/legal/epl-v10.html
7  *
8  * Contributors:
9  * IBM Corporation - initial API and implementation
10  *******************************************************************************/
11 package org.eclipse.swt.internal.image;
12
13 import java.io.*;
14
15 public class PngHuffmanTable {
16     CodeLengthInfo[] codeLengthInfo;
17     int[] codeValues;
18     
19     static final int MAX_CODE_LENGTH = 15;
20     static final int BAD_CODE = 0xFFFFFFF;
21     static final int incs[] = {1391376, 463792, 198768, 86961, 33936, 13776, 4592, 1968, 861, 336, 112, 48, 21, 7, 3, 1};
22
23 PngHuffmanTable (int[] lengths) {
24     super();
25     initialize(lengths);
26     generateTable(lengths);
27 }
28
29 private void initialize(int[] lengths) {
30     codeValues = new int[lengths.length];
31     for (int i = 0; i < codeValues.length; i++) {
32         codeValues[i] = i;
33     }
34     
35     // minCodesByLength[n] : The smallest Huffman code of length n + 1.
36 // maxCodesByLength[n] : The largest Huffman code of length n + 1.
37 // indexesByLength[n] : Index into the values array. First value with a code of length n + 1.
38 codeLengthInfo = new CodeLengthInfo[MAX_CODE_LENGTH];
39     for (int i = 0; i < MAX_CODE_LENGTH; i++) {
40         codeLengthInfo[i] = new CodeLengthInfo();
41         codeLengthInfo[i].length = i;
42         codeLengthInfo[i].baseIndex = 0;
43         codeLengthInfo[i].min = BAD_CODE;
44         codeLengthInfo[i].max = -1;
45     }
46 }
47     
48 private void generateTable(int[] lengths) {
49     // Sort the values using shellsort. Primary key is code size. Secondary key is value.
50 int codeValuesTemp;
51     for (int k = 0; k < 16; k++) {
52         for (int h = incs[k], i = h; i < lengths.length; i++) {
53             int v = lengths[i];
54             codeValuesTemp = codeValues[i];
55             int j = i;
56             while (j >= h && (lengths[j - h] > v || (lengths[j - h] == v && codeValues[j - h] > codeValuesTemp))) {
57                 lengths[j] = lengths[j - h];
58                 codeValues[j] = codeValues[j - h];
59                 j -= h;
60             }
61             lengths[j] = v;
62             codeValues[j] = codeValuesTemp;
63         }
64     }
65
66     // These values in these arrays correspond to the elements of the
67 // "values" array. The Huffman code for codeValues[N] is codes[N]
68 // and the length of the code is lengths[N].
69 int[] codes = new int[lengths.length];
70     int lastLength = 0;
71     int code = 0;
72     for (int i = 0; i < lengths.length; i++) {
73         while (lastLength != lengths[i]) {
74             lastLength++;
75             code <<= 1;
76         }
77         if (lastLength != 0) {
78             codes[i] = code;
79             code++;
80         }
81     }
82     
83     int last = 0;
84     for (int i = 0; i < lengths.length; i++) {
85         if (last != lengths[i]) {
86             last = lengths[i];
87             codeLengthInfo[last - 1].baseIndex = i;
88             codeLengthInfo[last - 1].min = codes[i];
89         }
90         if (last != 0) codeLengthInfo[last - 1].max = codes[i];
91     }
92 }
93
94 int getNextValue(PngDecodingDataStream stream) throws IOException {
95     int code = stream.getNextIdatBit();
96     int codelength = 0;
97
98     // Here we are taking advantage of the fact that 1 bits are used as
99 // a prefix to the longer codeValues.
100 while (codelength < MAX_CODE_LENGTH && code > codeLengthInfo[codelength].max) {
101         code = ((code << 1) | stream.getNextIdatBit());
102         codelength++;
103     }
104     if (codelength >= MAX_CODE_LENGTH) stream.error();
105
106     // Now we have a Huffman code of length (codelength + 1) that
107 // is somewhere in the range
108 // minCodesByLength[codelength]..maxCodesByLength[codelength].
109 // This code is the (offset + 1)'th code of (codelength + 1);
110 int offset = code - codeLengthInfo[codelength].min;
111
112     // indexesByLength[codelength] is the first code of length (codelength + 1)
113 // so now we can look up the value for the Huffman code in the table.
114 int index = codeLengthInfo[codelength].baseIndex + offset;
115     return codeValues[index];
116 }
117     
118 class CodeLengthInfo {
119     int length;
120     int max;
121     int min;
122     int baseIndex;
123 }
124
125 }
126
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