Trouble Implementing a Sliding Window in Rx

Question

I created a SlidingWindow operator for reactive extensions because I want to easily monitor things like rolling averages, etc. As a simple example, I want to subscribe to hear mouse events, but each time there's an event I want to receive the last three (rather than waiting for every third event to receive the last three). That's why the Window overloads I found don't seem to give me what I need out of the box.

This is what I came up with. I fear that it might not be the most performant solution, given its frequent List operations:

public static IObservable<List<T>> SlidingWindow<T>(this IObservable<T> seq, int length)
{
    var seed = new List<T>();

    Func<List<T>, T, List<T>> accumulator = (list, arg2) =>
    {
        list.Add(arg2);

        if (list.Count > length)
            list.RemoveRange(0, (list.Count - length));

        return list;
    };

    return seq.Scan(seed, accumulator)
                .Where(list => list.Count == length);
}

It can be called this way:

var rollingSequence = Observable.Range(1, 5).SlidingWindow().ToEnumerable();

However, to my great surprise, instead of receiving the expected results

1,2,3
2,3,4
3,4,5

I receive the results

2,3,4
3,4,5
3,4,5

Any insights would be much appreciated!

Answer 1

Using your original test, with an argument of 3 for count, this gives the desired results:

public static IObservable<IList<T>> SlidingWindow<T>(
    this IObservable<T> source, int count)
{
    return source.Buffer(count, 1)
                 .Where(list => list.Count == count);
}

Testing like this:

var source = Observable.Range(1, 5);
var query = source.SlidingWindow(3);
using (query.Subscribe(i => Console.WriteLine(string.Join(",", i))))
{

}

Output:

1,2,3
2,3,4
3,4,5

Answer 2

Just source.Window(count, 1) - or source.Buffer(count, 1) It be a window/buffer of "count" items, sliding by one.