CS 2420-20 Homework 22

Due: Tuesday, April 26th, 2011 9:10am

Start with field.zip, which implements a game simulation using Racket message passing.

The game simulation is just like the C lock-based version, except for one case: When a player tries to move to a position where a second player is standing, if the second player is involved in a contest, then the moving player is “bumped” and does not move. In the C lock-based version, the moving player instead waits until the contest is over, at which point it might have a chance to steal from or pass to the second player.

Fix the message-passing simulation to match the lock-based simulation.

Here’s one possible strategy:

• In addition to the players and contests lists of serve, keep a delays list. The lists contains requests that have been received, but that couldn’t be answered, yet. The new delays list will need to be passed around just like the players and contests list.
• In handle-player, instead of replying with (bump) when an adjacent player is busy, add a record to delays. The record should keep the request and the requesting player; do not reply to the request. Also, remove the requesting player from players.
• At the start of serve, before accepting any new requests, check whether an item in delays has a move that would hit a player that is now in players. In that case, a contest for the hit player must have completed, so call handle-player on the delayed request — removing the request from delays and adding the requesting player back into players.