Mergesort in Ruby

gistfile1.rb

class Array
  def merge_sort
    return self if length <= 1
    a, b = self.half.map { |e| e.merge_sort }
    merge(a, b)
  end
  def half
    mid = length / 2
    return slice(0...mid), slice(mid..-1)
  end
  def merge(a, b)
    result = []
    until a.empty? && b.empty?
      result <<
      case
      when a.empty? then b.shift
      when b.empty? then a.shift
      when a.first <= b.first then a.shift
      else b.shift
      end
    end
    result
  end
end

ar = [5,3,2,7,10,2,12]
p ar.merge_sort
# [2, 2, 3, 5, 7, 10, 12]