Some area has 50% chance of colliding

I used to think that bullet collide detection works like:

1. all bullets move 2. detect every collection

but it actually works like this: (can be found in robocode BulletPeer.java)

1. first bullet move, and detect collision with the rest 2. second bullet move, and detect collision with the rest 3. etc.

as the order is randomized each tick, each moved bullet has 50% chance of colliding with a bullet that is not yet moved.