accountant_7.ex

defmodule Accountant do
  @sum 2020
  def product_of_equals_twenty_twenty([]), do: :noop
  def product_of_equals_twenty_twenty([_head | tail] = list) do
    product = get_two(list)
    if product do
      product
    else
      product_of_equals_twenty_twenty(tail)
    end
  end

  defp get_two(list) when length(list) == 1, do: nil

  defp get_two([first | [second | _rest]]) when first + second == @sum do
    first * second
  end

  defp get_two([first | [_second | rest]]) do
    get_two([first | rest])
  end
end