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 * http://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 package org.apache.commons.collections4.iterators;
18
19 import java.util.ArrayList;
20 import java.util.Arrays;
21 import java.util.Collection;
22 import java.util.HashMap;
23 import java.util.Iterator;
24 import java.util.List;
25 import java.util.Map;
26 import java.util.NoSuchElementException;
27 import java.util.Objects;
28
29 /**
30 * This iterator creates permutations of an input collection, using the
31 * Steinhaus-Johnson-Trotter algorithm (also called plain changes).
32 * <p>
33 * The iterator will return exactly n! permutations of the input collection.
34 * The {@code remove()} operation is not supported, and will throw an
35 * {@code UnsupportedOperationException}.
36 * <p>
37 * NOTE: in case an empty collection is provided, the iterator will
38 * return exactly one empty list as result, as 0! = 1.
39 *
40 * @param <E> the type of the objects being permuted
41 *
42 * @since 4.0
43 */
44 public class PermutationIterator<E> implements Iterator<List<E>> {
45
46 /**
47 * Permutation is done on these keys to handle equal objects.
48 */
49 private final int[] keys;
50
51 /**
52 * Mapping between keys and objects.
53 */
54 private final Map<Integer, E> objectMap;
55
56 /**
57 * Direction table used in the algorithm:
58 * <ul>
59 * <li>false is left</li>
60 * <li>true is right</li>
61 * </ul>
62 */
63 private final boolean[] direction;
64
65 /**
66 * Next permutation to return. When a permutation is requested
67 * this instance is provided and the next one is computed.
68 */
69 private List<E> nextPermutation;
70
71 /**
72 * Standard constructor for this class.
73 * @param collection the collection to generate permutations for
74 * @throws NullPointerException if coll is null
75 */
76 public PermutationIterator(final Collection<? extends E> collection) {
77 Objects.requireNonNull(collection, "collection");
78 keys = new int[collection.size()];
79 direction = new boolean[collection.size()];
80 Arrays.fill(direction, false);
81 int value = 1;
82 objectMap = new HashMap<>();
83 for (final E e : collection) {
84 objectMap.put(Integer.valueOf(value), e);
85 keys[value - 1] = value;
86 value++;
87 }
88 nextPermutation = new ArrayList<>(collection);
89 }
90
91 /**
92 * Indicates if there are more permutation available.
93 * @return true if there are more permutations, otherwise false
94 */
95 @Override
96 public boolean hasNext() {
97 return nextPermutation != null;
98 }
99
100 /**
101 * Returns the next permutation of the input collection.
102 * @return a list of the permutator's elements representing a permutation
103 * @throws NoSuchElementException if there are no more permutations
104 */
105 @Override
106 public List<E> next() {
107 if (!hasNext()) {
108 throw new NoSuchElementException();
109 }
110
111 // find the largest mobile integer k
112 int indexOfLargestMobileInteger = -1;
113 int largestKey = -1;
114 for (int i = 0; i < keys.length; i++) {
115 if (direction[i] && i < keys.length - 1 && keys[i] > keys[i + 1] ||
116 !direction[i] && i > 0 && keys[i] > keys[i - 1]) {
117 if (keys[i] > largestKey) { // NOPMD
118 largestKey = keys[i];
119 indexOfLargestMobileInteger = i;
120 }
121 }
122 }
123 if (largestKey == -1) {
124 final List<E> toReturn = nextPermutation;
125 nextPermutation = null;
126 return toReturn;
127 }
128
129 // swap k and the adjacent integer it is looking at
130 final int offset = direction[indexOfLargestMobileInteger] ? 1 : -1;
131 final int tmpKey = keys[indexOfLargestMobileInteger];
132 keys[indexOfLargestMobileInteger] = keys[indexOfLargestMobileInteger + offset];
133 keys[indexOfLargestMobileInteger + offset] = tmpKey;
134 final boolean tmpDirection = direction[indexOfLargestMobileInteger];
135 direction[indexOfLargestMobileInteger] = direction[indexOfLargestMobileInteger + offset];
136 direction[indexOfLargestMobileInteger + offset] = tmpDirection;
137
138 // reverse the direction of all integers larger than k and build the result
139 final List<E> nextP = new ArrayList<>();
140 for (int i = 0; i < keys.length; i++) {
141 if (keys[i] > largestKey) {
142 direction[i] = !direction[i];
143 }
144 nextP.add(objectMap.get(Integer.valueOf(keys[i])));
145 }
146 final List<E> result = nextPermutation;
147 nextPermutation = nextP;
148 return result;
149 }
150
151 @Override
152 public void remove() {
153 throw new UnsupportedOperationException("remove() is not supported");
154 }
155
156 }