use std::sync::Mutex;
use std::time::Instant;

/// A thread-safe token-bucket rate limiter.
pub struct TokenBucket {
    capacity: f64,
    refill_per_second: f64,
    state: Mutex<State>,
}

struct State {
    tokens: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a full bucket with the given capacity and refill rate.
    pub fn new(capacity: u64, refill_per_second: f64) -> Self {
        assert!(capacity > 0, "capacity must be positive");
        assert!(
            refill_per_second.is_finite() && refill_per_second > 0.0,
            "refill rate must be finite and positive"
        );

        Self {
            capacity: capacity as f64,
            refill_per_second,
            state: Mutex::new(State {
                tokens: capacity as f64,
                last_refill: Instant::now(),
            }),
        }
    }

    /// Attempts to consume `amount` tokens without waiting.
    pub fn try_acquire(&self, amount: u64) -> bool {
        self.try_acquire_at(amount, Instant::now())
    }

    /// Returns the number of whole tokens currently available.
    pub fn available(&self) -> u64 {
        let now = Instant::now();
        let mut state = self.state.lock().unwrap_or_else(|e| e.into_inner());
        self.refill(&mut state, now);
        state.tokens.floor() as u64
    }

    fn try_acquire_at(&self, amount: u64, now: Instant) -> bool {
        if amount == 0 {
            return true;
        }
        if amount as f64 > self.capacity {
            return false;
        }

        let mut state = self.state.lock().unwrap_or_else(|e| e.into_inner());
        self.refill(&mut state, now);

        if state.tokens >= amount as f64 {
            state.tokens -= amount as f64;
            true
        } else {
            false
        }
    }

    fn refill(&self, state: &mut State, now: Instant) {
        let elapsed = now.saturating_duration_since(state.last_refill);
        state.tokens = (state.tokens + elapsed.as_secs_f64() * self.refill_per_second)
            .min(self.capacity);
        state.last_refill = now;
    }
}

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

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

        assert!(bucket.try_acquire(2));
        assert!(bucket.try_acquire(1));
        assert!(!bucket.try_acquire(1));
        assert_eq!(bucket.available(), 0);
    }

    #[test]
    fn refills_over_time_without_exceeding_capacity() {
        let bucket = TokenBucket::new(4, 2.0);
        let start = Instant::now();

        assert!(bucket.try_acquire_at(4, start));
        assert!(!bucket.try_acquire_at(1, start));
        assert!(bucket.try_acquire_at(2, start + Duration::from_secs(1)));
        assert!(!bucket.try_acquire_at(1, start + Duration::from_secs(1)));

        // Ten seconds would add twenty tokens, but capacity remains four.
        assert!(bucket.try_acquire_at(4, start + Duration::from_secs(11)));
        assert!(!bucket.try_acquire_at(1, start + Duration::from_secs(11)));
    }
}