/// <summary>
/// Heap's algorithm.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="source"></param>
/// <returns></returns>
public static IEnumerable<T[]> GetAllPermutations<T>(IEnumerable<T> source)
{
    var result = source.ToArray();

    yield return result;

    var copy = result.ToArray();
    var c = new int[copy.Length];

    int i = 0;

    while (i < copy.Length)
    {
        if (c[i] < i)
        {
            if (i % 2 == 0)
            {
                var a = copy[0];
                copy[0] = copy[i];
                copy[i] = a;
            }
            else
            {
                var a = copy[c[i]];
                copy[c[i]] = copy[i];
                copy[i] = a;
            }

            yield return copy.ToArray();

            c[i]++;
            i = 0;
        }
        else
        {
            c[i] = 0;
            i++;
        }
    }
}