use std::time::Duration;

#[derive(Debug, Clone)]
pub struct Backoff {
    initial: Duration,
    max: Duration,
    current: Option<Duration>,
}

impl Backoff {
    pub fn new(initial: Duration, max: Duration) -> Self {
        Self {
            initial,
            max,
            current: None,
        }
    }

    #[allow(clippy::should_implement_trait)]
    pub fn next(&mut self) -> Duration {
        let next = match self.current {
            None => self.initial,
            Some(d) => (d * 2).min(self.max),
        };
        self.current = Some(next);
        next
    }

    pub fn reset(&mut self) {
        self.current = None;
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn starts_at_initial() {
        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
        assert_eq!(b.next(), Duration::from_secs(1));
    }

    #[test]
    fn doubles_each_call() {
        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
        assert_eq!(b.next(), Duration::from_secs(1));
        assert_eq!(b.next(), Duration::from_secs(2));
        assert_eq!(b.next(), Duration::from_secs(4));
        assert_eq!(b.next(), Duration::from_secs(8));
    }

    #[test]
    fn caps_at_max() {
        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(5));
        for _ in 0..10 {
            b.next();
        }
        assert_eq!(b.next(), Duration::from_secs(5));
    }

    #[test]
    fn reset_starts_over() {
        let mut b = Backoff::new(Duration::from_secs(1), Duration::from_secs(30));
        b.next();
        b.next();
        b.next();
        b.reset();
        assert_eq!(b.next(), Duration::from_secs(1));
    }
}