package btools.codec;

/**
 * Encoder/Decoder for signed integers that automatically detects the typical
 * range of these numbers to determine a noisy-bit count as a very simple
 * dictionary
 *
 * Adapted for 3-pass encoding (counters -> statistics -> encoding )
 * but doesn't do anything at pass1
 */
public final class NoisyDiffCoder
{
  private int tot;
  private int[] freqs;
  private int noisybits;
  private StatCoderContext bc;
  private int pass;

  /**
   * Create a decoder and read the noisy-bit count from the gibe context
   */
  public NoisyDiffCoder( StatCoderContext bc )
  {
    noisybits = bc.decodeVarBits();
    this.bc = bc;
  }

  /**
   * Create an encoder for 3-pass-encoding
   */
  public NoisyDiffCoder()
  {
  }

  /**
   * encodes a signed int (pass3 only, stats collection in pass2)
   */
  public void encodeSignedValue( int value )
  {
    if ( pass == 3 )
    {
      bc.encodeNoisyDiff( value, noisybits );
    }
    else if ( pass == 2 )
    {
      count( value < 0 ? -value : value );
    }
  }

  /**
   * decodes a signed int
   */
  public int decodeSignedValue()
  {
    return bc.decodeNoisyDiff( noisybits );
  }

  /**
   * Starts a new encoding pass and (in pass3) calculates the noisy-bit count
   * from the stats collected in pass2 and writes that to the given context
   */
  public void encodeDictionary( StatCoderContext bc )
  {
    if ( ++pass == 3 )
    {
      // how many noisy bits?
      for ( noisybits = 0; noisybits < 14 && tot > 0; noisybits++ )
      {
        if ( freqs[noisybits] < ( tot >> 1 ) )
          break;
      }
      bc.encodeVarBits( noisybits );
    }
    this.bc = bc;
  }

  private void count( int value )
  {
    if ( freqs == null )
      freqs = new int[14];
    int bm = 1;
    for ( int i = 0; i < 14; i++ )
    {
      if ( value < bm )
        break;
      else
        freqs[i]++;
      bm <<= 1;
    }
    tot++;
  }
}