#!markdown

# Simple Moving Average and `IAsyncEnumerable`

Khalid said:

<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Hey <a href="https://twitter.com/andrewlocknet?ref_src=twsrc%5Etfw">@andrewlocknet</a>! It would be cool to see your Simple Moving Average take in a IAsyncEnumerable and recalculate the average as new input comes in.<a href="https://t.co/NIw8Ow5PA5">https://t.co/NIw8Ow5PA5</a></p>&mdash; Khalid 🪀 (@buhakmeh) <a href="https://twitter.com/buhakmeh/status/1390674414748979201?ref_src=twsrc%5Etfw">May 7, 2021</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

I decided to look into it!

First step, we have the existing Simple Moving Average calculator from [my post](https://andrewlock.net/creating-a-simple-moving-average-calculator-in-csharp-1-a-simple-moving-average-calculator/).

#!csharp

public class SimpleMovingAverage
{
    private readonly int _k;
    private readonly int[] _values;

    private int _index = 0;
    private int _sum = 0;

    public SimpleMovingAverage(int k)
    {
        if (k <= 0) throw new ArgumentOutOfRangeException(nameof(k), "Must be greater than 0");

        _k = k;
        _values = new int[k];
    }

    public double Update(int nextInput)
    {
        _sum = _sum - _values[_index] + nextInput;
        _values[_index] = nextInput;
        _index = (_index + 1) % _k;
        return ((double) _sum) / _k;
    }
}

#!markdown

Khalid said he wanted to take an `IAsyncEnumerable` of inputs, and retrieve an `IAsyncEnumerable` of outputs. This is essentially just a mapping from one `IAsyncEnumerable` to another.

There's a few ways we could do that, but to keep everything encapsulated, I created an `SmaConverter` class that wraps the `SimpleMovingAverage` to do the conversion:

#!csharp

public class SmaConverter
{
    private readonly SimpleMovingAverage _sma = new(k: 3);
    public async IAsyncEnumerable<double> GetAverage(IAsyncEnumerable<int> inputs)
    {
        await foreach (int input in inputs)
            yield return _sma.Update(input);
    }
}

#!markdown

That's all we need to do the conversion, so now lets create a class to get our `IAsyncEnumerable` input:

#!csharp

async IAsyncEnumerable<int> GetInputs()
{
    await Task.Yield();
    yield return 1;
    yield return 2;
    yield return 3;
    yield return 2;
    yield return 2;
    yield return 1;
    yield return 1;
    yield return 3;
    yield return 3;
    yield return 3;
}

#!markdown

And finally, lets put it all together

#!csharp

var converter = new SmaConverter();
var values = GetValues();
await foreach(var average in converter.GetAverage(values))
{
    Console.WriteLine($"Next average is {average:0.00}");
}

#!markdown

And voila!