Runlength.ex

defmodule Runlength do
  defp encode([], cur_char, n, cur_encoded), do: "#{cur_encoded}#{n}#{<<cur_char>>}"
  defp encode([h | tail], cur_char, n, cur_encoded) do
    case h do
      ^cur_char -> encode tail, h, n + 1, cur_encoded
      _         -> encode tail, h, 1    , "#{cur_encoded}#{n}#{<<cur_char>>}"
    end
  end

  def encode(string) do
    [h|tail] = String.to_charlist string
    encode tail, h, 1, ""
  end


  defp decode([], acc), do: acc
  defp decode([n_char, c | tail], acc) do
    {n, ""} = Integer.parse <<n_char>>
    decode tail, (acc <> String.duplicate(<<c>>, n))
  end

  def decode(string) do
    decode String.to_charlist(string), ""
  end
end