Conor, Fernando, Davide' s solution for run length encoder/decoder at github.com/marjaimate/runlength

runlength.ex

# Conor, Fernando, Davide + wisdom of the crowd solution for
# https://github.com/marjaimate/runlength

defmodule Runlength do
  def encode(string) do
    encode(string, "")
  end

  def encode("", acc) do
    acc
  end
  def encode(<<char :: binary-size(1), tail :: binary>> = whole_thing, acc) do
   remaining = String.trim_leading(whole_thing, char)

   occurrences = String.length(whole_thing) - String.length(remaining)

   encode(remaining, acc <> Integer.to_string(occurrences) <> char)
  end


  def decode(string) do
    decode(string, "")
  end

  def decode("", acc), do: acc
  def decode(string, acc) do
   {occurrences, rest} = Integer.parse(string)

   <<char :: binary-size(1), remaining :: binary>> = rest

   decode(remaining, acc <> String.duplicate(char, occurrences))
  end
end