using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program {

  private static IEnumerable<(T item, double average)> MovingAverage<T>(

    IEnumerable<T> source, Func<T, double> selector, int windowSize) {

​

    if (source is null)

      throw new ArgumentNullException(nameof(source));

​

    if (selector is null)

      throw new ArgumentNullException(nameof(selector));

​

    if (selector is null)

      throw new ArgumentNullException(nameof(selector));

​

    if (windowSize <= 0)

      throw new ArgumentOutOfRangeException(nameof(windowSize));

​

    double total = 0.0;

    var priors = new Queue<double>();

​

    foreach (var item in source) {

      double value = selector(item);

      priors.Enqueue(value);

      total += value;   

      while (priors.Count > windowSize)

        total -= priors.Dequeue();

​

      if (priors.Count == windowSize)    

        yield return (item, total / windowSize);

    }

  } 

  public static void Main() {

    (int id, char name, int value)[] data = new (int id, char name, int value)[] { 

      (1, 'A', 100),

      (2, 'X', 200),

      (3, 'D', 300),

      (5, 'B', 300),

      (6, 'Z', 300),

      (7, 'G', 300),

    };

    var result = string.Join(Environment.NewLine, MovingAverage(data, item => item.value, 3)

        .Select(item => $"{item.item} :: {Math.Round(item.average)}"));

    Console.WriteLine(result);  

    Console.WriteLine();

    var max = MovingAverage(data, item => item.value, 3)

      .MaxBy(pair => pair.average);   

    Console.WriteLine($"Max: {max.item} :: {max.average}");  

  }

}