solution.ex

defmodule Mumbling do
  
  def accum(s) do
    accum(String.graphemes(s), 1, [])
  end
  
  defp accum([], n, acc) do
    acc
    |> Enum.reverse()
    |> Enum.join("-")
  end

  defp accum([s | rest], n, acc) do
    accum(rest, n+1, [duplicate(s, n) | acc])
  end
   
  def duplicate(s, n) when n > 0, do: duplicate(s, n, [])
  
  defp duplicate(s, 1, acc) do
    List.to_string([String.upcase(s) | acc])
  end
  
  defp duplicate(s, n, acc) do
    duplicate(s, n-1, [String.downcase(s) | acc])
  end
  
end

test.ex

defmodule MumblingTest do
  
  use ExUnit.Case
  
  defp testing(numtest, s, ans) do 
    IO.puts("Test #{numtest} \n")
    assert Mumbling.accum(s) == ans
  end
 
  test "duplicate" do
    assert Mumbling.duplicate("p", 1) == "P"
    assert Mumbling.duplicate("z", 3) == "Zzz"
    assert Mumbling.duplicate("M", 5) == "Mmmmm"
  end
  
  test "accum" do 
    testing 1, "ZpglnRxqenU", "Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnnnnnnn-Uuuuuuuuuuu"
    testing 2, "NyffsGeyylB", "N-Yy-Fff-Ffff-Sssss-Gggggg-Eeeeeee-Yyyyyyyy-Yyyyyyyyy-Llllllllll-Bbbbbbbbbbb"
    testing 3, "MjtkuBovqrU", "M-Jj-Ttt-Kkkk-Uuuuu-Bbbbbb-Ooooooo-Vvvvvvvv-Qqqqqqqqq-Rrrrrrrrrr-Uuuuuuuuuuu"
    testing 4, "EvidjUnokmM", "E-Vv-Iii-Dddd-Jjjjj-Uuuuuu-Nnnnnnn-Oooooooo-Kkkkkkkkk-Mmmmmmmmmm-Mmmmmmmmmmm"
    testing 5, "HbideVbxncC", "H-Bb-Iii-Dddd-Eeeee-Vvvvvv-Bbbbbbb-Xxxxxxxx-Nnnnnnnnn-Cccccccccc-Ccccccccccc"
  end
  
end