Ruby Merge Sort

merge_sort.rb

def merge_sort(array)
  return array if array.length <= 1

  middle = array.length / 2
  left   = merge_sort(array[0...middle])
  right  = merge_sort(array[middle..])
  merge(left, right)
end

def merge(left, right)
  return left  if right.empty?
  return right if left.empty?

  if left.first < right.first
    [left.first] + merge(left[1..], right)
  else
    [right.first] + merge(left, right[1..])
  end
end