accountant_10.ex

defmodule Accountant do
  @sum 2020
  defguard equals_twenty_twenty(first, second) when first + second == @sum

  def product_of_equals_twenty_twenty(list, product \\ nil)
  def product_of_equals_twenty_twenty(_list, product) when product != nil, do: product

  def product_of_equals_twenty_twenty([_head | tail] = list, _product) do
    product_of_equals_twenty_twenty(tail, get_two(list))
  end

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

  defp get_two([first | [second | _rest]]) when equals_twenty_twenty(first, second) do
    first * second
  end

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