Scala token bucket rate limiter with bursts

final class TokenBucket(ratePerSecond: Double, capacity: Int):
  require(ratePerSecond > 0, "rate must be positive")
  require(capacity > 0, "capacity must be positive")
  private var tokens = capacity.toDouble
  private var lastRefill = System.nanoTime()

  def tryAcquire(amount: Int = 1): Boolean = synchronized {
    require(amount > 0 && amount <= capacity, "invalid amount")
    val now = System.nanoTime()
    val elapsed = (now - lastRefill).toDouble / 1e9
    tokens = math.min(capacity.toDouble, tokens + elapsed * ratePerSecond)
    lastRefill = now
    if tokens >= amount then { tokens -= amount; true } else false
  }