Heap sort in Ruby

heap_sort.rb

class HeapSort
	attr_accessor :input
	
	def initialize(arg)
		@input=arg
	end
	
	def heap_sort
	  heapify
	  the_end=input.length-1
	  while the_end > 0
		input[the_end],input[0]=input[0],input[the_end]
		the_end-=1
		shift_down(0,the_end)
	  end
	  input
	end
	
	def heapify
	    the_start=(input.length-2)/2
	     
	     while the_start >=0 
			shift_down(the_start,input.length-1)
			the_start-=1
	     end
	end
	
	def shift_down(the_start,the_end)
		root=the_start
		while root*2+1 <= the_end
			child=root*2+1
			swap=root
			
			if input[swap] < input[child]
				swap=child
			end
			
			if child+1 <= the_end && input[swap] < input[child+1]
				swap=child+1
			end
			
			if swap!=root
				input[root],input[swap]=input[swap],input[root]
				root=swap
			else
				return
			end
					
		end
	end
	
end