Merge Sort in Ruby

array.rb

# merge sort in ruby

class Array
  def merge_sort
    if length > 1
      halves = split.map do |half|
        half.merge_sort
      end
      combine(*halves)
    else
      self
    end
  end

  private

  def split
    half_length = length / 2
    [ shift(half_length), self ]
  end

  def combine(x, y)
    return y if x.empty? or x.nil?
    return x if y.empty? or y.nil?

    if x.first < y.first
      [x.shift] + combine(x, y)
    else
      [y.shift] + combine(x, y)
    end
  end
end