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

/// A simple token-bucket rate limiter.
///
/// The bucket starts full, refills over time up to `capacity`, and each
/// successful acquisition consumes tokens.
#[derive(Debug, Clone)]
pub struct TokenBucket {
    capacity: f64,
    refill_per_second: f64,
    tokens: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a bucket that holds at most `capacity` tokens and refills at
    /// `refill_per_second` tokens 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 consume one token.
    pub fn try_acquire(&mut self) -> bool {
        self.try_acquire_n(1)
    }

    /// Attempts to consume `count` tokens.
    pub fn try_acquire_n(&mut self, count: u64) -> bool {
        if count == 0 {
            return true;
        }

        self.refill();

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

    /// Returns the 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 wait_time_for(&mut self, count: u64) -> Option<Duration> {
        if count == 0 {
            return Some(Duration::ZERO);
        }

        let needed = count as f64;
        if needed > self.capacity {
            return None;
        }

        self.refill();

        if self.tokens >= needed {
            Some(Duration::ZERO)
        } else {
            let missing = needed - self.tokens;
            Some(Duration::from_secs_f64(missing / self.refill_per_second))
        }
    }

    fn refill(&mut self) {
        self.refill_at(Instant::now());
    }

    fn refill_at(&mut self, now: Instant) {
        let Some(elapsed) = now.checked_duration_since(self.last_refill) else {
            return;
        };

        if elapsed.is_zero() {
            return;
        }

        let added = elapsed.as_secs_f64() * 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_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_time_without_exceeding_capacity() {
        let now = Instant::now();
        let mut bucket = TokenBucket::new(3, 2.0);

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

        bucket.refill_at(now + Duration::from_millis(500));
        assert_eq!(bucket.available(), 1);

        bucket.refill_at(now + Duration::from_secs(10));
        assert_eq!(bucket.available(), 3);
    }

    #[test]
    fn reports_wait_time_for_missing_tokens() {
        let mut bucket = TokenBucket::new(5, 2.0);

        assert!(bucket.try_acquire_n(4));

        let wait = bucket.wait_time_for(3).unwrap();
        assert!(wait >= Duration::from_millis(900));
        assert!(wait <= Duration::from_millis(1100));

        assert_eq!(bucket.wait_time_for(6), None);
    }
}