Permutations and Anagrams

I needed to generate some anagrams, which I know falls into the area Permutations (combinations being altogether the wrong thing for this). There is a nice code sample on Wikipedia that shows how to generate these for an unordered collection.

I implemented a generic class to carry out permutations for me, I changed it to be based on an iterator pattern as often you don't want to generate the whole set, mainly because of the big-oh O(n!), class is therefore:

class Permutation<T>
{
  IEnumerable<T> set;

  private Permutation()
  {
  }

  public Permutation(IEnumerable<T> set)
  {
    this.set = set;
  }

  public IEnumerable<IEnumerable<T>> Mutations
  {
    get
    {
      int length = set.Count();

      int factorialK = (int)Factorial(length);

      for (int k = 0; k < factorialK; k++)
        yield return GeneratePermutation(k, set, length);

    }
  }

  private static IEnumerable<T> GeneratePermutation(long k, IEnumerable<T> set, int length)
  {
    T[] result = set.ToArray();
    for (int j = 2; j <= length; j++)
    {
      k = k / (j - 1);
      int dest = (int)(k % j);
      int source = j - 1;
      if (source != dest)
        Swap(ref result[dest], ref result[source]);
    }

    return result;
  }

  private static void Swap(ref T a, ref T b)
  {
    T temp = a;
    a = b;
    b = temp;
  }

  private static long Factorial(int n)
  {
    long result = 1L;

    for (int integer = 2; integer <= n; integer++)
      result *= integer;

    return result;
  }
}

which allowed me to write the following code to generate ananagrams:

static void Main(string[] args)
{
  string word = "ape";

  List<string> anagrams = GenerateAnagrams(word);

  anagrams.Sort();

  Console.WriteLine("{0} anagrams", anagrams.Count);
  anagrams.ForEach(anagram => Console.WriteLine(anagram));
}

private static List<string> GenerateAnagrams(string word)
{
  List<string> sets = new List<string>();

  Permutation<char> perm = new Permutation<char>(word);
  foreach (char[] combo in perm.Mutations)
    sets.Add(combo.ToString());

  return sets;
}

note that we are taking advantage of string being a char[], for other types such as int would need to be passed as an array, list or similar IEnumerable<T>