State Driven Agents
CIS 479/579
Bruce R. Maxim
UM-Dearborn
3/24/2016
1
Buckland Software
• The executables and source code available
http://www.wordware.com/files/ai
The Boost library is available from
http://www.boost.org/
• The Visual C++ environment need to be
configured to compile the source code by
setting the appropriate paths
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Paths
• Include file paths
C:\boost
C:\AI_Source\Common
C:\AI_Source\Common\lua-5.0\include
C:\AI_Source\Common\laubind
• Source file paths
C:\AI_Source\Common
C:\AI_Source\Common\lua-5.0
C:\AI_Source\Common\laubind
• Library file paths
C:\AI_Source\Common\lua-5.0\lib
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FSM Agent Advantages
•
•
•
•
Quick design and simple to code
Easy to debug
Low computational overhead
Intuitive so they make sense to domain
experts
• Flexible, easy for programmers to tweak to
modify behaviors
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FSM State Examples
• Pacman ghosts
– evade, chase
• Quake bots
– find armor, find health, seek cover, run away
• FIFA soccer players
– strike, dribble, chase ball, mark player
• NPC’s in real time strategy games
– move to position, patrol, follow path
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Switch Implementation - 1
• Define an enumerated type with the state names
enum StateType(RunAway, Patrol, Attack);
• Then define method that manages the transitions
void Agent::Update(StateType CurrentState)
{
switch (CurrentState)
{
case Runaway:
EvadeEnemy( );
if (Safe( ))
ChangeState(Patrol);
break;
…
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Switch Implementation - 2
• This works OK for simple FSM’s
• Becomes overly complicated for FSM’s with
lots of states and transitions
• Requires recompilation if states are changed
• Not easy to have simultaneous behaviors
implemented (run and shoot) without defining
lots of states
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State Transition Table -1
Current State
Condition
State Transition
Runaway
Safe
Patrol
Attack
Weaker than enemy
Runaway
Patrol
Threatened and
stronger than enemy
Attack
Patrol
Threatened and
weaker than enemy
Runaway
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State Transition Table - 2
• Agent queries table as needed
• Table can stored in file and edited without
recompiling the program
• Each state can be modeled as a separate
objects or functions called by the agent
• Fairly easy to extend and modify
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State Design Pattern - 1
• The state transition rules can be embedded in
the states themselves
• All states share a common interface using a
pure virtual class
class State
{
public:
virtual void Execute(Troll* troll) = 0;
}
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State Design Pattern - 2
class Troll
{
State* m_pCurrentState;
public:
void Update()
{
m_pCurrentState->Execute(this);
}
void ChangeState(const State* pNewState)
{
delete m_pCurrentState;
m_pCurrentState = pNewState;
}
}
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State Design Pattern - 3
class State_Runaway : public State
{
public:
void Execute(Troll* troll)
{
if (troll->isSafe())
troll->ChangeState(new State_Sleep());
else
troll->MoveAwayFromEnemy();
}
}
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State Design Pattern - 4
class State_Sleep : public State
{
public:
void Execute(Troll* troll)
{
if (troll->isThreatned())
troll->ChangeState(new State_RunAway());
else
troll->Snore();
}
}
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West World Miner States
pockets full
EnterMineDigForGold
thirsty
not thirsty
not wealthy
VisitBankDepositGold
QuenchThirstSaloon
rested
wealthy enough
GoHomeSleep
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Classes
• All West World inhabitants are derived from
the base class BaseGameEntity
• Each entity will be assigned a unique
identifier
• The Miner class is an example of one such
class
• The State class is the base abstract class
and each of the four Miner states needs its
own class definition
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Reusable Classes
• The State base class and the Miner owned
classes would be more easily reused if were
templates
• Creating a StateMachine class with ways of
tracking the Miner’s current, previous, and
global states gives the FSM a crude memory
to allow more complex (context sensitive)
behaviors
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Adding New NPC
• Elsa only visits two states VisitBathroom
(1 chance in 10) or DoHouseWork
• This makes use of the template version of the
State class and required the definition of a
new global state WifeGlobalState to deal
with the bathroom visits
• This requires adding classes MinersWife
and MinersWifeOwnedStates
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Adding NPC Communication
• Good games make use of event driven
architectures
• When events occur they are broadcast to
other game objects who are programmed to
react to the event or ignore it
• Intelligent game agents can send messages
to each other to allow cooperative behaviors
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Telegram Structure
struct Telegram
{
//the entity that sent this telegram
int
Sender;
//the entity that is to receive this telegram
int
Receiver;
//the message itself. These are all enumerated in the file
//"MessageTypes.h"
int
Msg;
//If a delay is necessary this field is stamped with the time
//the message should be dispatched.
double
DispatchTime;
//any additional information that may accompany the message
void*
ExtraInfo;
// constructors not shown here
}
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Message Types
#include <string>
enum message_type
{
Msg_HiHoneyImHome,
Msg_StewReady,
};
inline std::string MsgToStr(int msg)
{
switch (msg)
{
case 1:
return "HiHoneyImHome";
case 2:
return "StewReady";
default:
return "Not recognized!";
}
}
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Elsa’s New States
CookStew
1 chance in 10
Previous state
Delayed message to self
“Stew Ready”
VisitBathroom
1 chance in 10
Receive “honey I’m home”
previous state
DoHouseWork
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Message Management
• Creation, dispatch, and management of
telegrams is handled by the class
MessageDispatcher
• The method DispatchMessage is used by
every agent to send a message to entity with
a known ID
• The database of ID’s are managed by the
class EntityManager
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Use Priority Queue
• Telegrams with delays are store in a priority
queue ordered by their Time Stamp data
• Telegrams without delays (e.g. “I just shot
you with my crossbow”) are sent right away
• The front queue message is removed long
with any unprocessed expired telegrams
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FSM Changes
• The BaseEntity class needs to be modified
(add HandleMessage) so that any derived
class can receive messages
• The State class needs to be modified so
that BaseEntity states can choose to
accept and handle messages (add
OnMessage method)
• The StateMachine class needs its own
version of HandleMessage
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Elsa Cooks Dinner
• Miner enters shack and sends “honey I’m home”
message
• Elsa receives “honey I’m home” and changes state to
CookStew
• Elsa puts Stew in oven and sends delayed message
“Stew ready”
• Elsa receives “Stew ready”, takes out stew, and
sends Miner “Dinner ready” message
• Miner receives “Dinner ready” and changes state to
EatStew
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What happens?
• In between messages the NPC’s are free to
respond to all state appropriate inputs
• This may means that the Miner has left the
building before receiving the “Dinner ready”
message
• Sometimes you need two FSM’s working in
parallel (one for character movement and one
for character’s weapon)
• Sometimes you need nested state machines
(aka hierarchical FSM’s)
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