rangecoder.h

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// Compression/RangeCoder.h
// This code is based on Eugene Shelwien's Rangecoder code


#ifndef __COMPRESSION_RANGECODER_H
#define __COMPRESSION_RANGECODER_H

#include "InByte.h"

const int kNumTopBits = 24;
const UINT32 kTopValue = (1 << kNumTopBits);

class CRangeDecoder
{
public:
  CInByte m_Stream;
  UINT32 m_Range;
  UINT32 m_Code;
  UINT32 m_Word;
  void Normalize()
  {
    while (m_Range < kTopValue)
    {
      m_Code = (m_Code << 8) | m_Stream.ReadByte();
      m_Range <<= 8;
    }
  }
  
  void Init(const BYTE *buffer, int size)
  {
    m_Stream.Init(buffer, size);
    m_Code = 0;
    m_Range = UINT32(-1);
    for(int i = 0; i < 5; i++)
      m_Code = (m_Code << 8) | m_Stream.ReadByte();
  }

  UINT32 DecodeDirectBits(UINT32 numTotalBits)
  {
    UINT32 range = m_Range;
    UINT32 code = m_Code;        
    UINT32 result = 0;
    for (UINT32 i = numTotalBits; i > 0; i--)
    {
      range >>= 1;
      /*
      result <<= 1;
      if (code >= range)
      {
        code -= range;
        result |= 1;
      }
      */
      UINT32 t = (code - range) >> 31;
      code -= range & (t - 1);
      // range = aRangeTmp + ((range & 1) & (1 - t));
      result = (result << 1) | (1 - t);

      if (range < kTopValue)
      {
        code = (code << 8) | m_Stream.ReadByte();
        range <<= 8; 
      }
    }
    m_Range = range;
    m_Code = code;
    return result;
  }

  int DecodeBit(UINT32 size0, UINT32 numTotalBits)
  {
    UINT32 newBound = (m_Range >> numTotalBits) * size0;
    int symbol;
    if (m_Code < newBound)
    {
      symbol = 0;
      m_Range = newBound;
    }
    else
    {
      symbol = 1;
      m_Code -= newBound;
      m_Range -= newBound;
    }
    Normalize();
    return symbol;
  }
};

#endif