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

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

impl TokenBucket {
    /// Creates a bucket with `capacity` tokens and a refill rate in tokens/sec.
    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 greater than zero"
        );

        Self {
            capacity: capacity as f64,
            tokens: capacity as f64,
            refill_per_second,
            last_refill: Instant::now(),
        }
    }

    /// Attempts to take `amount` tokens immediately.
    pub fn try_acquire(&mut self, amount: u64) -> bool {
        self.try_acquire_at(amount, Instant::now())
    }

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

    /// Returns how long until `amount` tokens are expected to be available.
    pub fn retry_after(&mut self, amount: u64) -> Duration {
        assert!(amount > 0, "amount must be greater than zero");

        self.refill(Instant::now());

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

    fn try_acquire_at(&mut self, amount: u64, now: Instant) -> bool {
        assert!(amount > 0, "amount must be greater than zero");

        self.refill(now);

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

    fn refill(&mut self, now: Instant) {
        if now <= self.last_refill {
            return;
        }

        let elapsed = now.duration_since(self.last_refill).as_secs_f64();
        self.tokens = (self.tokens + elapsed * self.refill_per_second).min(self.capacity);
        self.last_refill = now;
    }
}

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

    #[test]
    fn starts_full_and_rejects_when_empty() {
        let mut bucket = TokenBucket::new(3, 1.0);

        assert!(bucket.try_acquire(1));
        assert!(bucket.try_acquire(2));
        assert!(!bucket.try_acquire(1));
    }

    #[test]
    fn refills_over_time_without_exceeding_capacity() {
        let start = Instant::now();
        let mut bucket = TokenBucket {
            capacity: 5.0,
            tokens: 0.0,
            refill_per_second: 2.0,
            last_refill: start,
        };

        assert!(!bucket.try_acquire_at(3, start + Duration::from_secs(1)));
        assert!(bucket.try_acquire_at(2, start + Duration::from_secs(1)));
        assert_eq!(bucket.available(), 0);

        bucket.refill(start + Duration::from_secs(10));
        assert_eq!(bucket.tokens, 5.0);
    }
}