title: Scroll backward circular array
body: <p>I have an array and I want to scroll the positions in it backward.</p>
  
  <p>I have a boolean array and I need to count how many true are there from a given cell going forward and backward.</p>
  
  <p>I know that if N is the number of elements into the array, I can go ahead doing i=(i+1)%N.
  But what if I need to scroll it in the opposite way?</p>
  
  <p>Thank you in advance.</p>