6.5 Implementing a State Machine Language

State Machine in game AI The most used software pattern Simple to program Easy to comprehend Easy to debug Completely general to any problem Might not always provide the best solution But few can deny that they get the job done with minimal risk to the project

State Machine in game AI Disadvantage of state machine No consistent structure Development cycle churns on Poor structure This article presents Robust way to structure your state machine with a simple language Make programming games much easier

Game Developer-Style State Machine void RunLogic (int *state) { switch(*state){ case 0: //Wander Wander(); if(SeeEnemy()){ if(GetRandomChance() <0.8) *state =1; else *state =2; } if(Dead()) *state =3; break; case 1: //Attack Attack(); if(Dead()) *state =3; break; case 2: //RunAway RunAway(); if(Dead()) *state =3; break; case 3: //Dead SlowlyRot(); break; }

Game Developer-Style State Machine Serious weaknesses The state changes are poorly regulated States are of type int and would be more robust and debuggable as enums The omission of a single break keyword would couse hard- to-find bugs Redundant logic appears in multiple states No way to tell that a state has been entered for the first time No way to monitor or log how the state machine has behaved over time What is a poor game programmer to do? Provide some structure

A State Machine Language Be created with the help of macros Have six keyword BeginStateMachine EndStateMachine State OnEnter OnExit OnUpdate

The State Machine Language Example structure of the State Machine Language BeginStateMachine State(STATE_Wander) OnEnter // C or C++ code for state entry OnUpdate // C or C++ code executed every tick OnExit // C or C++ code for state clean-up State(STATE_Attack) OnEnter // C or C++ code for state entry EndStateMachine

Actual Implementation The six macro keywords #define BeginStateMachineif(state < 0){if(0){ #define EndStateMachine return(true);}}else(assert(0); \ return(false);}return(false); #define State(a) return(true);}}else if(a == state){if(0){ #define OnEvent(a) return(true);}else if(a == event){ #define OnEnterOnEvent(EVENT_Enter) #define OnUpdateOnEvent(EVENT_Update) #define OnExitOnEvent(EVENT_Exit)

Using macro Nice properties Expand in a building-block fashion Handled event Provide an easy way to monitor how the state machine is Can't mess up the state machine by forgetting something like a break keyword. Easier to read

Support function void StateMachine::Process(StateMachineEvent event){ States(event, m_currentState); int safetyCount = 10; while (m_stateChange && (-safetyCount >=0)) { assert ( safetyCount > 0 && "States are flip-flopping."); m_stateChange = false; //Let the last state clean-up States(EVENT_Exit, m_currentState); //Set the new state m_currentState = m_nextState; //Let the new state initialize States(EVENT_Entr, m_currentState); }

Integrating this Solution How is it actually integrated within a game? class Robot: public StateMachine{ public: Robot(void){}; ~Robot(void){}; private: virtual bool States(StateMachineEvent event, int state); // Put private robot specific variables here }; bool Robot::States (StateMachineEvent event, int state){ BeginStateMachine // Put any number of states and event responses. // Refer to the code on the CD for more examples. EndStateMachine }

Conclusion State Machine Language provides Simple enforced structure Excellent readability Natural to debug Full power of C/C++ within the state machine Easy to add code for entering or exiting a state State changes are more formal and protected Error checking for flip-flopping state changes No need to spend months developing a custom language

6.6 Enhancing a State Machine Language through Messaging

The Concept of Messages Message A number of enumerated type Communicate with messages between game objects The technique of messages Event-driven behavior enum MSG_Name { MSG_Attacked, MSG_Damaged, MSG_Healed, MSG_Poisoned };

Messages as Letters class MSG_Object { public: MSG_Namem_Name; // Message name(enumeration) float m_Data; // Extra data for the message objectID m_Sender; // Object tht sent the message objectID m_Receiver; // Object that will get the message float m_DeliveryTime; // Time at which to send the message };

Messages as Events Incorporate messages into the State Machine Language A message event BeginStateMachine State(STATE_Wander) OnMsg(MSG_Attacked) SetState(STATE_RunAway); State(STATE_RunAway) OnEnter //run away EndStateMachine

Message Timers When a game object sends a message to itself to create an internal event BeginStateMachine State(STATE_Wander) OnMsg(MSG_Attacked) SendDelayedMsgToMe(0.5, MSG_TimeOut); OnMsg(MSG_TimeOut) SetState(STATE_RunAway); State(STATE_RunAway) OnEnter //run away EndStateMachine

Scoping Messages Scoping Something is only valid within a certain context Scoping messages By defining a scope for messages Won't be misinterpreted in the wrong context The extraneous queued timeout messages are ignored

Scoping Messages BeginStateMachine State(STATE_Wander) OnMsg(MSG_Attacked) SendDelayedMsgToMe(0.5, MSG_TimeOut, SCOPE_TO_THIS_STATE); OnMsg(MSG_TimeOut) SetState(STATE_RunAway); State(STATE_RunAway) OnEnter SendDelayedMsgToMe(7.0, MSG_TimeOut); OnMsg(MSG_TimeOut) SetState(STATE_Wander); EndStateMachine

Redundant Message Policy Redundant Messages Multiple delayed messages All basically the same messages Accumulate in the message router Standard rules for redundant messages 1. Ignore the new message 2. Replace the old message with the new one 3. Store the new message, letting redundant messages accumulate The best policy is #1

Global Event Responses BeginStateMachine OnMsg(MSG_Dead) // Global response -Triggered regardless of state SetState(STATE_Dead); State(STATE_Wander) OnEnter //Wander State(STATE_RunAway) OnEnter //Run away State(STATE_Dead) OnEnter //Die OnMsg(MSG_Dead) //Ignore msg - this overrids the global response EndStateMachine

Recording Behavior for Debugging The need for good debugging of state machines is critical To monitor and record every event That's handled and not handled by each state machine Embed logging function Calls within the macro keywords themselves #define BeginStateMachine if(state < 0){char statename[64]= "STATE_Global"; if(0){ #define State(a) return(true);}}else if(a == state) \ {char statename[64] =#a; if(0){ #define OnMsg(a) return(true);}else if(EVENT_Message == \ event && msg && a == msg->GetMsgName()){ \ g_debuflog.LogStateMachineEvent( \ m_Owner->GetID(),msg,statename,#a,true);

Partial listing of the main function // Create game object GameObject myGameObject (g_database.GetNewObjectID()); g_database.Store(myGameObject); //Give the game object a state machine myGameObject.SetStateMachine (new Robot(&myGameObject)); myGameObject.GetStateMachine()>Initialized(); //Game Loop while(1) { g_time.MarkTimeThisTick(); myGameObject.GetStateMachine()->Update(); g_msgroute.DeliverDelayedMessages(); }

Summary Make games easier and more robust to code State machines Easy to read Easy to debug Easy to build and prototype A way to communicate with other game objects Global event responses to reduce redundant code A way to set event timers Allow transparent monitoring and logging of all events

Summary Allow execution of special code when first entered Allow execution of special code when exited Conclusion For simplicity For maintainability For robustness For ease of debugging