Elixir Quiz #2: Run Length Encoding

run_length_encoder.ex

# Given a string of uppercase characters in the range A-Z, 
# replace runs of sequential characters with a single 
# instance of that value preceded by the number of items 
# in the run.
defmodule RunLengthEncoder do

  def encode(string) do
    encode_aux(String.codepoints(string), [])
  end

  def encode_aux([], result) do
    result 
    |> Enum.reverse
    |> Enum.map(fn {char, freq} -> "#{freq}#{char}" end)
    |> to_string
  end

  def encode_aux([h1|t1], []) do
    encode_aux(t1, [{h1,1}])
  end

  def encode_aux([h1|t1], [{h1,n}|t2]) do
    encode_aux(t1, [{h1,n+1}|t2])
  end

  def encode_aux([h1|t1], result) do
    encode_aux(t1, [{h1,1}|result])
  end

end

defmodule RunLengthEncoderTest do
  use ExUnit.Case, async: true

  test "encoding works with all uppercase" do

    input  = "JJJTTWPPMMMMYYYYYYYYYVVVVVV"
    output = "3J2T1W2P4M9Y6V"

    assert RunLengthEncoder.encode(input) == output
  end

  test "encoding works with all unique letters" do

    input  = "HORSE"
    output = "1H1O1R1S1E"

    assert RunLengthEncoder.encode(input) == output
  end
end