divisible.ex

defmodule Divisible do

  def sum_reversed(number, target) do
    enumerate(number, target)
    |> Stream.filter(&(is_divisible_reversed(&1, number)))
    |> Enum.sum
  end

  def enumerate(number, target) do
    Stream.iterate(number, &(&1 + number))
    |> Stream.take_while(&(&1 <= target))
  end

  def is_divisible_reversed(number, divisor) do
    number
    |> to_char_list
    |> Enum.reverse
    |> List.to_integer
    |> rem(divisor) === 0
  end
end

divisible_test.exs

defmodule DivisibleTest do
  use ExUnit.Case

  test "sum multiples of 7, up to 100" do
    assert Divisible.sum_reversed(7, 100) == 154
  end

  test "sum multiples of 7, up to 1000" do
    assert Divisible.sum_reversed(7, 1000) == 10787
  end

  test "enumeration of multiples of 7, up to 100" do
    assert Divisible.enumerate(7, 100) |> Enum.take(15) == [7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98]
  end

  test "reversed numbers divisible by the multiplier" do
    assert Divisible.is_divisible_reversed(7, 7)
    assert Divisible.is_divisible_reversed(161, 7)
    refute Divisible.is_divisiible_reversed(49, 7)
  end
end