Artificial Intelligence - Game Programming Algorithms and Techniques - page 238

Ghost AI in Pac-Man
At first glance, it may seem like the AI for the arcade classic Pac-Man
is very basic. The ghosts appear to either chase after the player or run
from the player, so one might think that it's just a binary state machine.
However, it turns out that the ghost AI is relatively complex.
Toru Iwatani, designer of Pac-Man , spoke with Susan Lammers on this
very topic in her 1986 topic Programmers at Work . He “wanted each
ghostlyenemytohaveaspecificcharacteranditsownparticularmove-
ments, so they weren't all just chasing after Pac-Man...which would
have been tiresome and flat.”
Each ofthe fourghosts have fourdifferent behaviors to define different
target points in relation to Pac-Man or the maze. The ghosts also al-
ternate between phases of attacking and dispersing, with the attack
phases increasing proportionally as the player progresses to further and
further levels.
An excellent write up on the implementation of the Pac-Man ghost AI
is available on the Game Internals blog at http://tinyurl.com/238l7km .
Basic State Machine Implementation
A state machine can be implemented in several ways. A minimum requirement is
that when the AI updates itself, the correct update action must be performed based
on the current state of the AI. Ideally, we would also like our state machine imple-
mentation to support enter and exit actions.
If the AI only has two states, we could simply use a Boolean check in the AI's
Update function.Butthissolutionisnotveryrobust.Aslightlymoreflexibleim-
plementation, often used in simple games, is to have an enumeration ( enum ) that
represents all of the possible states. For instance, an enum for the states in Figure
9.9 would be the following:
enum AIState
Patrol,
Death,
Attack
end