Wow, I did not know about it till last week and when I read about it, I was like where has this been all my life 🙂
Anyways its called Pairing Functions, which allows you to reduce 2 positive numbers into 1 and back.
Uses: Imagine you had a set of objects called Pair(long,long) and you wanted to search if a particular pair exists in that set, the usual way would be implement hashcode and equals . But with a pairing function you can reduce it to a set of longs.
In my experiments I saw an improvement of 30-40%.
For more info