merge and count inversions in array

gistfile1.rb

def sort_and_count(list)
  return [0, list] if list.size <= 1

  mid = list.size / 2
  left  = list[0, mid]
  right = list[mid, list.size-mid]

  left_result = sort_and_count(left)
  right_result = sort_and_count(right)
  merge_result = merge_and_count(left_result[1], right_result[1])

  [left_result[0] + right_result[0] + merge_result[0], merge_result[1]]
end

def merge_and_count(left, right)
  inversions = 0
  sorted = []

  until left.empty? || right.empty?
    if left.first <= right.first
      sorted << left.shift
    else
      sorted << right.shift
      inversions += left.size
    end
  end

  sorted.concat(right).concat(left)

  [inversions, sorted]
end

result = sort_and_count []
puts "number of inverstions: #{result[0]}"
puts "ordered array: #{result[1]}"