Simple FPS watcher

fps.rs

/// Simple FPS watchcer
#[derive(Debug, Clone, Default)]
pub struct Fps {
    /// Average duration per frame in seconds
    avg: f64,
    /// Square of spike FPS in seconds
    spike: f64,
}

impl Fps {
    const K: f64 = 0.05;

    /// Update FPS counts with real time
    pub fn update(&mut self, frame_time: Duration) {
        self.update_avg(frame_time.as_secs_f64());
        self.update_spike(frame_time.as_secs_f64());
    }

    /// Average FPS
    pub fn avg(&self) -> f64 {
        1.0 / self.avg
    }

    /// Spike FPS
    pub fn spike(&self) -> f64 {
        self.spike.sqrt()
    }

    fn update_avg(&mut self, dt: f64) {
        self.avg *= 1.0 - Self::K;
        self.avg += dt * Self::K;
    }

    // FIXME:
    fn update_spike(&mut self, dt: f64) {
        self.spike *= 1.0 - Self::K;
        self.spike += (dt * dt) * Self::K;
    }
}