Immutable token bucket rate limiter in Elixir

defmodule TokenBucket do
  @default_rate 5.0
  @default_burst 5

  def new(rate \\ @default_rate, burst \\ @default_burst) do
    %{rate: rate, burst: burst, tokens: burst * 1.0, at: System.monotonic_time(:millisecond)}
  end

  def take(bucket, count \\ 1) do
    now = System.monotonic_time(:millisecond)
    elapsed = max(now - bucket.at, 0) / 1000
    tokens = min(bucket.burst * 1.0, bucket.tokens + elapsed * bucket.rate)
    if tokens >= count do
      {:ok, %{bucket | tokens: tokens - count, at: now}}
    else
      {:error, tokens, %{bucket | tokens: tokens, at: now}}
    end
  end
end