/**
   * Optimization hook. Override this to make SequenceFile.Sorter's scream.
   *
   * <p>The default implementation reads the data into two {@link WritableComparable}s (using {@link
   * Writable#readFields(DataInput)}, then calls {@link
   * #compare(WritableComparable,WritableComparable)}.
   */
  public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
    try {
      buffer.reset(b1, s1, l1); // parse key1
      key1.readFields(buffer);

      buffer.reset(b2, s2, l2); // parse key2
      key2.readFields(buffer);

    } catch (IOException e) {
      throw new RuntimeException(e);
    }

    return compare(key1, key2); // compare them
  }
  /** Used by child copy constructors. */
  protected synchronized void copy(Writable other) {
    if (other != null) {
      try {
        DataOutputBuffer out = new DataOutputBuffer();
        other.write(out);
        DataInputBuffer in = new DataInputBuffer();
        in.reset(out.getData(), out.getLength());
        readFields(in);

      } catch (IOException e) {
        throw new IllegalArgumentException("map cannot be copied: " + e.getMessage());
      }

    } else {
      throw new IllegalArgumentException("source map cannot be null");
    }
  }