defmodule GenTimer.Timer do
  @type t :: %__MODULE__{
          duration: non_neg_integer(),
          state: :initial | :paused | :active | :finished,
          time_passed: non_neg_integer()
        }

  @enforce_keys [:duration]
  defstruct [:duration, state: :initial, time_passed: 0]

  @spec new(non_neg_integer()) :: t()
  def new(duration) do
    struct!(__MODULE__, duration: duration)
  end

  @spec start(t()) :: t()
  def start(timer) do
    change_state(timer, :active)
  end

  @spec increment(t()) :: t()
  def increment(%__MODULE__{state: state} = timer) when state !== :active, do: timer

  def increment(%__MODULE__{time_passed: time_passed, duration: duration} = timer)
      when time_passed == duration - 1 do
    timer
    |> increment_time_passed()
    |> change_state(:finished)
  end

  def increment(timer) do
    increment_time_passed(timer)
  end

  @spec pause(t()) :: t()
  def pause(timer) do
    change_state(timer, :paused)
  end

  @spec reset(t()) :: t()
  def reset(timer) do
    timer
    |> reset_counter()
    |> change_state(:initial)
  end

  defp change_state(timer, next_state) do
    Map.replace(timer, :state, next_state)
  end

  defp reset_counter(timer) do
    Map.replace(timer, :time_passed, 0)
  end

  defp increment_time_passed(timer) do
    Map.update!(timer, :time_passed, &(&1 + 1))
  end
end