tor-browser

The Tor Browser
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RangeCoderBit.cs (3272B)

      1 using System;
      2 
      3 namespace SevenZip.Compression.RangeCoder
      4 {
      5 struct BitEncoder
      6 {
      7 	public const int kNumBitModelTotalBits = 11;
      8 	public const uint kBitModelTotal = (1 << kNumBitModelTotalBits);
      9 	const int kNumMoveBits = 5;
     10 	const int kNumMoveReducingBits = 2;
     11 	public const int kNumBitPriceShiftBits = 6;
     12 
     13 	uint Prob;
     14 
     15 	public void Init() { Prob = kBitModelTotal >> 1; }
     16 
     17 	public void UpdateModel(uint symbol)
     18 	{
     19 		if (symbol == 0)
     20 			Prob += (kBitModelTotal - Prob) >> kNumMoveBits;
     21 		else
     22 			Prob -= (Prob) >> kNumMoveBits;
     23 	}
     24 
     25 	public void Encode(Encoder encoder, uint symbol)
     26 	{
     27 		// encoder.EncodeBit(Prob, kNumBitModelTotalBits, symbol);
     28 		// UpdateModel(symbol);
     29 		uint newBound = (encoder.Range >> kNumBitModelTotalBits) * Prob;
     30 		if (symbol == 0)
     31 		{
     32 			encoder.Range = newBound;
     33 			Prob += (kBitModelTotal - Prob) >> kNumMoveBits;
     34 		}
     35 		else
     36 		{
     37 			encoder.Low += newBound;
     38 			encoder.Range -= newBound;
     39 			Prob -= (Prob) >> kNumMoveBits;
     40 		}
     41 		if (encoder.Range < Encoder.kTopValue)
     42 		{
     43 			encoder.Range <<= 8;
     44 			encoder.ShiftLow();
     45 		}
     46 	}
     47 
     48 	private static UInt32[] ProbPrices = new UInt32[kBitModelTotal >> kNumMoveReducingBits];
     49 
     50 	static BitEncoder()
     51 	{
     52 		const int kNumBits = (kNumBitModelTotalBits - kNumMoveReducingBits);
     53 		for (int i = kNumBits - 1; i >= 0; i--)
     54 		{
     55 			UInt32 start = (UInt32)1 << (kNumBits - i - 1);
     56 			UInt32 end = (UInt32)1 << (kNumBits - i);
     57 			for (UInt32 j = start; j < end; j++)
     58 				ProbPrices[j] = ((UInt32)i << kNumBitPriceShiftBits) +
     59 					(((end - j) << kNumBitPriceShiftBits) >> (kNumBits - i - 1));
     60 		}
     61 	}
     62 
     63 	public uint GetPrice(uint symbol)
     64 	{
     65 		return ProbPrices[(((Prob - symbol) ^ ((-(int)symbol))) & (kBitModelTotal - 1)) >> kNumMoveReducingBits];
     66 	}
     67   public uint GetPrice0() { return ProbPrices[Prob >> kNumMoveReducingBits]; }
     68 	public uint GetPrice1() { return ProbPrices[(kBitModelTotal - Prob) >> kNumMoveReducingBits]; }
     69 }
     70 
     71 struct BitDecoder
     72 {
     73 	public const int kNumBitModelTotalBits = 11;
     74 	public const uint kBitModelTotal = (1 << kNumBitModelTotalBits);
     75 	const int kNumMoveBits = 5;
     76 
     77 	uint Prob;
     78 
     79 	public void UpdateModel(int numMoveBits, uint symbol)
     80 	{
     81 		if (symbol == 0)
     82 			Prob += (kBitModelTotal - Prob) >> numMoveBits;
     83 		else
     84 			Prob -= (Prob) >> numMoveBits;
     85 	}
     86 
     87 	public void Init() { Prob = kBitModelTotal >> 1; }
     88 
     89 	public uint Decode(RangeCoder.Decoder rangeDecoder)
     90 	{
     91 		uint newBound = (uint)(rangeDecoder.Range >> kNumBitModelTotalBits) * (uint)Prob;
     92 		if (rangeDecoder.Code < newBound)
     93 		{
     94 			rangeDecoder.Range = newBound;
     95 			Prob += (kBitModelTotal - Prob) >> kNumMoveBits;
     96 			if (rangeDecoder.Range < Decoder.kTopValue)
     97 			{
     98 				rangeDecoder.Code = (rangeDecoder.Code << 8) | (byte)rangeDecoder.Stream.ReadByte();
     99 				rangeDecoder.Range <<= 8;
    100 			}
    101 			return 0;
    102 		}
    103 		else
    104 		{
    105 			rangeDecoder.Range -= newBound;
    106 			rangeDecoder.Code -= newBound;
    107 			Prob -= (Prob) >> kNumMoveBits;
    108 			if (rangeDecoder.Range < Decoder.kTopValue)
    109 			{
    110 				rangeDecoder.Code = (rangeDecoder.Code << 8) | (byte)rangeDecoder.Stream.ReadByte();
    111 				rangeDecoder.Range <<= 8;
    112 			}
    113 			return 1;
    114 		}
    115 	}
    116 }
    117 }