The server and client have shared code. To be sure to have the same algorithm and not send by network (both server and client calculate the same, then no network communication needed).
- You walk into the grass, thanks to the random list pre-loaded by the client, the client and server have same random list, and use number to do the random match into the grass
- When it's append, both client and server know it, and switch into match mode, without any communication
- Both use the same random list to define the details (monster, level, ...)
No communication to random step into the grass, no to start the match.
See the random list.
Single player case
That's allow to do single player game too.