Bounded LRU cache with explicit Elixir state

defmodule LruCache do
  def new(capacity) when is_integer(capacity) and capacity > 0, do: %{capacity: capacity, data: %{}, order: []}

  def get(cache, key) do
    case Map.fetch(cache.data, key) do
      {:ok, value} -> {value, touch(cache, key)}
      :error -> {nil, cache}
    end
  end

  def put(cache, key, value) do
    data = Map.put(cache.data, key, value)
    order = [key | List.delete(cache.order, key)]
    {dropped, kept} = Enum.split(order, cache.capacity)
    {Enum.reverse(dropped), %{cache | data: Map.drop(data, dropped), order: kept}}
  end

  defp touch(%{order: order} = cache, key), do: %{cache | order: [key | List.delete(order, key)]}
end