Run Length Encoding in Elixir.

run_length.exs

defmodule RunLength do

  def encode(data) do
    String.graphemes(data) |> do_encode
  end

  defp do_encode([]) do
    ""
  end

  defp do_encode([h|t]) do
    do_encode(1, h, t)
  end

  defp do_encode(1, c, []) do
    "#{c}"
  end

  defp do_encode(n, c, []) do
    "#{n}#{c}"
  end

  defp do_encode(n, c, [c|t]) do
    do_encode(n + 1, c, t)
  end

  defp do_encode(n, c, [h|t]) do
    do_encode(n, c, []) <> do_encode(1, h, t)
  end

  def decode(data) do
    String.graphemes(data) |> do_decode
  end

  defp do_decode([]) do
    ""
  end

  defp do_decode([h|[]]) do
    h
  end

  defp do_decode([first|t]) do
    [second|_] = t
    case Integer.parse(first) do
      :error -> first <> do_decode t
      {i, _} -> cond do
                  i > 2 -> second <> do_decode [Integer.to_string(i-1)|t]
                  i == 2 -> second <> do_decode t
                end
    end
  end
end

ExUnit.start

defmodule RunLengthTest do
  use ExUnit.Case, async: true

  test "encode" do
    x = RunLength.encode("ELIIIIXIIIIR")
    assert x == "EL4IX4IR"
  end

  test "encode empty returns empty" do
    x = RunLength.encode("")
    assert x == ""
  end

  test "decode" do
    x = RunLength.decode("EL4IX4IR")
    assert x == "ELIIIIXIIIIR"
  end
end