Method decodeSymbol

src/arithmeticdecoder.cpp:184–235

}

U32 ArithmeticDecoder::decodeSymbol(ArithmeticModel* m)
{
  U32 n, sym, x, y = length;

  if (m->decoder_table) {             // use table look-up for faster decoding

    unsigned dv = value / (length >>= DM__LengthShift);
    unsigned t = dv >> m->table_shift;

    sym = m->decoder_table[t];      // initial decision based on table look-up
    n = m->decoder_table[t+1] + 1;

    while (n > sym + 1) {                      // finish with bisection search
      U32 k = (sym + n) >> 1;
      if (m->distribution[k] > dv) n = k; else sym = k;
    }
                                                           // compute products
    x = m->distribution[sym] * length;
    if (sym != m->last_symbol) y = m->distribution[sym+1] * length;
  }

  else {                                  // decode using only multiplications

    x = sym = 0;
    length >>= DM__LengthShift;
    U32 k = (n = m->symbols) >> 1;
                                                // decode via bisection search
    do {
      U32 z = length * m->distribution[k];
      if (z > value) {
        n = k;
        y = z;                                             // value is smaller
      }
      else {
        sym = k;
        x = z;                                     // value is larger or equal
      }
    } while ((k = (sym + n) >> 1) != sym);
  }

  value -= x;                                               // update interval
  length = y - x;

  if (length < AC__MinLength) renorm_dec_interval();        // renormalization

  ++m->symbol_count[sym];
  if (--m->symbols_until_update == 0) m->update();    // periodic model update

  assert(sym < m->symbols);

  return sym;
}

U32 ArithmeticDecoder::readBit()
{