sum via recursion.ex

defmodule Summer do
  def sum_of_threes_and_fives(n) when n <= 0 do
    0
  end

  def sum_of_threes_and_fives(n) when rem(n, 5) == 0 or rem(n, 3) == 0 do
    n + sum_of_threes_and_fives(n - 1)
  end

  def sum_of_threes_and_fives(n) do
    sum_of_threes_and_fives(n - 1)
  end
end

defmodule HelloTest do
  use ExUnit.Case

  test "the truth" do
    assert Summer.sum_of_threes_and_fives(3) == 3
    assert Summer.sum_of_threes_and_fives(5) == 8
    assert Summer.sum_of_threes_and_fives(20) == 98
    assert Summer.sum_of_threes_and_fives(999) == 233168
  end
end