Question

I have a method that outputs all the permutations of an array using a recursive function:

    /// <summary>
    /// Yields a sequence of all permutations in lexical order
    /// </summary>
    /// <typeparam name="T">Type of item in the input sequence</typeparam>
    /// <param name="input">The initial sequence</param>
    /// <returns>A sequence of all permutations in lexical order</returns>
    public IEnumerable<IEnumerable<T>> Permute<T>(IEnumerable<T> input) 
    {
        var list = input.ToList();
        list.Sort(); // into lexical order

        if (list.Count > 2)
        {
            foreach (var item in list)
            {
                var itemArray = new[] {item};
                T[] otherItems = list.Except(itemArray).ToArray();
                foreach (var permutation in Permute(otherItems))
                    yield return itemArray.Concat(permutation).ToArray();
            }
        }
        else 
        {
            yield return new[] {list[0], list[1]};
            yield return new[] {list[1], list[0]};
        }
    }

However, when I run this function in an NUnit test, it terminates far earlier than I think it should:

    [Test]
    public void Can_print_all_permutations()
    {
        foreach (var p in Permute("123456789"))
        {
            Console.WriteLine(new string(p.ToArray()));
        }
    }

Here’s the last lines printed by the test (I separated them by commas for posting purposes):

349527816,
349527861,
349528167,
349528176,
349528617,
3

The abrupt termination makes me think that buffering and flushing by the console is a component of the issue, but the last line that should be printed is 987654321, so I feel like the loop is terminating early.

If I include a calculation in the loop, it terminates even earlier (in the 24… range).

Is there something in my implementation that explains this behavior? Am I pushing the limits of the stack?

Answer 1

Most likely test infrastructure you are using have some sort of “maximum time for test to run” restriction. Enumerating all permutations does take non-trivial amount of time.