Multiplayer Interactive-Fiction Game-Design Blog

04 Combat AI goals

The AI that handles combat is a bit tricky, so this section

will be a tad long...

Every AI (derived from cAI) remembers a list of dangerous enemies,

as well as friends that will help in combat. (This list is different

than the like/dislike list also supplied for AI's.)

Characters that a NPC meets are rated from 10.0 to -10.0 on the

enemies list. Values between 10.0 and 1.0 mean that the character

is an enemy that will attack the NPC. .999 and -.999 is a neutral

character whose intentions are unknown. -1.0 to -10.0 is a trusted

friend that will help defend the NPC in combat.

When a NPC first encounters a character, a call

to AIEnemiesListGuess() is called to see if the

NPC considers the character an enemy. The default behavior

for this function is to:

1. If the AI has pAIEnemyOfAllPCs then the AI will

   automatically be an enemy of any character controlled by

   a player. This is a good setting for a monster.

   
   
   




2. If the AI was spawned by the same room as another AI then

   it will be a friend with the other AI.
   
   





3. If the AI is the same class as the other AI

   it's an automatic friend too. Thus, if a number of tigers

   are spawned in different rooms of the wilderness, they

   will act as friends.
   
   

You can provide your own AIEnemiesListGuess() function for

an AI.

The enemies rating is affected by what the AI sees:

1. When a NPC is attacked, the attacker is almost invariably added

   to the NPC's enemies list.

   See cAI.PerceiveAttack for the code.

   
   
   




2. When the NPC see a friend attacked,

   the attacker is likewise added (but not always).

   See cAI.PerceiveAttack for the code.

   
   
   




3. If a character

   attacks a NPC's enemy, they are (usually) made a friend.

   See cAI.PerceiveAttack for the code.

   
   
   




4. If an AI starts yelling out for help

   using AICombatCallForHelp(), the

   code in cAI.PerceiveSpeak() may cause any

   AI's that hear the yell to become enemies of the attackers.
   
   

You can programatically set and read the enemies' list values

using the following methods:

• 
  
  AIEnemiesCalculateTheOdds() - Used by AIs to

  determine if they're likely to win a battle of if their

  enemies are likely to win. It's based on AIEnemiesListUpdate().

  You probably won't need to use this method directly.

  
  
  



• 
  
  AIEnemiesListGet() - Returns the enemies rating,

  from 10 to -10, for a character. If the character has never

  been met it returns NULL.
  
  




• 
  
  AIEnemiesListGuess() - Call this if no rating

  is set. It will return a rating to begin with, or Undefined

  if there's no opinion whatsoever. Calling ToNumber (

  AIEnemiesListGuess()) will always return a number, since

  Undefined will be converted to 0.
  
  




• 
  
  AIEnemiesListSet() - Sets a new rating for the

  enemy or friend.

  
  
  



• 
  
  AIEnemiesListUpdate() - Finds all the characters

  in a room and checks their enemies' list score. If it's

  unknown, then it calls AIEnemiesListGuess() to fill them in.

  This method can optionally return a list of enemies, friends,

  or both enemies and friends.
  
  

AI's check for enemies in the room when:

• The AI walks into a new room.

  This code is on cAI.PerceiveInNewRoom().
  
  





• When a character walks into the AI's room.

  This code is on cAI.PerceiveActorEnter().
  
  





• When an AI sees one character attacking another.

  This code is on cAI.PerceiveAttack().
  
  

When danger might just have appeared, the AI code

calls AIPotentialConflict(). This checks for any

enemies in the room.

If there are enemies in the room, the code adds several

goals to the AI's list. The goals used are

in pAIGoalsConflict. If the goal is already in

the AI's list then it is not added a second

time.

The default goals that are added by pAIGoalsConflict are:

1. 
   
   oAIGoalCombatRetreat - This causes the AI to test

   out it's odds of winning every once in awhile. If it thinks its

   odds are still good the AI suspends the retreat goal for another

   10 seconds. If there are not enemies left, the retreat goal

   finishes.
   
   


If the goal decides that the character is in danger,

based on a call to AIEnemiesCalculateTheOdds(),

then the AI will run away. pAICombatBravery is

used to determine how much the odds have to be stacked against

the AI before it runs, as well

as pAICombatWoundsRetreat to see if

it will selfishly abandon it's friends when its wounded.

If pAICombatRunDrop is set to TRUE, the AI will

drop an item to distract its attackers.

If pAICombatRunCallForHelp is set to TRUE the

AI will yell for help as it runs.

If pAICombatRunForGuard is set to TRUE, the AI

will run to the nearest guard and ask it for help. (This

one is particularly useful for villagers, which should run at

the slightest sign of danger and seek a guard.)


2. 
   
   oAIGoalCombatCallForHelp - This causes the NPC

   to call out for help once in awhile. AI's listen

   for calls for help in their PerceiveSpeak() methods.

   If they detect a call, they determine if the calling NPC

   is a friend, enemy, or neutral. If it's a friend, they

   may run to the friend's aid

   if pAICombatCallForHelpRespondsFriends is not 0.

   Likewise, if they hear an enemy calling for help, they may

   decide to join in the kill.

   The pAICombatCallForHelpRespondsEnemies property

   affects this chance.
   
   


If you don't wish a NPC to call for help, override

its pAIGoalsConflict so it doesn't include the goal.





3. 
   
   oAIGoalAttackEnemies - This goal causes the NPC

   to attack any enemies in the room. It picks an enemy, by

   various means, and attacks with a weapon. The code also

   has the AI ready a weapon or potentially pick one up.

   
   
   



4. 
   
   oAIGoalMoveChaseEnemies - If all the enemies

   in a room are killed, oAIGoalAttackEnemies will finish.

   The next goal in line causes the NPC to chase after enemies

   in neighboring rooms, just in case some of the enemies

   ran away. It finishes when no more enemies

   are left in any neighboring rooms.

   
   
   


5. 
   
   oAIGoalLootDeadBodies - Once all the enemies

   are gone, this goal will have the NPC loot all the dead bodies.

   It picks up the most valuable items first. It will hold onto

   them for 30-60 minutes, after which point they're automatically

   deleted. That means that characters who are killed by NPCs have

   30-60 minutes to find the NPC and kill it, or they won't

   get their gear back.
   
   




6. 
   
   oAIGoalMoveToPreCombat - Finally, after all the

   bodies have been looted, oAIGoalMoveToPreCombat causes the

   NPC to return to the room it was in before combat began.

   Once there, this goal finishes and the AI continues its

   normal routine.
   
   

When a NPC is first created, it automatically adds the following

goal, as specified in pAIGoalsFirstActions.

• 
  
  oAIGoalObjectEquipAll - Causes the NPC to equip

  any weapons and armor that it has. This ensures it will be

  ready for combat when the time comes.
  
  

Of course, you can

change pAIGoalsConflict and pAIGoalsFirstActions to

whatever goals you wish.

Some other combat-related goals that I haven't discussed yet

are:

• 
  
  oAIGoalAttackEnemy - Has the NPC take a swing

  at an enemy. This is called from oAIGoalAttackEnemies.

  
  
  



• 
  
  oAIGoalCombatAim - Call this to have the

  NPC change his aim (body part) location to a specific value.

  
  
  



• 
  
  oAIGoalCombatAimChoose - Call this to have the

  NPC change his aim (body part) location automatically, based on

  various personality properties.
  
  




• 
  
  oAIGoalCombatDefend - Call this to have the

  NPC change his defense location to a specific value.

  
  
  



• 
  
  oAIGoalCombatDefendChoose - Call this to have the

  NPC change his defense location automatically, based on

  various personality properties.
  
  




• 
  
  oAIGoalCombatEnemy - Call this to have the

  NPC change target a new enemy, based on a passed-in parameter.

  
  
  



• 
  
  oAIGoalCombatEnemyTargetChoose - Call this to have the

  NPC change his targeted enemy, based on

  various personality properties.
  
  




• 
  
  oAIGoalCombatDodgeParry - Call this to have the

  NPC change his dodge/parry to a specific value.

  
  
  



• 
  
  oAIGoalCombatDodgeParryChoose - Call this to have the

  NPC change his dodge/parry, based on

  various personality properties.
  
  




• 
  
  oAIGoalCombatRetreatCallForHelp - This goal

  causes the NPC to keep calling out for help until the goal

  is killed. It's added and killed when the NPC retreats

  in oAIGoalCombatRetreat.

  
  
  



• 
  
  oAIGoalObjectEquip - The NPC will equip

  or unequip a single

  object. It's called by oAIGoalObjectEquipAll.
  
  




• 
  
  oAIGoalObjectGetDrop - The combat AI uses this

  goal to pick up weapons it wishes to equip, to loot bodies,

  or to drop treasure in the hopes of distracting an attacker.
  
  




• 
  
  oAIGoalObjectGetWeapon - The AI will pick up

  a weapon from the ground (or a dead body) if it's better than

  its current weapon, and if it's usable by the NPC. This

  is used by oAIGoalAttackEnemies if the AI's weapon breaks

  or gets dropped.
  
  

Many of the combat-related goals rely on properties that affect

how the AI reacts in combat:

• 
  
  pAICombatAttackEnemySpeed - How rapidly the AI attacks

  compared to the ideal attack speech for his weapon.

  
  
  



• 
  
  pAICombatAttackEnemySpeedVar - Amount of variation in

  the AI's attack speed.

  
  
  



• 
  
  pAICombatAimLocation - Location where the AI tends to aim for.
  
  



• 
  
  pAICombatAimSwitch - Likelihood of the AI aiming for

  a different body part.

  
  
  



• 
  
  pAICombatDefendBodyPart - Preferred defence location.
  
  



• 
  
  pAICombatDefendJustHit - How likely it is that the AI

  will defend a body part that was just hit.

  
  
  



• 
  
  pAICombatDefendSwitch - Likelihood of changing defence locations.
  
  



• 
  
  pAICombatEnemyTargetAttacker - Likelihood that the AI

  will attack the last character that attacked it.

  
  
  



• 
  
  pAICombatEnemyTargetSame - How likely the AI will keep

  attacking the same enemy and not switch.

  
  
  



• 
  
  pAICombatEnemyTargetWeaker - Causes the AI to

  attack weaker opponents in preference to stronger opponents.

  
  
  



• 
  
  pAICombatEnemyTargetWounded - Likelihood that the AI

  will attack the most wounded character.

  
  
  

• Because each NPC will have their own combat tactics, you might

  wish to provide a AIGoalOverride() method that

  overrides some standard combat goals,

  such as oIAIGoalCombatDefendChoose, with a custom goal. For example:

  You could use this to make the NPC only defend one part of his

  body.