groovy.util.PermutationGenerator Maven / Gradle / Ivy
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* to you under the Apache License, Version 2.0 (the
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
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*/
package groovy.util;
import org.codehaus.groovy.runtime.DefaultGroovyMethods;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
/**
* Systematically generate permutations.
*
* Adapted from Java Code by Michael Gilleland (released with no restrictions) using an algorithm described here:
* Kenneth H. Rosen, Discrete Mathematics and Its Applications, 2nd edition (NY: McGraw-Hill, 1991), pp. 282-284
*/
public class PermutationGenerator implements Iterator> {
private int[] a;
private BigInteger numLeft;
private BigInteger total;
private List items;
/**
* WARNING: Don't make n too large.
* Recall that the number of permutations is n!
* which can be very large, even when n is as small as 20 --
* 20! = 2,432,902,008,176,640,000 and
* 21! is too big to fit into a Java long, which is
* why we use BigInteger instead.
*
* @param items the items to permute
*/
public PermutationGenerator(Collection items) {
this.items = new ArrayList(items);
int n = items.size();
if (n < 1) {
throw new IllegalArgumentException("At least one item required");
}
a = new int[n];
total = getFactorial(n);
reset();
}
public PermutationGenerator(Iterable items) {
this(DefaultGroovyMethods.asCollection(items));
}
public void reset() {
for (int i = 0; i < a.length; i++) {
a[i] = i;
}
numLeft = new BigInteger(total.toString());
}
public BigInteger getTotal() {
return total;
}
public boolean hasNext() {
return numLeft.compareTo(BigInteger.ZERO) == 1;
}
/**
* Compute factorial (TODO: expose this)
*
* @param n the input integer
* @return the factorial for n
*/
private static BigInteger getFactorial(int n) {
BigInteger fact = BigInteger.ONE;
for (int i = n; i > 1; i--) {
fact = fact.multiply(new BigInteger(Integer.toString(i)));
}
return fact;
}
/**
* Generate next permutation (algorithm from Rosen p. 284)
*
* @return the items permuted
*/
public List next() {
if (numLeft.equals(total)) {
numLeft = numLeft.subtract(BigInteger.ONE);
return items;
}
int temp;
// Find largest index j with a[j] < a[j+1]
int j = a.length - 2;
while (a[j] > a[j + 1]) {
j--;
}
// Find index k such that a[k] is smallest integer
// greater than a[j] to the right of a[j]
int k = a.length - 1;
while (a[j] > a[k]) {
k--;
}
// Interchange a[j] and a[k]
temp = a[k];
a[k] = a[j];
a[j] = temp;
// Put tail end of permutation after jth position in increasing order
int r = a.length - 1;
int s = j + 1;
while (r > s) {
temp = a[s];
a[s] = a[r];
a[r] = temp;
r--;
s++;
}
numLeft = numLeft.subtract(BigInteger.ONE);
List ans = new ArrayList(a.length);
for (int index : a) {
ans.add(items.get(index));
}
return ans;
}
public void remove() {
throw new UnsupportedOperationException("remove() not allowed for PermutationGenerator");
}
}