DNA Encoding

	defmodule DNA do
	  def encode_nucleotide(? ), do: 0
	  def encode_nucleotide(?A), do: 1
	  def encode_nucleotide(?C), do: 2
	  def encode_nucleotide(?G), do: 4
	  def encode_nucleotide(?T), do: 8

	  def decode_nucleotide(0), do: ? 
	  def decode_nucleotide(1), do: ?A
	  def decode_nucleotide(2), do: ?C
	  def decode_nucleotide(4), do: ?G
	  def decode_nucleotide(8), do: ?T

	  def encode(dna) do
		to_charlist(dna)
		|> do_encode(<<>>) 
	  end

	  defp do_encode([], count), do: count
	  defp do_encode([char | rest], count), do: do_encode(rest, <<count::bitstring, encode_nucleotide(char)::4>>)

	  def decode(dna) do
		do_decode(dna, []) 
		|> reverse
	  end

	  defp do_decode(<<>>, count), do: count
	  defp do_decode(<<ele :: 4, rest :: bitstring>>, count) do
		nucleotide = decode_nucleotide(ele)
		do_decode(rest, [nucleotide | count])
	  end

	  defp reverse(list) do
		reverse_new(list, [])
	  end
	  defp reverse_new([], count), do: count
	  defp reverse_new([head | tail], count), do: reverse_new(tail, [head | count])
	end