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//! A Frames Per Second counter.
extern crate instant;
use instant::Instant;
use std::{collections::VecDeque, time::Duration};

/// Measures Frames Per Second (FPS).
#[derive(Debug)]
pub struct FPSCounter {
    /// The last registered frames.
    last_second_frames: VecDeque<Instant>,
}

impl Default for FPSCounter {
    fn default() -> Self {
        FPSCounter::new()
    }
}

impl FPSCounter {
    /// Creates a new FPSCounter.
    pub fn new() -> FPSCounter {
        FPSCounter {
            last_second_frames: VecDeque::with_capacity(128),
        }
    }

    /// Updates the FPSCounter and returns number of frames.
    pub fn tick(&mut self) -> usize {
        let now = Instant::now();
        let a_second_ago = now - Duration::from_secs(1);

        while self
            .last_second_frames
            .front()
            .map_or(false, |t| *t < a_second_ago)
        {
            self.last_second_frames.pop_front();
        }

        self.last_second_frames.push_back(now);
        self.last_second_frames.len()
    }
}