Random playlist algorithm

Question

I need to create a list of numbers from a range (for example from x to y) in a random order so that every order has an equal chance.

I need this for a music player I

Answer 1

It is pretty much impossible to do it without allocating extra memory. If you're worried about the amount of extra memory allocated, you could always pick a random subset and shuffle between those. You'll get repeats before every song is played, but with a sufficiently large subset I'll warrant few people will notice.

const int MaxItemsToShuffle = 20;
public static IEnumerable RandomIndexes(int count)
{
    Random random = new Random();

    int indexCount = Math.Min(count, MaxItemsToShuffle);
    int[] indexes = new int[indexCount];

    if (count > MaxItemsToShuffle)
    {
        int cur = 0, subsetCount = MaxItemsToShuffle;
        for (int i = 0; i < count; i += 1)
        {
            if (random.NextDouble() <= ((float)subsetCount / (float)(count - i + 1)))
            {
                indexes[cur] = i;
                cur += 1;
                subsetCount -= 1;
            }
        }
    }
    else
    {
        for (int i = 0; i < count; i += 1)
        {
            indexes[i] = i;
        }
    }

    for (int i = indexCount; i > 0; i -= 1)
    {
        int curIndex = random.Next(0, i);
        yield return indexes[curIndex];

        indexes[curIndex] = indexes[i - 1];
    }
}