Range Extractor from Codewars

range_extractor.exs

# http://www.codewars.com/kata/51ba717bb08c1cd60f00002f

defmodule RangeExtractor do
  def solution([head | tail]) do
    _solution(tail, [head], [])
  end
    
  defp _solution([], [], acc) do
    acc 
    |> Enum.reverse 
    |> Enum.join(", ")
  end
  
  defp _solution([], range, acc) when length(range) >= 3 do
    _solution([], [], [format_range(range) | acc])
  end
  
  defp _solution([], range, acc) do
    _solution([], [], range ++ acc)
  end
  
  defp _solution([head | tail], range, acc) when (hd(range) + 1) == head do
    _solution(tail, [head | range], acc)
  end

  defp _solution([head | tail], range, acc) when length(range) >= 3 do
    _solution(tail, [head], [format_range(range) | acc])
  end

  defp _solution([head | tail], range, acc) do
    _solution(tail, [head], range ++ acc)
  end
    
  defp format_range(range) do
    last = hd(range)
    first = range |> Enum.reverse |> hd
    "#{first}-#{last}"
  end
end

list = [-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]

list 
|> RangeExtractor.solution 
|> IO.puts