Decoder.java

package sevenzip.compression.rangecoder;

import java.io.IOException;


/**
 * Range decoder.
 */
public class Decoder {

    static final int kTopMask = ~((1<<24)-1);

    static final int kNumBitModelTotalBits = 11;

    static final int kBitModelTotal = (1<<kNumBitModelTotalBits);

    static final int kNumMoveBits = 5;

    int Range;

    int Code;

    java.io.InputStream Stream;


    public final void SetStream(java.io.InputStream stream) {
        Stream = stream;
    }


    public final void ReleaseStream() {
        Stream = null;
    }


    public final void Init() throws IOException {
        Code = 0;
        Range = -1;
        for (int i = 0; i<5; i++) {
            Code = (Code<<8)|Stream.read();
        }
    }


    public final int DecodeDirectBits(int numTotalBits) throws IOException {
        int result = 0;
        for (int i = numTotalBits; i!=0; i--) {
            Range >>>= 1;
            int t = ((Code-Range)>>>31);
            Code -= Range&(t-1);
            result = (result<<1)|(1-t);

            if ((Range&kTopMask)==0) {
                Code = (Code<<8)|Stream.read();
                Range <<= 8;
            }
        }
        return result;
    }


    public int DecodeBit(short[] probs, int index) throws IOException {
        int prob = probs[index];
        int newBound = (Range>>>kNumBitModelTotalBits)*prob;
        int result = 0;
        if ((Code^0x80000000)<(newBound^0x80000000)) {
            Range = newBound;
            probs[index] = (short) (prob+((kBitModelTotal-prob)>>>kNumMoveBits));
            if ((Range&kTopMask)==0) {
                Code = (Code<<8)|Stream.read();
                Range <<= 8;
            }
        } else {
            Range -= newBound;
            Code -= newBound;
            probs[index] = (short) (prob-((prob)>>>kNumMoveBits));
            if ((Range&kTopMask)==0) {
                Code = (Code<<8)|Stream.read();
                Range <<= 8;
            }
            result = 1;
        }
        return result;
    }


    public static void InitBitModels(short[] probs) {
        for (int i = 0; i<probs.length; i++) {
            probs[i] = (kBitModelTotal>>>1);
        }
    }

}