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

/// A thread-safe token-bucket rate limiter.
///
/// Tokens accumulate at `refill_per_second` up to `capacity`.
/// Each successful acquisition consumes the requested number of tokens.
#[derive(Debug)]
pub struct TokenBucket {
    capacity: f64,
    refill_per_second: f64,
    state: Mutex<State>,
}

#[derive(Debug)]
struct State {
    tokens: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a full token bucket.
    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 rate must be finite and greater than zero"
        );

        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 blocking.
    pub fn try_acquire(&self, amount: u64) -> bool {
        self.try_acquire_at(amount, Instant::now())
    }

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

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

    fn refill(&self, state: &mut State, now: Instant) {
        if let Some(elapsed) = now.checked_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 consumes_until_empty() {
        let bucket = TokenBucket::new(3, 1.0);

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

    #[test]
    fn refills_over_time_and_respects_capacity() {
        let bucket = TokenBucket::new(4, 2.0);
        let start = bucket.state.lock().unwrap().last_refill;

        assert!(bucket.try_acquire_at(4, start));
        assert!(!bucket.try_acquire_at(1, start));

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

        let much_later = start + Duration::from_secs(10);
        assert!(bucket.try_acquire_at(4, much_later));
        assert!(!bucket.try_acquire_at(1, much_later));
    }
}