permutations.cs

namespace Alvornithms
{
    class Itertools
    {
        static IEnumerable<T[]> Permutations<T>(T[] ts) where T : System.IComparable<T>
        {
            Array.Sort(ts);
            while (true)
            {
                yield return (T[])ts.Clone();
                int pivot = ts.Length - 2;
                while (ts[pivot].CompareTo(ts[pivot + 1]) >= 0)
                {
                    pivot--;
                    if (pivot < 0) { yield break; }
                }
               
                int swap = ts.Length - 1;
                while (ts[pivot].CompareTo(ts[swap]) >= 0)
                {
                    swap--;
                }
                SwapArrayIndexes(ts, pivot, swap);
                int swap_counts = (ts.Length - pivot - 1) / 2;
                for (int k = 1; k <= swap_counts; k++)
                {
                    SwapArrayIndexes(ts, pivot + k, ts.Length - k);
                }
            }
        }
        
        static void SwapArrayIndexes<T>(T[] ts, int a, int b)
        {
            T temp = ts[a];
            ts[a] = ts[b];
            ts[b] = temp;
        }
    }
}