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

/// A simple token-bucket rate limiter.
///
/// The bucket holds up to `capacity` tokens and refills continuously at
/// `refill_per_second`. Each successful acquisition removes tokens.
#[derive(Debug, Clone)]
pub struct TokenBucket {
    capacity: f64,
    refill_per_second: f64,
    tokens: f64,
    last_refill: Instant,
}

impl TokenBucket {
    /// Creates a full bucket.
    pub fn new(capacity: u32, refill_per_second: f64) -> Self {
        assert!(capacity > 0, "capacity must be positive");
        assert!(
            refill_per_second.is_finite() && refill_per_second > 0.0,
            "refill_per_second must be a positive finite number"
        );

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

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

    /// Attempts to take `amount` tokens immediately.
    pub fn try_acquire_n(&mut self, amount: u32) -> bool {
        self.try_acquire_n_at(amount, Instant::now())
    }

    /// Returns how long until `amount` tokens are available.
    pub fn time_until_available(&mut self, amount: u32) -> Option<Duration> {
        self.time_until_available_at(amount, Instant::now())
    }

    /// Returns the current token count after refilling.
    pub fn available_tokens(&mut self) -> f64 {
        self.refill(Instant::now());
        self.tokens
    }

    fn try_acquire_n_at(&mut self, amount: u32, now: Instant) -> bool {
        assert!(amount > 0, "amount must be positive");
        assert!(
            amount as f64 <= self.capacity,
            "amount cannot exceed bucket capacity"
        );

        self.refill(now);

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

    fn time_until_available_at(&mut self, amount: u32, now: Instant) -> Option<Duration> {
        assert!(amount > 0, "amount must be positive");

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

        self.refill(now);

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

    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;
    }
}

fn main() {}

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

    #[test]
    fn consumes_until_empty() {
        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 start = Instant::now();
        let mut bucket = TokenBucket {
            capacity: 3.0,
            refill_per_second: 2.0,
            tokens: 0.0,
            last_refill: start,
        };

        assert!(!bucket.try_acquire_n_at(2, start + Duration::from_millis(500)));
        assert!(bucket.try_acquire_n_at(2, start + Duration::from_secs(1)));
        assert_eq!(bucket.tokens, 0.0);
    }

    #[test]
    fn reports_wait_time() {
        let start = Instant::now();
        let mut bucket = TokenBucket {
            capacity: 5.0,
            refill_per_second: 4.0,
            tokens: 1.0,
            last_refill: start,
        };

        let wait = bucket
            .time_until_available_at(3, start)
            .expect("request fits in bucket");

        assert_eq!(wait, Duration::from_millis(500));
        assert_eq!(bucket.time_until_available_at(6, start), None);
    }
}