src/main/java/org/apache/datasketches/frequencies/Util.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.datasketches.frequencies;

final class Util {

  private Util() {}

  /**
   * The following constant controls the size of the initial data structure for the 
   * frequencies sketches and its value is somewhat arbitrary.
   */
  static final int LG_MIN_MAP_SIZE = 3;

  /**
   * This constant is large enough so that computing the median of SAMPLE_SIZE
   * randomly selected entries from a list of numbers and outputting
   * the empirical median will give a constant-factor approximation to the 
   * true median with high probability.
   */
  static final int SAMPLE_SIZE = 1024;

  /**
   * @param key to be hashed
   * @return an index into the hash table This hash function is taken from the internals of 
   * Austin Appleby's MurmurHash3 algorithm. It is also used by the Trove for Java libraries.
   */
  static long hash(long key) {
    key ^= key >>> 33;
    key *= 0xff51afd7ed558ccdL;
    key ^= key >>> 33;
    key *= 0xc4ceb9fe1a85ec53L;
    key ^= key >>> 33;
    return key;
  }

}