bittreedecoder.java

一个7ZIP的解压源码。比较详细。里面含有四种语言的实现代码。

package SevenZip.Compression.RangeCoder;

public class BitTreeDecoder
{
	short[] Models;
	int NumBitLevels;
	
	public BitTreeDecoder(int numBitLevels)
	{
		NumBitLevels = numBitLevels;
		Models = new short[1 << numBitLevels];
	}
	
	public void Init()
	{
		Decoder.InitBitModels(Models);
	}
	
	public int Decode(Decoder rangeDecoder) throws java.io.IOException
	{
		int m = 1;
		for (int bitIndex = NumBitLevels; bitIndex != 0; bitIndex--)
			m = (m << 1) + rangeDecoder.DecodeBit(Models, m);
		return m - (1 << NumBitLevels);
	}
	
	public int ReverseDecode(Decoder rangeDecoder) throws java.io.IOException
	{
		int m = 1;
		int symbol = 0;
		for (int bitIndex = 0; bitIndex < NumBitLevels; bitIndex++)
		{
			int bit = rangeDecoder.DecodeBit(Models, m);
			m <<= 1;
			m += bit;
			symbol |= (bit << bitIndex);
		}
		return symbol;
	}
	
	public static int ReverseDecode(short[] Models, int startIndex,
			Decoder rangeDecoder, int NumBitLevels) throws java.io.IOException
	{
		int m = 1;
		int symbol = 0;
		for (int bitIndex = 0; bitIndex < NumBitLevels; bitIndex++)
		{
			int bit = rangeDecoder.DecodeBit(Models, startIndex + m);
			m <<= 1;
			m += bit;
			symbol |= (bit << bitIndex);
		}
		return symbol;
	}
}