Hey, I could not understand the solution of a problem and I need some help.

Here's the problem I am talking about. BOI 2009, Candy Machine.

In this solution, I understood the O(n^2) algo with OWSP. Can you please explain what the O(n log n) solution (the code part) is doing.

Why is it sorting on basis of (t + p, t — p)? And what is the binary search on?

Thanks.