quicksort.rb

def quick_sort(array, first, last)
  if first < last
    q = partition(array, first, last)
    quick_sort(array, first, q)
    quick_sort(array, q+1, last)
  end
  array
end

def partition(array, low_index, high_index)
  i = low_index - 1
  j = high_index + 1
  pivot = array[low_index]
  
  loop do
    begin
      i += 1
      break if i == high_index
    end until array[i] >= pivot

    begin 
      j -= 1
      break if j == low_index
    end until array[j] <= pivot
    
    # break the loop if pointers cross    
    break if i >= j
    array[i], array[j] = array[j], array[i]
  end
  
  return j
end

p quick_sort([7,3,11,1,2,6,10,8,9,4,5],0,10)