pnghuffmantable.java

源码为Eclipse开源开发平台桌面开发工具SWT的源代码,

/*******************************************************************************
 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
 * are made available under the terms of the Common Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.swt.internal.image;


public class PngHuffmanTable {
	CodeLengthInfo[] codeLengthInfo;
	int[] codeValues;
	
	static final int MAX_CODE_LENGTH = 15;
	static final int BAD_CODE = 0xFFFFFFF;

PngHuffmanTable (int[] lengths) {
	super();
	initialize(lengths);
	generateTable(lengths);
}

private void initialize(int[] lengths) {
	codeValues = new int[lengths.length];
	for (int i = 0; i < codeValues.length; i++) {
		codeValues[i] = i;
	}
	
	// minCodesByLength[n] : The smallest Huffman code of length n + 1.
	// maxCodesByLength[n] : The largest Huffman code of length n + 1.
	// indexesByLength[n] : Index into the values array. First value with a code of length n + 1.
	codeLengthInfo = new CodeLengthInfo[MAX_CODE_LENGTH];
	for (int i = 0; i < MAX_CODE_LENGTH; i++) {
		codeLengthInfo[i] = new CodeLengthInfo();
		codeLengthInfo[i].length = i;
		codeLengthInfo[i].baseIndex = 0;
		codeLengthInfo[i].min = BAD_CODE;
		codeLengthInfo[i].max = -1;
	}
}
	
private void generateTable(int[] lengths) {
	// Sort the values. Primary key is code size. Secondary key is value.
	for (int i = 0; i < lengths.length - 1; i++) {
		for (int j = i + 1; j < lengths.length; j++) {
			if (lengths[j] < lengths[i]
				|| (lengths[j] == lengths[i]
				&& codeValues[j] < codeValues[i]))
			{
				int tmp;
				tmp = lengths[j];
				lengths[j] = lengths[i];
				lengths[i] = tmp;
				tmp = codeValues[j];
				codeValues[j] = codeValues[i];
				codeValues[i] = tmp;
			}
		}
	}

	// These values in these arrays correspond to the elements of the
	// "values" array. The Huffman code for codeValues[N] is codes[N]
	// and the length of the code is lengths[N].
	int[] codes = new int[lengths.length];
	int lastLength = 0;
	int code = 0;
	for (int i = 0; i < lengths.length; i++) {
		while (lastLength != lengths[i]) {
			lastLength++;
			code <<= 1;
		}
		if (lastLength != 0) {
			codes[i] = code;
			code++;
		}
	}
	
	int last = 0;
	for (int i = 0; i < lengths.length; i++) {
		if (last != lengths[i]) {
			last = lengths[i];
			codeLengthInfo[last - 1].baseIndex = i;
			codeLengthInfo[last - 1].min = codes[i];
		}
		if (last != 0) codeLengthInfo[last - 1].max = codes[i];
	}
}

int getNextValue(PngDecodingDataStream stream) {
	int code = stream.getNextIdatBit();
	int codelength = 0;

	// Here we are taking advantage of the fact that 1 bits are used as
	// a prefix to the longer codeValues.
	while (codelength < MAX_CODE_LENGTH && code > codeLengthInfo[codelength].max) {
		code = ((code << 1) | stream.getNextIdatBit());
        codelength++;
	}
	if (codelength >= MAX_CODE_LENGTH) stream.error();

	// Now we have a Huffman code of length (codelength + 1) that
	// is somewhere in the range
	// minCodesByLength[codelength]..maxCodesByLength[codelength].
	// This code is the (offset + 1)'th code of (codelength + 1);
	int offset = code - codeLengthInfo[codelength].min;

	// indexesByLength[codelength] is the first code of length (codelength + 1)
	// so now we can look up the value for the Huffman code in the table.
	int index = codeLengthInfo[codelength].baseIndex + offset;
	return codeValues[index];
}	
	
class CodeValuePair {
	int value;
	int code;
}
class CodeLengthInfo {
	int length;
	int max;
	int min;
	int baseIndex;
}

}