1. Finite State Machines in Games
Jarret Raim
2004
Based on
AI Game Programming Wisdom 2

2. Problem: Can’t remember what we were doing
FSM Example
States
Attack
Chase
Spawn
Wander
Events
E: see an enemy
S: hear a sound
D: die
Attack ~E ~S
Chase E S D D E
Wander E ~E
Spawn D
Problem: Can’t remember what we were doing

3. FSM Implementation - Code
Simplest method
After an action, the state might change.
Requires a recompile for changes
No pluggable AI
Not accessible to non-programmers
No set structure
Can be a bottleneck.
void RunLogic( int *state ) {
switch( *state ) {
case 0: //Wander
Wander();
if( SeeEnemy() )
*state = 1;
if( Dead() )
*state = 2;
break;
case 1: //Attack
Attack();
*state = 0;
case 3: //Dead
SlowlyRot() }

4. FSM Example Problem: Can’t remember what we were doing
Attack
Problem: Can’t remember what we were doing ~E ~S
Chase E S D D E
Wander E ~E
Spawn D
Solution: Add an stack to keep track of the states I have visited

5. Stack FSM – Thief 3
Stack allows AI to move back and forth between states.
Leads to more realistic behavior without increasing FSM complexity.