Rust token bucket rate limiter with burst capacity

use std::time::Instant;

struct TokenBucket { capacity: f64, tokens: f64, rate: f64, last: Instant }

impl TokenBucket {
    fn new(capacity: f64, rate: f64) -> Self {
        Self { capacity, tokens: capacity, rate, last: Instant::now() }
    }
    fn try_acquire(&mut self, cost: f64) -> bool {
        let now = Instant::now();
        let gained = now.duration_since(self.last).as_secs_f64() * self.rate;
        self.tokens = (self.tokens + gained).min(self.capacity);
        self.last = now;
        if self.tokens >= cost {
            self.tokens -= cost;
            true
        } else {
            false
        }
    }
}