runlength.ex

runlength.ex

#
# A solution to the problem defined at https://github.com/marjaimate/runlength/
#

defmodule Runlength do
  def encode(string) do
    String.codepoints(string)
    |> Enum.reverse
    |> Enum.reduce([], &char_to_tuples/2)
    |> Enum.reduce("", fn ({char, count}, acc) -> "#{acc}#{Integer.to_string(count)}#{char}" end)
  end

  def char_to_tuples(char, []) do
    [{char, 1}]
  end

  def char_to_tuples(char, [{char, count} | t]) do
    [{char, count + 1} | t]
  end

  def char_to_tuples(char, acc) do
    [{char, 1} | acc]
  end


  def decode(string) do
    string = String.codepoints(string)
    expand(string, "")
  end

  def expand([], acc) do acc end

  def expand([count_str | [char | tail]], acc) do
    {count, _} = Integer.parse(count_str)
    next = String.duplicate(char, count)
    expand(tail, "#{acc}#{next}")
  end
end