use super::{RateCounter, RateCounterImmut};
use std::time::{Duration, Instant};

/// A very basic non-rolling update counter. It counts n updates, calculates, and
/// then resets (where n is the sample rate), which means that it takes at least
/// n updates to react to a change in rate appropriately.
///
/// Generally, the RollingRateCounter is a better instrument, but it has to recalculate
/// its measured rate every time it is queried whereas the DiscreteRateCounter only
/// recalculates every n cycles.
///
/// # Usage
///
/// Call `.update()` every time your system starts a new update/cycle; for
/// instance, an FPS counter would call this at the beginning of every frame.
/// The sample rate (set with `set_sample_rate()` and in the first argument to
/// `new()`) governs how many `.update()` calls are required before a
/// meaningful result is produced.
///
/// You can also use .update_immut() for this. Since DiscreteRateCounter is
/// small and easily copyable, this is negligibly less efficient.
#[derive(Clone, Copy)]
pub struct DiscreteRateCounter {
    updates_since_clear: u64,
    time_at_last_clear: Instant,
    rate: f64,
    samples: u64,
}

impl DiscreteRateCounter {
    /// Create a new DiscreteRateCounter which calculates the update rate every
    /// `samples` cycles.  Until that many cycles occur, `rate()` will
    /// return a useless value, typically 0.0.
    ///
    /// If this isn't acceptable, one strategy is to start the `samples` value at 0
    /// and keep ramping it up until it reaches your target `samples` value;
    /// however, the data near the beginning will not be useful.
    pub fn new(samples: u64) -> Self {
        DiscreteRateCounter {
            updates_since_clear: 0,
            time_at_last_clear: Instant::now(),
            rate: 0.0,
            samples: samples,
        }
    }

    /// Return the number of cycles since the rate was last recalculated.
    pub fn rate_age_cycles(&self) -> u64 {
        self.updates_since_clear
    }

    /// Return the amount of time since the rate was last recalculated. This
    /// requires examining the system clock and is thus relatively expensive.
    pub fn rate_age_duration(&self) -> Duration {
        self.time_at_last_clear.elapsed()
    }
}

impl RateCounter for DiscreteRateCounter {
    fn samples(&self) -> u64 {
        self.samples
    }

    fn set_samples(&mut self, samples: u64) {
        self.samples = samples
    }

    fn update(&mut self) {
        self.updates_since_clear += 1;

        if self.updates_since_clear >= self.samples {
            let elapsed = self.time_at_last_clear.elapsed();
            // Compose a f64 of the amount of time elapsed since the last
            // update; that's seconds plus nanos
            let real_time_since_clear =
                elapsed.as_secs() as f64 + elapsed.subsec_nanos() as f64 * 1.0e-9;
            // The rate is the number of updates, over the amount of time it's
            // taken to do them
            self.rate = self.updates_since_clear as f64 / real_time_since_clear;

            // Reset the structure
            self.time_at_last_clear = Instant::now();
            self.updates_since_clear = 0;
        }
    }

    fn rate(&self) -> f64 {
        self.rate
    }
}

impl RateCounterImmut for DiscreteRateCounter {
    /// Consumes the struct and returns an updated version.
    /// Call this at the beginning of each cycle of the periodic activity being
    /// measured.
    /// # Examples
    ///
    /// ```
    /// use update_rate::DiscreteRateCounter;
    /// use update_rate::{RateCounter, RateCounterImmut};
    /// let c = DiscreteRateCounter::new(5);
    /// for i in 1..101 {
    ///     let c = c.update_immut();
    ///     if i % 10 == 0 {println!("Rate: {}", c.rate())}
    ///     // Do work here
    /// }
    /// ```
    fn update_immut(self) -> Self {
        let mut new = self;
        new.update();
        new
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_discrete_rate_counter() {
        let mut c = DiscreteRateCounter::new(10);
        assert!(
            c.rate() == 0.0,
            "Counter should have no data before it gets enough samples."
        );

        let sample_period = Duration::from_millis(10);
        for i in 1..11 {
            // Use busy-wait because sleeping is extremely inaccurate
            let start = Instant::now();
            while start.elapsed() < sample_period {}

            c.update();

            // Mod 10 because rate_age_cycles will go back to 0 at sample_rate which is 10
            assert!(
                c.rate_age_cycles() == i % 10,
                "Rate age not in sync with cycle loop! {} loops but ras = {}",
                i,
                c.rate_age_cycles()
            );
        }

        // Rate should be 100 Hz with 10 ms/update
        let difference = 100.0 - c.rate();
        println!("Rate was {}", c.rate());
        assert!(
            difference < 10.0,
            "Counter rate should be closer to actual rate."
        );
    }
    #[test]
    fn test_discrete_rate_counter_immut() {
        let mut c = DiscreteRateCounter::new(10);
        assert!(
            c.rate() == 0.0,
            "Counter should have no data before it gets enough samples."
        );

        let sample_period = Duration::from_millis(10);
        for i in 1..11 {
            // Use busy-wait because sleeping is extremely inaccurate
            let start = Instant::now();
            while start.elapsed() < sample_period {}

            c = c.update_immut();

            // Mod 10 because rate_age_cycles will go back to 0 at sample_rate which is 10
            assert!(
                c.rate_age_cycles() == i % 10,
                "Rate age not in sync with cycle loop! {} loops but ras = {}",
                i,
                c.rate_age_cycles()
            );
        }

        // Rate should be 100 Hz with 10 ms/update
        let difference = 100.0 - c.rate();
        println!("Rate was {}", c.rate());
        assert!(
            difference < 10.0,
            "Counter rate should be closer to actual rate."
        );
    }
}