brouter/brouter-codec/src/main/java/btools/codec/StatCoderContext.java

package btools.codec;

import java.util.Map;
import java.util.TreeMap;

import btools.util.BitCoderContext;

public final class StatCoderContext extends BitCoderContext {
  private static Map<String, long[]> statsPerName;
  private long lastbitpos = 0;


  private static final int[] noisy_bits = new int[1024];

  static {
    // noisybits lookup
    for (int i = 0; i < 1024; i++) {
      int p = i;
      int noisybits = 0;
      while (p > 2) {
        noisybits++;
        p >>= 1;
      }
      noisy_bits[i] = noisybits;
    }
  }


  public StatCoderContext(byte[] ab) {
    super(ab);
  }

  /**
   * assign the de-/encoded bits since the last call assignBits to the given
   * name. Used for encoding statistics
   *
   * @see #getBitReport
   */
  public void assignBits(String name) {
    long bitpos = getWritingBitPosition();
    if (statsPerName == null) {
      statsPerName = new TreeMap<String, long[]>();
    }
    long[] stats = statsPerName.get(name);
    if (stats == null) {
      stats = new long[2];
      statsPerName.put(name, stats);
    }
    stats[0] += bitpos - lastbitpos;
    stats[1] += 1;
    lastbitpos = bitpos;
  }

  /**
   * Get a textual report on the bit-statistics
   *
   * @see #assignBits
   */
  public static String getBitReport() {
    if (statsPerName == null) {
      return "<empty bit report>";
    }
    StringBuilder sb = new StringBuilder();
    for (String name : statsPerName.keySet()) {
      long[] stats = statsPerName.get(name);
      sb.append(name + " count=" + stats[1] + " bits=" + stats[0] + "\n");
    }
    statsPerName = null;
    return sb.toString();
  }

  /**
   * encode an unsigned integer with some of of least significant bits
   * considered noisy
   *
   * @see #decodeNoisyNumber
   */
  public void encodeNoisyNumber(int value, int noisybits) {
    if (value < 0) {
      throw new IllegalArgumentException("encodeVarBits expects positive value");
    }
    if (noisybits > 0) {
      int mask = 0xffffffff >>> (32 - noisybits);
      encodeBounded(mask, value & mask);
      value >>= noisybits;
    }
    encodeVarBits(value);
  }

  /**
   * decode an unsigned integer with some of of least significant bits
   * considered noisy
   *
   * @see #encodeNoisyNumber
   */
  public int decodeNoisyNumber(int noisybits) {
    int value = decodeBits(noisybits);
    return value | (decodeVarBits() << noisybits);
  }

  /**
   * encode a signed integer with some of of least significant bits considered
   * noisy
   *
   * @see #decodeNoisyDiff
   */
  public void encodeNoisyDiff(int value, int noisybits) {
    if (noisybits > 0) {
      value += 1 << (noisybits - 1);
      int mask = 0xffffffff >>> (32 - noisybits);
      encodeBounded(mask, value & mask);
      value >>= noisybits;
    }
    encodeVarBits(value < 0 ? -value : value);
    if (value != 0) {
      encodeBit(value < 0);
    }
  }

  /**
   * decode a signed integer with some of of least significant bits considered
   * noisy
   *
   * @see #encodeNoisyDiff
   */
  public int decodeNoisyDiff(int noisybits) {
    int value = 0;
    if (noisybits > 0) {
      value = decodeBits(noisybits) - (1 << (noisybits - 1));
    }
    int val2 = decodeVarBits() << noisybits;
    if (val2 != 0) {
      if (decodeBit()) {
        val2 = -val2;
      }
    }
    return value + val2;
  }

  /**
   * encode a signed integer with the typical range and median taken from the
   * predicted value
   *
   * @see #decodePredictedValue
   */
  public void encodePredictedValue(int value, int predictor) {
    int p = predictor < 0 ? -predictor : predictor;
    int noisybits = 0;

    while (p > 2) {
      noisybits++;
      p >>= 1;
    }
    encodeNoisyDiff(value - predictor, noisybits);
  }

  /**
   * decode a signed integer with the typical range and median taken from the
   * predicted value
   *
   * @see #encodePredictedValue
   */
  public int decodePredictedValue(int predictor) {
    int p = predictor < 0 ? -predictor : predictor;
    int noisybits = 0;
    while (p > 1023) {
      noisybits++;
      p >>= 1;
    }
    return predictor + decodeNoisyDiff(noisybits + noisy_bits[p]);
  }

  /**
   * encode an integer-array making use of the fact that it is sorted. This is
   * done, starting with the most significant bit, by recursively encoding the
   * number of values with the current bit being 0. This yields an number of
   * bits per value that only depends on the typical distance between subsequent
   * values and also benefits
   *
   * @param values  the array to encode
   * @param offset  position in this array where to start
   * @param subsize number of values to encode
   * @param nextbit bitmask with the most significant bit set to 1
   * @param mask    should be 0
   */
  public void encodeSortedArray(int[] values, int offset, int subsize, int nextbit, int mask) {
    if (subsize == 1) { // last-choice shortcut
      while (nextbit != 0) {
        encodeBit((values[offset] & nextbit) != 0);
        nextbit >>= 1;
      }
    }
    if (nextbit == 0) {
      return;
    }

    int data = mask & values[offset];
    mask |= nextbit;

    // count 0-bit-fraction
    int i = offset;
    int end = subsize + offset;
    for (; i < end; i++) {
      if ((values[i] & mask) != data) {
        break;
      }
    }
    int size1 = i - offset;
    int size2 = subsize - size1;

    encodeBounded(subsize, size1);
    if (size1 > 0) {
      encodeSortedArray(values, offset, size1, nextbit >> 1, mask);
    }
    if (size2 > 0) {
      encodeSortedArray(values, i, size2, nextbit >> 1, mask);
    }
  }

  /**
   * @param values  the array to encode
   * @param offset  position in this array where to start
   * @param subsize number of values to encode
   * @param nextbit bitmask with the most significant bit set to 1
   * @param value   should be 0
   * @see #encodeSortedArray
   */
  public void decodeSortedArray(int[] values, int offset, int subsize, int nextbitpos, int value) {
    if (subsize == 1) { // last-choice shortcut
      if (nextbitpos >= 0) {
        value |= decodeBitsReverse(nextbitpos + 1);
      }
      values[offset] = value;
      return;
    }
    if (nextbitpos < 0) {
      while (subsize-- > 0) {
        values[offset++] = value;
      }
      return;
    }

    int size1 = decodeBounded(subsize);
    int size2 = subsize - size1;

    if (size1 > 0) {
      decodeSortedArray(values, offset, size1, nextbitpos - 1, value);
    }
    if (size2 > 0) {
      decodeSortedArray(values, offset + size1, size2, nextbitpos - 1, value | (1 << nextbitpos));
    }
  }

}