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

/// A simple token-bucket rate limiter.
///
/// The bucket refills continuously at `refill_rate_per_second` up to `capacity`.
/// Calls to `try_acquire` consume tokens immediately when enough are available.
#[derive(Debug, Clone)]
pub struct TokenBucket {
    capacity: f64,
    refill_rate_per_second: f64,
    tokens: f64,
    last_refill: Instant,
}

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

        let capacity = capacity as f64;

        Self {
            capacity,
            refill_rate_per_second,
            tokens: capacity,
            last_refill: Instant::now(),
        }
    }

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

    /// Attempts to acquire `n` tokens.
    pub fn try_acquire_n(&mut self, n: u32) -> bool {
        if n == 0 {
            return true;
        }

        self.refill();

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

    /// Returns the current number of whole available tokens.
    pub fn available(&mut self) -> u32 {
        self.refill();
        self.tokens.floor() as u32
    }

    /// Returns how long until `n` tokens are available.
    pub fn wait_time_for(&mut self, n: u32) -> Duration {
        if n == 0 {
            return Duration::ZERO;
        }

        self.refill();

        let requested = n as f64;
        if self.tokens >= requested {
            Duration::ZERO
        } else {
            Duration::from_secs_f64((requested - self.tokens) / self.refill_rate_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_rate_per_second).min(self.capacity);
            self.last_refill = now;
        }
    }
}

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

    #[test]
    fn consumes_tokens_until_empty() {
        let mut bucket = TokenBucket::new(2, 1.0);

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

    #[test]
    fn refills_over_time_without_exceeding_capacity() {
        let mut bucket = TokenBucket::new(3, 2.0);

        assert!(bucket.try_acquire_n(3));
        assert!(!bucket.try_acquire());

        bucket.last_refill = bucket
            .last_refill
            .checked_sub(Duration::from_secs(2))
            .unwrap();

        assert_eq!(bucket.available(), 3);
        assert!(bucket.try_acquire_n(3));
        assert!(!bucket.try_acquire());
    }
}