Elixir Run-Length Encoding

run_length.ex

defmodule RunLength do
  def run(string) do
    compress(String.slice(string, 1..-1), String.slice(string, 0, 1), 1, "")
  end

  defp compress("", char, count, result) do
    result <> Integer.to_string(count) <> char
  end

  defp compress(string, char, count, result) do
    next = String.slice(string, 0, 1)
    if next == char do
      compress(String.slice(string, 1..-1), char, count+1, result)
    else
      compress(
        String.slice(string, 1..-1),
        next, 
        1,
        result <> Integer.to_string(count) <> char
      )
    end
  end
end