nl_buffer.ex

iex

alias Sandbox.Buffer

{:ok, pid} = Buffer.start_link([])
Buffer.push(pid, %{created_at: 5})
Buffer.push(pid, %{created_at: 7})
Buffer.push(pid, %{created_at: 1})
Buffer.inspect(pid)
Buffer.pop(pid) # nil
Process.sleep(3100)
Buffer.pop(pid) # created_at: 1
Buffer.pop(pid) # created_at: 5
Buffer.pop(pid) # created_at: 7
Buffer.pop(pid) # nil

nl_buffer.ex

defmodule Sandbox.Buffer do
  use GenServer

  # Client

  def start_link(default) when is_list(default) do
    GenServer.start_link(__MODULE__, default)
  end

  def push(pid, element) do
    GenServer.cast(pid, {:push, element})
  end

  def pop(pid) do
    GenServer.call(pid, :pop)
  end

  # Server

  @impl true
  def init(state) do
    {:ok, state}
  end

  @impl true
  def handle_cast({:push, element}, state) do
    # TODO: pass mapping function when creating buffer
    new_element = Map.put(element, :buffered_at, DateTime.utc_now())
    new_state = Enum.sort_by([new_element | state], fn item -> item.created_at end)
    {:noreply, new_state}
  end

  @impl true
  def handle_call(:pop, _from, []) do
    {:reply, nil, []}
  end

  def handle_call(:pop, _from, [head | tail]) do
    # TODO: pass condition function when creating buffer
    condition = fn item -> buffer_age(item) >= :timer.seconds(3) end

    if condition.(head) do
      {:reply, head, tail}
    else
      {:reply, nil, [head | tail]}
    end
  end

  def buffer_age(item) do
    DateTime.diff(DateTime.utc_now(), item.buffered_at) * 1000
  end
end