sneak/Shared/FpsCounter.cs

using System;

namespace SemiColinGames {
  class FpsCounter {
    private readonly int[] frameTimes = new int[60];
    private double fps = 0;
    private int idx = 0;

    public int Fps {
      get => (int) Math.Ceiling(fps);
    }

    public void Update() {
      var now = Environment.TickCount;  // ms
      if (frameTimes[idx] != 0) {
        var timeElapsed = now - frameTimes[idx];
        fps = 1000.0 * frameTimes.Length / timeElapsed;
      }
      frameTimes[idx] = now;
      idx = (idx + 1) % frameTimes.Length;
    }
  }
}