Shuffle an array's elements, without using Ruby's built-in methods.

shuffle_array.rb

class Array
  def mixup
    length = self.length
    while length > 0
      length -= 1
      break if length == 0
      randomIndex = (rand * (length - 1)).floor
      self[length], self[randomIndex] = self[randomIndex], self[length]
    end
  end
end