net.jqwik.engine.support.combinatorics.PermutationIterator Maven / Gradle / Ivy
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package net.jqwik.engine.support.combinatorics;
import org.jspecify.annotations.*;
import java.util.*;
// Taken and adapted from https://codereview.stackexchange.com/questions/119969/
public class PermutationIterator implements Iterator> {
private final List values = new ArrayList<>();
private @Nullable List next;
private int @Nullable [] indices;
public PermutationIterator(List values) {
this.values.addAll(values);
initializeIndices(values);
if (values.isEmpty()) {
next = null;
} else {
next = new ArrayList<>(this.values);
}
}
private void initializeIndices(List values) {
this.indices = new int[values.size()];
for (int i = 0; i < indices.length; ++i) {
indices[i] = i;
}
}
@Override
public boolean hasNext() {
return next != null;
}
@Override
public List next() {
if (next == null) {
throw new NoSuchElementException();
}
List current = next;
this.next = findNext();
return current;
}
private List findNext() {
int index = nextIndexToChange();
if (index == -1) {
// No more new permutations.
return null;
}
generateNextIndices(index);
return current();
}
private int nextIndexToChange() {
int i = indices.length - 2;
while (i >= 0 && indices[i] > indices[i + 1]) {
i--;
}
return i;
}
private void generateNextIndices(int i) {
int j = i + 1;
int min = indices[j];
int minIndex = j;
while (j < indices.length) {
if (indices[i] < indices[j] && indices[j] < min) {
min = indices[j];
minIndex = j;
}
j++;
}
swap(indices, i++, minIndex);
j = indices.length - 1;
while (i < j) {
swap(indices, i++, j--);
}
}
private List current() {
List newPermutation = new ArrayList<>(indices.length);
for (int i : indices) {
newPermutation.add(values.get(i));
}
return newPermutation;
}
private static void swap(int[] array, int a, int b) {
int tmp = array[a];
array[a] = array[b];
array[b] = tmp;
}
}