use std::time::{Duration, Instant};

/// A simple token-bucket rate limiter.
///
/// The bucket starts full. Tokens are refilled continuously over time up to
/// `capacity`; each successful acquire removes tokens from the bucket.
#[derive(Debug, Clone)]
pub struct TokenBucket {
    capacity: f64,
    refill_per_second: f64,
    tokens: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a bucket that holds up to `capacity` tokens and refills at
    /// `refill_per_second`.
    pub fn new(capacity: u64, refill_per_second: f64) -> Self {
        assert!(capacity > 0, "capacity must be greater than zero");
        assert!(
            refill_per_second.is_finite() && refill_per_second > 0.0,
            "refill_per_second must be finite and positive"
        );

        let capacity = capacity as f64;

        Self {
            capacity,
            refill_per_second,
            tokens: capacity,
            last_refill: Instant::now(),
        }
    }

    /// Attempts to acquire one token.
    pub fn try_acquire(&mut self) -> bool {
        self.try_acquire_n(1)
    }

    /// Attempts to acquire `count` tokens.
    ///
    /// Returns `true` when enough tokens were available, otherwise returns
    /// `false` and leaves the bucket unchanged except for normal refilling.
    pub fn try_acquire_n(&mut self, count: u64) -> bool {
        if count == 0 {
            return true;
        }

        self.refill();

        let requested = count as f64;
        if self.tokens >= requested {
            self.tokens -= requested;
            true
        } else {
            false
        }
    }

    /// Returns the number of currently available whole tokens.
    pub fn available(&mut self) -> u64 {
        self.refill();
        self.tokens.floor() as u64
    }

    /// Returns how long until `count` tokens are available.
    pub fn time_until_available(&mut self, count: u64) -> Duration {
        if count == 0 {
            return Duration::ZERO;
        }

        self.refill();

        let requested = count as f64;
        if self.tokens >= requested {
            Duration::ZERO
        } else {
            let missing = requested - self.tokens;
            Duration::from_secs_f64(missing / self.refill_per_second)
        }
    }

    fn refill(&mut self) {
        let now = Instant::now();
        let elapsed = now.duration_since(self.last_refill).as_secs_f64();

        if elapsed > 0.0 {
            let added = elapsed * self.refill_per_second;
            self.tokens = (self.tokens + added).min(self.capacity);
            self.last_refill = now;
        }
    }
}

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

    #[test]
    fn consumes_tokens_until_empty() {
        let mut bucket = TokenBucket::new(2, 1.0);

        assert!(bucket.try_acquire());
        assert!(bucket.try_acquire());
        assert!(!bucket.try_acquire());
        assert_eq!(bucket.available(), 0);
    }

    #[test]
    fn refills_over_elapsed_time() {
        let mut bucket = TokenBucket::new(3, 2.0);

        assert!(bucket.try_acquire_n(3));
        assert_eq!(bucket.available(), 0);

        bucket.last_refill = bucket
            .last_refill
            .checked_sub(Duration::from_secs(1))
            .expect("test duration should not underflow");

        assert_eq!(bucket.available(), 2);
        assert!(bucket.try_acquire_n(2));
        assert!(!bucket.try_acquire());
    }
}