OCaml token bucket rate limiter over Unix time

type bucket = {
  rate : float;
  burst : float;
  mutable tokens : float;
  mutable last : float;
}

let create ~rate ~burst =
  if rate <= 0. || burst <= 0. then invalid_arg "bucket: rate and burst must be positive";
  { rate; burst; tokens = burst; last = Unix.gettimeofday () }

let refill bucket now =
  let elapsed = Float.max 0. (now -. bucket.last) in
  bucket.tokens <- Float.min bucket.burst (bucket.tokens +. (elapsed *. bucket.rate));
  bucket.last <- now

let take ?(amount = 1.) bucket =
  refill bucket (Unix.gettimeofday ());
  if bucket.tokens >= amount then (bucket.tokens <- bucket.tokens -. amount; true) else false