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/*
 * 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.solr.util.hll;

/** A collection of bit utilities. */
class BitUtil {
  /**
   * The set of least-significant bits for a given byte. -1 is used if no
   * bits are set (so as to not be confused with "index of zero" meaning that the least significant
   * bit is the 0th (1st) bit).
   *
   * @see #leastSignificantBit(long)
   */
  private static final int[] LEAST_SIGNIFICANT_BIT = {
    -1, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
    5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
  };

  /**
   * Computes the least-significant bit of the specified long that is set to 1
   * . Zero-indexed.
   *
   * @param value the long whose least-significant bit is desired.
   * @return the least-significant bit of the specified long. -1 is
   *     returned if there are no bits set.
   */
  // REF:  http://stackoverflow.com/questions/757059/position-of-least-significant-bit-that-is-set
  // REF:  http://www-graphics.stanford.edu/~seander/bithacks.html
  public static int leastSignificantBit(final long value) {
    if (value == 0L) return -1 /*by contract*/;
    if ((value & 0xFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 0) & 0xFF)] + 0;
    if ((value & 0xFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 8) & 0xFF)] + 8;
    if ((value & 0xFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 16) & 0xFF)] + 16;
    if ((value & 0xFFFFFFFFL) != 0)
      return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 24) & 0xFF)] + 24;
    if ((value & 0xFFFFFFFFFFL) != 0)
      return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 32) & 0xFF)] + 32;
    if ((value & 0xFFFFFFFFFFFFL) != 0)
      return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 40) & 0xFF)] + 40;
    if ((value & 0xFFFFFFFFFFFFFFL) != 0)
      return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 48) & 0xFF)] + 48;
    return LEAST_SIGNIFICANT_BIT[(int) ((value >>> 56) & 0xFFL)] + 56;
  }
}