1 /* 2 * RangeDecoder 3 * 4 * Authors: Lasse Collin <lasse.collin (at) tukaani.org> 5 * Igor Pavlov <http://7-zip.org/> 6 * 7 * This file has been put into the public domain. 8 * You can do whatever you want with this file. 9 */ 10 11 package org.tukaani.xz.rangecoder; 12 13 import java.io.DataInputStream; 14 import java.io.IOException; 15 16 public abstract class RangeDecoder extends RangeCoder { 17 int range = 0; 18 int code = 0; 19 20 public abstract void normalize() throws IOException; 21 22 public int decodeBit(short[] probs, int index) throws IOException { 23 normalize(); 24 25 int prob = probs[index]; 26 int bound = (range >>> BIT_MODEL_TOTAL_BITS) * prob; 27 int bit; 28 29 // Compare code and bound as if they were unsigned 32-bit integers. 30 if ((code ^ 0x80000000) < (bound ^ 0x80000000)) { 31 range = bound; 32 probs[index] = (short)( 33 prob + ((BIT_MODEL_TOTAL - prob) >>> MOVE_BITS)); 34 bit = 0; 35 } else { 36 range -= bound; 37 code -= bound; 38 probs[index] = (short)(prob - (prob >>> MOVE_BITS)); 39 bit = 1; 40 } 41 42 return bit; 43 } 44 45 public int decodeBitTree(short[] probs) throws IOException { 46 int symbol = 1; 47 48 do { 49 symbol = (symbol << 1) | decodeBit(probs, symbol); 50 } while (symbol < probs.length); 51 52 return symbol - probs.length; 53 } 54 55 public int decodeReverseBitTree(short[] probs) throws IOException { 56 int symbol = 1; 57 int i = 0; 58 int result = 0; 59 60 do { 61 int bit = decodeBit(probs, symbol); 62 symbol = (symbol << 1) | bit; 63 result |= bit << i++; 64 } while (symbol < probs.length); 65 66 return result; 67 } 68 69 public int decodeDirectBits(int count) throws IOException { 70 int result = 0; 71 72 do { 73 normalize(); 74 75 range >>>= 1; 76 int t = (code - range) >>> 31; 77 code -= range & (t - 1); 78 result = (result << 1) | (1 - t); 79 } while (--count != 0); 80 81 return result; 82 } 83 } 84
