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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      https://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  
18  /*
19   * This is not the original file distributed by the Apache Software Foundation
20   * It has been modified by the Hipparchus project
21   */
22  package org.hipparchus.util;
23  
24  import org.hipparchus.exception.MathIllegalArgumentException;
25  
26  /**
27   * A strategy to pick a pivoting index of an array for doing partitioning.
28   */
29  public enum PivotingStrategy {
30  
31      /**
32       * A mid point strategy based on the average of begin and end indices.
33       */
34      CENTRAL {
35          /**
36           * {@inheritDoc}
37           * This in particular picks a average of begin and end indices
38           * @return The index corresponding to a simple average of
39           * the first and the last element indices of the array slice
40           * @throws MathIllegalArgumentException when indices exceeds range
41           */
42          @Override
43          public int pivotIndex(final double[] work, final int begin, final int end)
44              throws MathIllegalArgumentException {
45              MathArrays.verifyValues(work, begin, end-begin);
46              return begin + (end - begin)/2;
47          }
48      },
49  
50      /**
51       * Classic median of 3 strategy given begin and end indices.
52       */
53      MEDIAN_OF_3 {
54          /**
55           * {@inheritDoc}
56           * This in specific makes use of median of 3 pivoting.
57           * @return The index corresponding to a pivot chosen between the
58           * first, middle and the last indices of the array slice
59           * @throws MathIllegalArgumentException when indices exceeds range
60           */
61          @Override
62          public int pivotIndex(final double[] work, final int begin, final int end)
63              throws MathIllegalArgumentException {
64              MathArrays.verifyValues(work, begin, end-begin);
65              final int inclusiveEnd = end - 1;
66              final int middle = begin + (inclusiveEnd - begin) / 2;
67              final double wBegin = work[begin];
68              final double wMiddle = work[middle];
69              final double wEnd = work[inclusiveEnd];
70  
71              if (wBegin < wMiddle) {
72                  if (wMiddle < wEnd) {
73                      return middle;
74                  } else {
75                      return wBegin < wEnd ? inclusiveEnd : begin;
76                  }
77              } else {
78                  if (wBegin < wEnd) {
79                      return begin;
80                  } else {
81                      return wMiddle < wEnd ? inclusiveEnd : middle;
82                  }
83              }
84          }
85      };
86  
87      /**
88       * Find pivot index of the array so that partition and K<sup>th</sup>
89       * element selection can be made
90       * @param work data array
91       * @param begin index of the first element of the slice
92       * @param end index after the last element of the slice
93       * @return the index of the pivot element chosen between the
94       * first and the last element of the array slice
95       * @throws MathIllegalArgumentException when indices exceeds range
96       */
97      public abstract int pivotIndex(double[] work, int begin, int end)
98          throws MathIllegalArgumentException;
99  
100 }