Functional Katas, Elixir Special! 8-6-2017

runlength.ex

defmodule Runlength do
  def encode(string) do
    String.codepoints(string)
    |> List.foldl([], &count/2)
    |> List.foldr("", &format/2)
  end

  defp count(char, [{char,n}|tail]) do
    [{char, n+1}|tail]
  end
  defp count(char, acc) do
    [{char, 1}|acc]
  end
  defp format({char, occ}, output) do
    output <> Integer.to_string(occ) <> char
  end

  def decode(string) do
    String.codepoints(string)
    |> expand("")
  end

  defp expand([], acc) do acc end
  defp expand([occ,char|tail], acc) do
    String.duplicate(char, String.to_integer(occ)) <> expand(tail, acc)
  end
end