defmodule Grains do
  @doc """
  Calculate two to the power of the input minus one.
  """
  @spec square(pos_integer()) :: {:ok, pos_integer()} | {:error, String.t()}
  def square(number) when number < 1 or number > 64 do
    {:error, "The requested square must be between 1 and 64 (inclusive)"}
  end
  def square(number), do: {:ok, 2**(number-1)}

  @doc """
  Adds square of each number from 1 to 64.
  """
  @spec total :: {:ok, pos_integer()}
  def total do
    Enum.reduce_while(1..64, {:ok, 0}, fn n, {:ok, sum} ->
      case square(n) do
        {:ok, squared} -> {:cont, {:ok, squared + sum}}
        {:error, error} -> {:halt, {:error, error}}
      end
    end)
  end
end