defmodule TaskAwaiter do
  use GenServer

  defstruct [:state, :condition_verifier, :result_acceptor, :awaiter_notifier]

  # API
  def start_link(initial_state, initializer, condition_verifier, result_acceptor, awaiter_notifier, gen_server_options \\ []) do
    GenServer.start_link(__MODULE__, [initial_state, initializer, condition_verifier, result_acceptor, awaiter_notifier], gen_server_options)
  end

  def task_completed(server, result) do
    server |> GenServer.cast({:accept_result, result})
  end

  # Callbacks
  def init([initial_state, initializer, condition_verifier, result_acceptor, awaiter_notifier]) do
    if initializer do
      initializer.(initial_state)
    else
      initial_state
    end
    {:ok, %TaskAwaiter{state: initial_state, condition_verifier: condition_verifier, result_acceptor: result_acceptor, awaiter_notifier: awaiter_notifier}}
  end

  def handle_cast({:accept_result, result}, %TaskAwaiter{state: awaiter_state, result_acceptor: result_acceptor, condition_verifier: condition_verifier, awaiter_notifier: awaiter_notifier} = state) do
    new_awaiter_state = awaiter_state |> result_acceptor.(result) |> coalesce(condition_verifier, awaiter_notifier)
    {:noreply, %{state|state: new_awaiter_state}}
  end

  def handle_info({ref, _result} = msg, state) when ref |> is_reference() do
    self() |> task_completed(msg)
    {:noreply, state}
  end

  def handle_info(_, state) do
    {:noreply, state}
  end

  # Helper functions
  def coalesce(value, condition, transformation) do
    if condition.(value) do
      transformation.(value)
    else
      value
    end
  end
end