NYCJUG/2011-05-10/ExamplePermutationGeneratorInJava

Here's an example of a permutation generator written in Java, from here (with my own, idiosyncratic code formatting.) I bring this up in the context of a discussion about generating permutations which is addressed at length in this elaboration of generating a single permutation.

Java Permutation Generator

Introduction

The PermutationGenerator Java class systematically generates permutations. It relies on the fact that any set with n elements can be placed in one-to-one correspondence with the set {1, 2, 3, ..., n}. The algorithm is described by Kenneth H. Rosen, Discrete Mathematics and Its Applications, 2nd edition (NY: McGraw-Hill, 1991), pp. 282-284.

The class is very easy to use. Suppose that you wish to generate all permutations of the strings "a", "b", "c", and "d". Put them into an array. Keep calling the permutation generator's getNext () method until there are no more permutations left. The getNext () method returns an array of integers, which tell you the order in which to arrange your original array of strings. Here is a snippet of code which illustrates how to use the PermutationGenerator class.

int[] indices;
String[] elements = {"a", "b", "c", "d"};
PermutationGenerator x = new PermutationGenerator (elements.length);
StringBuffer permutation;
while (x.hasMore ())
{  permutation = new StringBuffer ();
  indices = x.getNext ();
  for (int i = 0; i < indices.length; i++)
  { permutation.append (elements[indices[i]]);
  }
  System.out.println (permutation.toString ());
}

One caveat. Don't use this class on large sets. Recall that the number of permutations of a set containing n elements is n factorial, which is a very large number even when n is as small as 20. 20! is 2,432,902,008,176,640,000.

Source Code

The source code is free for you to use in whatever way you wish.

//--------------------------------------
// Systematically generate permutations.
//--------------------------------------

import java.math.BigInteger;

public class PermutationGenerator
{ private int[] a;
  private BigInteger numLeft;
  private BigInteger total;

// Constructor. 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.
  public PermutationGenerator (int n)
  { if (n < 1) { throw new IllegalArgumentException ("Min 1"); }
    a = new int[n]; total = getFactorial (n);
    reset ();
  }

  // Reset
  public void reset () { for (int i = 0; i < a.length; i++) {a[i] = i;}
    numLeft = new BigInteger (total.toString ());
  }

  // Return number of permutations not yet generated
  public BigInteger getNumLeft () {return numLeft;}

  // Return total number of permutations
  public BigInteger getTotal () {return total;}

  // Are there more permutations?
  public boolean hasMore () {return numLeft.compareTo (BigInteger.ZERO) == 1;}

  // Compute factorial
  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)
  public int[] getNext ()
  { if (numLeft.equals (total))
    { numLeft = numLeft.subtract (BigInteger.ONE);
      return a;
    }

    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);
    return a;
  }
}