ExUnit.start(seed: 42)

defmodule CounterLabTest do
  use ExUnit.Case, async: false

  defp with_counter(fun) do
    {:ok, counter} = Agent.start_link(fn -> 0 end)
    try do
      fun.(counter)
    after
      Agent.stop(counter)
    end
  end

  test "separate get and update lose an update despite one owner" do
    with_counter(fn counter ->
      parent = self()
      tasks =
        for _ <- 1..2 do
          Task.async(fn ->
            old = Agent.get(counter, & &1)
            send(parent, {:ready, self()})
            receive do
              :go -> Agent.update(counter, fn _current -> old + 1 end)
            after
              5_000 -> raise "barrier timeout"
            end
          end)
        end
      clients =
        for _ <- 1..2 do
          receive do
            {:ready, pid} -> pid
          after
            5_000 -> raise "client timeout"
          end
        end
      Enum.each(clients, &send(&1, :go))
      Enum.each(tasks, &Task.await(&1, 5_000))
      assert Agent.get(counter, & &1) == 1
    end)
  end

  test "one update callback owns the entire increment" do
    with_counter(fn counter ->
      1..20
      |> Enum.map(fn _ -> Task.async(fn -> Agent.update(counter, &(&1 + 1)) end) end)
      |> Enum.each(&Task.await(&1, 5_000))
      assert Agent.get(counter, & &1) == 20
    end)
  end
end