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

/// A simple token-bucket rate limiter.
///
/// Tokens refill continuously over time up to `capacity`.
/// Each successful acquisition consumes one or more tokens.
#[derive(Debug, Clone)]
pub struct TokenBucket {
    capacity: f64,
    tokens: f64,
    refill_per_second: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a bucket initially filled to capacity.
    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"
        );

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

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

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

        self.refill();

        let amount = amount as f64;
        if amount > self.capacity || self.tokens < amount {
            return false;
        }

        self.tokens -= amount;
        true
    }

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

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

        self.refill();

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

        if self.tokens >= amount {
            Some(Duration::ZERO)
        } else {
            let missing = amount - self.tokens;
            Some(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 {
            self.tokens = (self.tokens + elapsed * self.refill_per_second).min(self.capacity);
            self.last_refill = now;
        }
    }
}

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

    #[test]
    fn consumes_available_tokens() {
        let mut bucket = TokenBucket::new(2, 10.0);

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

    #[test]
    fn refills_over_time() {
        let mut bucket = TokenBucket::new(2, 10.0);

        assert!(bucket.try_acquire_n(2));
        assert!(!bucket.try_acquire());

        bucket.last_refill -= Duration::from_millis(100);

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