Now that weapons and entities in general are covered. This chapter is about exploring another kind of entities: monsters.

Creating a monster or NPC (shortcut for "Non-Player Character") is one of the most difficult entities to program because they have to behave in a certain way to make a game fun. Artificial Intelligence ("AI") is the component responsible for manipulating that behavior.

We will explore in this page the concepts of Half-Life's AI before we get to create a monster itself. AI and monsters programming always happen in the server project, you will likely never touch the client project. For the sake of clarity, important terminology is in bold.

Classes

Let us do the same thing as we did for the weapons: look at the class declaration of scientist (CScientist), the human grunt (CHGrunt) and the headcrab (CHeadcrab) and determine the class hierarchy.

The reasons we are looking at those three monsters is that they have distinct traits: one is a "simple" monster, another one is a talkative friend and the last one has the ability to be part and communicate within a squad.

With the knowledge and experience you gained in the previous chapters of this book and/or from looking at the HL SDK by yourself, you should be able to do this on your own.

Need some help? The files you should be looking at are scientist.cpp, hgrunt.cpp and headcrab.cpp. Some IDEs like Visual Studio on Windows provide a "class viewer" to navigate through classes rather than files and this could help you when navigating through the code.

Have you finished? Here is the class hierarchy you should have found:

CHGrunt
└ CSquadMonster
CScientist
└ CTalkMonster
  CHeadcrab
  └ CBaseMonster
    └ CBaseToggle
      └ CBaseAnimating
        └ CBaseDelay
          └ CBaseEntity

If you found the correct hierarchy, congratulations! If not, do not get discouraged, learn from your mistakes and try again, that's how you gain knowledge and experience.

We can determine that those three monsters uses three different bases:

• CBaseMonster (basemonster.h) being the very basic one that is itself the base for the others two.
• CSquadMonster (squadmonster.h) that provide squad functionality to the monster.
• CTalkMonster (talkmonster.h) for talkative monsters with dialogues such as questions, answers and so on.

Everything has its beginning

Whenever a monster spawn through Spawn(), a call to MonsterInit() is made which is responsible of checking if the monster is allowed to spawn using the returned value of BOOL CGameRules::FAllowMonsters() which returns TRUE or FALSE depending on the game rules implementation used in g_pGameRules.

The monster remove itself immediately if that is not the case. Otherwise, it initialize some variables which some of them are described in this page.

That same method make a call to StartMonster() which is more interesting because it initialize the monster's "thinking" to MonsterThink() creating a loop of calls to RunAI() until the monster dies (IsAlive() returns FALSE).

RunAI() has several responsibilities such as:

• Calling Look() to gather visual information like a monster has spotted an enemy, friend, nemesis.
• Calling Listen() to gather audio information like footsteps, grenade explosions, shooting firearms.
• Seeking enemies with GetEnemy() and determine the best one to attack based on relationship and other factors.
• Check ammo if needed with CheckAmmo(), it is overridden and used by human grunts only.

These functions are responsible of toggling on and off condition bits of the monster.

Condition bits

You might have seen in the code of monsters constants like bits_COND_LIGHT_DAMAGE. Those are conditions bits, they are used as a way to give information to the AI like "this monster has taken light damage" for example.

They are either disabled or enabled in the monster's m_afConditions attribute. This is done through bitwise operation using the OR operator (|).

In the previous section, we said that several methods such as Look() and GetEnemy() toggle on and/or off these conditions bits. Look() handle all condition bits that involves sighting such like bits_COND_SEE_FEAR meaning "I see an enemy that I'm afraid of".

GetEnemy() also does it with bits_COND_NEW_ENEMY for "I have a new enemy to deal with" for example. Likewise, CheckAmmo() does it with bits_COND_NO_AMMO_LOADED ("my firearm is empty").

Here is a list of condition bits common to all monsters, keep in mind that a monster may use its own (schedule.h):

Constant	Value	What does it mean for the monster
bits_COND_NO_AMMO_LOADED	(1 << 0)	My firearm is empty.
bits_COND_SEE_HATE	(1 << 1)	I see an enemy that I hate (more on this later).
bits_COND_SEE_FEAR	(1 << 2)	I see an enemy that I am afraid of (more on this later).
bits_COND_SEE_DISLIKE	(1 << 3)	I see an enemy that I dislike (more on this later).
bits_COND_SEE_ENEMY	(1 << 4)	I see an enemy.
bits_COND_ENEMY_OCCLUDED	(1 << 5)	Enemy is occluded by the world.
bits_COND_SMELL_FOOD	(1 << 6)	I smell food.
bits_COND_ENEMY_TOOFAR	(1 << 7)	My enemy is too far.
bits_COND_LIGHT_DAMAGE	(1 << 8)	I took light damage, just a flesh wound.
bits_COND_HEAVY_DAMAGE	(1 << 9)	I took heavy damage, that hurts!
bits_COND_CAN_RANGE_ATTACK1	(1 << 10)	I can perform one kind of ranged attack.
bits_COND_CAN_MELEE_ATTACK1	(1 << 11)	I can perform one kind of melee attack.
bits_COND_CAN_RANGE_ATTACK2	(1 << 12)	I can perform another kind of ranged attack.
bits_COND_CAN_MELEE_ATTACK2	(1 << 13)	I can perform another kind of melee attack.
bits_COND_PROVOKED	(1 << 15)	A friendly (likely the player) provoked me.
bits_COND_NEW_ENEMY	(1 << 16)	I have a new enemy.
bits_COND_HEAR_SOUND	(1 << 17)	I hear something interesting.
bits_COND_SMELL	(1 << 18)	I smell something interesting.
bits_COND_ENEMY_FACING_ME	(1 << 19)	My enemy is facing me.
bits_COND_ENEMY_DEAD	(1 << 20)	My enemy is dead.
bits_COND_SEE_CLIENT	(1 << 21)	I see a client (a bot or a player).
bits_COND_SEE_NEMESIS	(1 << 22)	I see an enemy which I consider as my nemesis (more on this later).
bits_COND_SPECIAL1	(1 << 28)	First special condition specific to my type of monster.
bits_COND_SPECIAL2	(1 << 29)	Second special condition specific to my type of monster.
bits_COND_TASK_FAILED	(1 << 30)	I failed to perform a task (more on this later).
bits_COND_SCHEDULE_DONE	(1 << 31)	I completed a schedule (more on this later).

It is worth mentioning that bits_COND_ALL_SPECIAL exists and is a shortcut to include both special conditions ((bits_COND_SPECIAL1 | bits_COND_SPECIAL2)).

Same with bits_COND_CAN_ATTACK which regroup all kinds of attack ((bits_COND_CAN_RANGE_ATTACK1 | bits_COND_CAN_MELEE_ATTACK1 | bits_COND_CAN_RANGE_ATTACK2 | bits_COND_CAN_MELEE_ATTACK2)).

These are common methods used to handle condition bits with an added description of what they do:

// Returns the conditions ignored by this monster (useful to ignore flinching while attacking for example)
virtual int IgnoreConditions();

// Set the condition(s) specified in "iConditions" for this monster
inline void SetConditions( int iConditions )
{
    m_afConditions |= iConditions;
}

// Clear the condition(s) specified in "iConditions" for this monster
inline void ClearConditions( int iConditions )
{
    m_afConditions &= ~iConditions;
}

// Returns TRUE if any condition in "iConditions" is met for this monster, FALSE otherwise
inline BOOL HasConditions( int iConditions )
{
    if ( m_afConditions & iConditions )
        return TRUE;

    return FALSE;
}

// Returns TRUE if all conditions in "iConditions" are met for this monster, FALSE otherwise
inline BOOL HasAllConditions( int iConditions )
{
    if ( (m_afConditions & iConditions) == iConditions )
        return TRUE;

    return FALSE;
}

Sound bits

The goal of the Listen() method (called by RunAI()) is to make the monster hear sounds he could be interested in and classify them using sound bits such as bits_COND_HEAR_DANGER meaning "I heard a dangerous sound". They work in the same fashion as condition bits. Here is a table of common sound bits (and "smell bits") and what they mean (soundent.h):

Constant	Value	What does it mean for the monster
bits_SOUND_NONE	0	Nothing worth needing my attention.
bits_SOUND_COMBAT	(1 << 0)	I hear combat, could be explosions or gunshots.
bits_SOUND_WORLD	(1 << 1)	I hear doors opening/closing, glass breaking.
bits_SOUND_PLAYER	(1 << 2)	I hear the player making noise (running, walking, jumping...)
bits_SOUND_CARCASS	(1 << 3)	I smell a dead body.
bits_SOUND_MEAT	(1 << 4)	I smell gibs or pork chop.
bits_SOUND_DANGER	(1 << 5)	I hear danger such as a grenade bouncing, barrel about to explode...
bits_SOUND_GARBAGE	(1 << 6)	I smell trash cans, banana peels, old fast food bags.

Likewise, remember that monsters may use their own.

One particular method to note here is int ISoundMask() which returns all the bits that the monster should care about that could influence its behavior. There is also BOOL CSound::FIsSound() and BOOL CSound::FIsScent() to distinguish between sound and smell bits.

You also might have seen calls to CSoundEnt::InsertSound( int iType, const Vector &vecOrigin, int iVolume, float flDuration ) in various places of the HL SDK that is used to insert sounds (and smells) in the world for monsters to notice.

Memory

All monsters have a m_afMemory attribute which act as memory for the monster to remember various events. They also work like condition and sound/smell bits.

Again, here is a table of common memory bits (monsters.h):

Constant	Value	What does it mean for the monster
bits_MEMORY_PROVOKED	(1 << 0)	A friendly (likely the player) provoked me.
bits_MEMORY_INCOVER	(1 << 1)	I am in a spot for cover.
bits_MEMORY_SUSPICIOUS	(1 << 2)	A friendly (likely the player) accidently or tried to provoke me, I should be careful.
bits_MEMORY_PATH_FINISHED	(1 << 3)	I finished moving on a specific path (used by Big Momma only).
bits_MEMORY_ON_PATH	(1 << 4)	I'm moving on a specific path.
bits_MEMORY_MOVE_FAILED	(1 << 5)	I already failed to move somewhere.
bits_MEMORY_FLINCHED	(1 << 6)	I already flinched.
bits_MEMORY_KILLED	(1 << 7)	I'm already dead (Valve recognized this as a programming hack).
bits_MEMORY_CUSTOM4	(1 << 8)	Fourth special memory specific to my type of monster.
bits_MEMORY_CUSTOM3	(1 << 9)	Third special memory specific to my type of monster.
bits_MEMORY_CUSTOM2	(1 << 10)	Second special memory specific to my type of monster.
bits_MEMORY_CUSTOM1	(1 << 11)	First special memory specific to my type of monster.

You probably guessed already: monsters may have their own as well.

Barney is going to help us demonstrate the usage of memory for monsters: if he is not in combat and you hurt him, he's going to remember that you provoked him (bits_MEMORY_PROVOKED) making him stopping following you and try to kill you.

If he is in combat and you "accidently" hurt him, he is going to get suspicious of you (bits_MEMORY_SUSPICIOUS) and remind you he's a friendly. If you do it again, he will get provoked (bits_MEMORY_PROVOKED) and you already know what is going to happen.

As usual, there are methods to manipulate all of this (with added description):

// Store the bit(s) of memory specified in "iMemory" for this monster
inline void Remember( int iMemory )
{
    m_afMemory |= iMemory;
}

// Remove the bit(s) of memory specified in "iMemory" for this monster
inline void Forget( int iMemory )
{
    m_afMemory &= ~iMemory;
}

// Returns TRUE if this monster remember anything in "iConditions", FALSE otherwise
inline BOOL HasMemory( int iMemory )
{
    if ( m_afMemory & iMemory )
        return TRUE;

    return FALSE;
}

// Returns TRUE if this monster remember everything in "iConditions", FALSE otherwise
inline BOOL HasAllMemories( int iMemory )
{
    if ( (m_afMemory & iMemory) == iMemory )
        return TRUE;

    return FALSE;
}

Capabilities

A capability basically describe what a monster can and can't do. It is stored in the m_afCapability attribute and works the same as the other kinds of bits.

Here is a table of the common ones (cbase.h):

Constant	Value	What does it mean for the monster
bits_CAP_DUCK	(1 << 0)	I can duck.
bits_CAP_JUMP	(1 << 1)	I can jump.
bits_CAP_STRAFE	(1 << 2)	I can strafe.
bits_CAP_SQUAD	(1 << 3)	I can join, form and communicate in a squad.
bits_CAP_SWIM	(1 << 4)	I can swim.
bits_CAP_CLIMB	(1 << 5)	I can climb ladders, ropes.
bits_CAP_USE	(1 << 6)	I can open doors, push buttons, pull levers.
bits_CAP_HEAR	(1 << 7)	I can hear sounds.
bits_CAP_AUTO_DOORS	(1 << 8)	I can use automated doors.
bits_CAP_OPEN_DOORS	(1 << 9)	I can open "manual" doors.
bits_CAP_TURN_HEAD	(1 << 10)	I can turn my head around (if my bone controller at index 0 is correct).
bits_CAP_RANGE_ATTACK1	(1 << 11)	I can perform one kind of ranged attack.
bits_CAP_RANGE_ATTACK2	(1 << 12)	I can perform another kind of ranged attack.
bits_CAP_MELEE_ATTACK1	(1 << 13)	I can perform one kind of melee attack.
bits_CAP_MELEE_ATTACK2	(1 << 14)	I can perform another kind of melee attack.
bits_CAP_FLY	(1 << 15)	I can fly.

Again, a monster might have its own capabilities. There is also bits_CAP_DOORS_GROUP available that is a shortcut for bits_CAP_USE, bits_CAP_AUTO_DOORS and bits_CAP_OPEN_DOORS together.

However, there are no methods to manipulate those. You basically set m_afCapability in the monster's Spawn() method. If for some reason you need to query if the monster is capable of something, you check directly if its bit is set (if ( m_afCapability & bits_CAP_SOMETHING )).

Monster state

All monster have a monster state stored in m_monsterState describing in general their current state.

That state impact several things such as tasks and schedules selection, some generic behavior, behavior when performing scripted events ((ai)scripted_sequence) and so on.

Here is a table of possible states and what they mean (util.h):

Constant	Value	What does the monster do?
MONSTERSTATE_NONE	0	Nothing.
MONSTERSTATE_IDLE	1	Idling.
MONSTERSTATE_COMBAT	2	In combat.
MONSTERSTATE_ALERT	3	Staying alert.
MONSTERSTATE_HUNT	4	On the hunt (doesn't seems to be used).
MONSTERSTATE_PRONE	5	Being grabbed by a barnacle or repeling down a rope (human grunt).
MONSTERSTATE_SCRIPT	6	Performing a script ((ai)scripted_sequence).
MONSTERSTATE_PLAYDEAD	7	Play dead (doesn't seems to be used).
MONSTERSTATE_DEAD	8	Dying/dead.

The core of Half-Life's AI: tasks & schedules

If you go back to looking at the code of RunAI(), you can see there is a call to MaintainSchedule(). If you look at this method, you will see things about schedules, tasks, completion and failure.

Schedules and tasks are the two main components of the Half-Life's AI. A task is a set of specific actions executed by the monster like playing a sequence/animation, reload, throw a grenade and so on. A schedule is a list of tasks executed by the monster to achieve a bigger goal such as moving to a precise location, wave a hand, shoot and take cover to reload...

The Task_t structure in the code represent a task, here is its definition:

struct Task_t
{
    int iTask;    // The ID of this task (TASK_WAIT for example)
    float flData; // Additional data that this task may use (time to wait for TASK_WAIT for example)
};

Same thing with Schedule_t for a schedule:

struct Schedule_t
{
    Task_t *pTasklist;  // Array of tasks for this schedule
    int cTasks;         // The number of tasks (always use "ARRAYSIZE" of the value you use in "pTasklist" to be safe)
    int iInterruptMask; // Condition bit(s) allowed to interrupt this schedule if needed
    int iSoundMask;     // Sound bit(s) allowed to interrupt this schedule if needed
    const char *pName;  // Name of the schedule shown in the console when testing/debugging
};

Let us take a deeper look at how declared and defined a task and a schedule are by looking at how a monster make a small flinch:

Task_t tlSmallFlinch[] =
{
    { TASK_REMEMBER,     (float)bits_MEMORY_FLINCHED }, // Remember in my memory that I flinched
    { TASK_STOP_MOVING,  0                           }, // Stop moving
    { TASK_SMALL_FLINCH, 0                           }, // Make a small flinch
};

Schedule_t slSmallFlinch[] =
{
    {
        tlSmallFlinch,              // This schedule use the "tlSmallFlinch" tasks array
        ARRAYSIZE( tlSmallFlinch ), // "ARRAYSIZE" will return 3 here and tell this schedule has 3 tasks
        0,                          // No condition bit can interruot this schedule
        0,                          // No sound bit can interrupt this schedule
        "Small Flinch"              // This schedule is named "Small Flinch"
    },
};

How the monster is aware that it should make a small flinch? This is where Schedule_t *GetSchedule() intervenes:

Schedule_t *CBaseMonster::GetSchedule()
{
    switch ( m_MonsterState )
    {

    // [...]

    case MONSTERSTATE_COMBAT:
        {
            // [...]

            else if ( HasConditions( bits_COND_LIGHT_DAMAGE ) && !HasMemory( bits_MEMORY_FLINCHED ) )
            {
                return GetScheduleOfType( SCHED_SMALL_FLINCH );
            }

            // [...]

            break;
        }
        default:
        {
            ALERT( at_aiconsole, "Invalid State for GetSchedule!\n" );
            break;
        }
    }

    return &slError[0];
}

In the above example, if the monster is in combat, has the "took light damage" condition bit enabled and does not remember in it's memory "having already flinched", then he will try to perform a schedule tied to SCHED_SMALL_FLINCH.

Now let's look at Schedule_t *GetScheduleOfType( int Type ) to see how the schedule is actually executed:

Schedule_t *CBaseMonster::GetScheduleOfType( int Type )
{
    switch ( Type )
    {

    // [...]

    case SCHED_SMALL_FLINCH:
        {
            return &slSmallFlinch[0];
        }
    default:
        {
            ALERT( at_console, "GetScheduleOfType()\nNo CASE for Schedule Type %d!\n", Type );

            return &slIdleStand[0];
            break;
        }
    }

    return nullptr;
}

You can notice that SCHED_ constants are used to identify schedules and tie them to a Schedule_t.

How about tasks? The StartTask( Task_t *pTask ) method is called whenever a task is requested to be performed. Let us look at task startup for small flinch:

void CBaseMonster::StartTask( Task_t *pTask )
{
    switch ( pTask->iTask )
    {

    // [...]

    case TASK_REMEMBER:
        {
            Remember( (int)pTask->flData );
            TaskComplete();
            break;
        }

    // [...]

    case TASK_STOP_MOVING:
        {
            if ( m_IdealActivity == m_movementActivity )
            {
                m_IdealActivity = GetStoppedActivity();
            }

            RouteClear();
            TaskComplete();
            break;
        }

    // [...]

    case TASK_SMALL_FLINCH:
        {
            m_IdealActivity = GetSmallFlinchActivity();
            break;
        }

    // [...]

    default:
        {
            ALERT( at_aiconsole, "No StartTask entry for %d\n", (SHARED_TASKS)pTask->iTask );
            break;
        }
    }
}

There are two interesting things to note here:

• For TASK_REMEMBER, pTask->flData is passed as value to the only parameter of the Remember() method which is bits_MEMORY_FLINCHED as defined in the task itself.
• TASK_REMEMBER and TASK_STOP_MOVING notify they are completed successfully through TaskComplete() because those are tasks that don't need another "thinking tick" to be completed (or failed). This is not the case for TASK_SMALL_FLINCH.

How does TASK_SMALL_FLINCH notifies the AI that it has finished? This is the job of RunTask( Task_t *pTask ) which is called by RunAI(). A reminder that RunAI() is called every "thinking tick" until the monster dies.

void CBaseMonster::RunTask( Task_t *pTask )
{
    switch ( pTask->iTask )
    {

    // [...]

    case TASK_SMALL_FLINCH:
        {
            if ( m_fSequenceFinished )
            {
                TaskComplete();
            }
        }
        break;

    // [...]

    }
}

You can see here that the task will be marked as completed successfully when the sequence/animation has finished playing. We can also determine that the duration of the flinch is determined by the animation/sequence itself.

As you might have already guessed, there are scenarios where a monster cannot complete a task successfully as intended (a monster trying to follow a target but that same target is too far and/or in a place impossible to reach). In that case, the task is marked as failed through TaskFail().

There are handy methods about tasks available such as int TaskIsComplete() and int TaskIsRunning(). And you guessed it already, monsters can have their own tasks and schedules as well. You can find most of the common tasks in the defaultai.cpp file.

For the tasks that involves "cover": cover searches are made within a 764 units radius unless float CoverRadius() is overridden. There is also "normal cover" (BOOL FindCover( Vector vecThreat, Vector vecViewOffset, float flMinDist, float flMaxDist )) and "lateral cover" (BOOL FindLateralCover( const Vector &vecThreat, const Vector &vecViewOffset )), the difference between them is that the latter try to find cover directly to the left or right of the monster.

Activities

An activity allow tying sequences/animations in the model (MDL) with an action in the game code like idling, running, walking and so on.

If you look at any monster model with Solokiller's Half-Life Model Viewer (decompiling the model and looking at the QC also works), you can see when browsing the sequences/animations that they may or may not be tied to a specific activity. Idle sequences/animations are likely tied to ACT_IDLE, walking sequences/animations are likely tied to ACT_WALK and so on.

Sometimes, they are not tied to an activity but they may be used by the game code in a different way, if you look at certain monsters code, you can see the usage of int LookupSequence( const char *label ) instead of int LookupActivity( int activity ). Sequences/animations not tied to an activity are also likely used for scripting purposes ((ai)scripted_sequence).

It is important to note that activities are tied to an index through an "activity map" (activitymap.h). Here is a list of common activities (activity.h):

Constant	Value	What does it mean for the monster
ACT_RESET	0	Set m_Activity to this invalid value to force a reset to m_IdealActivity.
ACT_IDLE	1	Normal idling.
ACT_GUARD	2	Idling in a guarding stance.
ACT_WALK	3	Normal walking.
ACT_RUN	4	Normal running.
ACT_FLY	5	Fly (and flap if appropriate).
ACT_SWIM	6	Swim.
ACT_HOP	7	Do a vertical jump.
ACT_LEAP	8	Do a long forward jump.
ACT_FALL	9	Fall.
ACT_LAND	10	Land from falling or after a jump.
ACT_STRAFE_LEFT	11	Strafe to the left.
ACT_STRAFE_RIGHT	12	Strafe to the right.
ACT_ROLL_LEFT	13	Tuck and roll to the left.
ACT_ROLL_RIGHT	14	Tuck and roll to the right.
ACT_TURN_LEFT	15	Turn quickly to the left while being stationary.
ACT_TURN_RIGHT	16	Turn quickly to the right while being stationary.
ACT_CROUCH	17	Crouch down from a standing position.
ACT_CROUCHIDLE	18	Stay crouched (loops).
ACT_STAND	19	Stand up from a crouched position.
ACT_USE	20	Use something.
ACT_SIGNAL1	21	First way of making a signal.
ACT_SIGNAL2	22	Second way of making a signal.
ACT_SIGNAL3	23	Third way of making a signal.
ACT_TWITCH	24	Twitch.
ACT_COWER	25	Cower.
ACT_SMALL_FLINCH	26	Do a small flinch after a light damage.
ACT_BIG_FLINCH	27	Do a big flinch after a heavy damage.
ACT_RANGE_ATTACK1	28	Do the first type of ranged attack.
ACT_RANGE_ATTACK2	29	Do the second type of ranged attack.
ACT_MELEE_ATTACK1	30	Do the first type of melee attack.
ACT_MELEE_ATTACK2	31	Do the second type of melee attack.
ACT_RELOAD	32	Reload the firearm.
ACT_ARM	33	Draw the firearm.
ACT_DISARM	34	Holster the firearm.
ACT_EAT	35	Eat some tasty food (loop).
ACT_DIESIMPLE	36	Simple death.
ACT_DIEBACKWARD	37	Die and fall backward.
ACT_DIEFORWARD	38	Die and fall forward.
ACT_DIEVIOLENT	39	Violent death.
ACT_BARNACLE_HIT	40	Barnacle tongue hits a monster.
ACT_BARNACLE_PULL	41	Barnacle is lifting the monster (loop).
ACT_BARNACLE_CHOMP	42	Barnacle latches on to the monster.
ACT_BARNACLE_CHEW	43	Barnacle is holding the monster in its mouth (loop).
ACT_SLEEP	44	Take a nap.
ACT_INSPECT_FLOOR	45	Look at something on or near the floor.
ACT_INSPECT_WALL	46	Look at something directly ahead of you (doesn't have to be a wall or on a wall).
ACT_IDLE_ANGRY	47	Pissed off idling (loop).
ACT_WALK_HURT	48	Wounded walking (loop).
ACT_RUN_HURT	49	Wounded running (loop).
ACT_HOVER	50	Idle while in flight.
ACT_GLIDE	51	Fly (don't flap).
ACT_FLY_LEFT	52	Turn left in flight.
ACT_FLY_RIGHT	53	Turn right in flight.
ACT_DETECT_SCENT	54	Smells a scent carried by the air.
ACT_SNIFF	55	Sniff something in front of the monster.
ACT_BITE	56	Eat something in a single bite (the difference with ACT_EAT is that this one does not loop).
ACT_THREAT_DISPLAY	57	Demonstrate without attacking that I'm angry (yelling for example).
ACT_FEAR_DISPLAY	58	Just saw something scary of feared.
ACT_EXCITED	59	Show excitement (like seeing some very tasty food to eat).
ACT_SPECIAL_ATTACK1	60	First special attack.
ACT_SPECIAL_ATTACK2	61	Second special attack.
ACT_COMBAT_IDLE	62	Agitated idle.
ACT_WALK_SCARED	63	Scared walking.
ACT_RUN_SCARED	64	Scared running.
ACT_VICTORY_DANCE	65	Do a victory dance (after killing a player for example).
ACT_DIE_HEADSHOT	66	Die from a hit in the head.
ACT_DIE_CHESTSHOT	67	Die from a hit in the chest.
ACT_DIE_GUTSHOT	68	Die from a hit in the gut.
ACT_DIE_BACKSHOT	69	Die from a hit in the back.
ACT_FLINCH_HEAD	70	Flinch from a hit in the head.
ACT_FLINCH_CHEST	71	Flinch from a hit in the chest.
ACT_FLINCH_STOMACH	72	Flinch from a hit in the stomach.
ACT_FLINCH_LEFTARM	73	Flinch from a hit in the left arm.
ACT_FLINCH_RIGHTARM	74	Flinch from a hit in the right arm.
ACT_FLINCH_LEFTLEG	75	Flinch from a hit in the left leg.
ACT_FLINCH_RIGHTLEG	76	Flinch from a hit in the right leg.

Surprisingly, no monsters have their own activities.

It is possible to have multiple sequences/animations tied to a same activity. In that scenario, the algorithm (simplified) is:

• Forget sequences/animations not tied to the requested activity.
• Determine the highest activity weight in the potential candidates.
• Forget the sequences/animations that do not have that highest weight.
• Randomly pick one of the remaining candidates.

An use case for this is choosing a random idle sequence/animation from three possible choices whenever a monster is idling.

All monsters have three attributes named m_Activity, m_IdealActivity and m_movementActivity. They indicate the current activity, ideal next one and the one to use for movement respectively. For methods, there are SetActivity( Activity newActivity ) and Stop() that you might find useful.

Animation events

A model can contains events allowing executing specific things such as showing a muzzle flash, play a sound and more. For monsters, it is a way to communicate with the game code and it is an important aspect to consider.

If you look at the scientist model (models/scientist.mdl), more specifically the give_shot animation, we can see some valuable information: there is a tie to the ACT_MELEE_ATTACK1 activity and there is one animation event tied to this sequence/animation. If we look at the event's information, it is triggered on the 17th frame and has the ID #1. HandleAnimEvent( MonsterEvent_t *pEvent ) is the method responsible for handling animation events. If we look at the scientist's version, we can see this code:

// For the context:
// "SCIENTIST_AE_HEAL"      = 1
// "SCIENTIST_AE_NEEDLEON"  = 2
// "SCIENTIST_AE_NEEDLEOFF" = 3
// "NUM_SCIENTIST_HEADS"    = 4

void CScientist::HandleAnimEvent( MonsterEvent_t *pEvent )
{
    switch ( pEvent->event )
    {
    case SCIENTIST_AE_HEAL: // Heal my target (if within range)
        Heal();
        break;
    case SCIENTIST_AE_NEEDLEON:
        {
        int oldBody = pev->body;
        pev->body = (oldBody % NUM_SCIENTIST_HEADS) + NUM_SCIENTIST_HEADS * 1;
        }
        break;
    case SCIENTIST_AE_NEEDLEOFF:
        {
        int oldBody = pev->body;
        pev->body = (oldBody % NUM_SCIENTIST_HEADS) + NUM_SCIENTIST_HEADS * 0;
        }
        break;
    default:
        CTalkMonster::HandleAnimEvent( pEvent );
    }
}

Taking again our event as example, it will call HandleAnimEvent( MonsterEvent_t *pEvent ) with a value of "1" for pEvent->event thus calling Heal() to heal the player (if conditions are still met). You can also note that there are animation events to show and hide the syringe itself as well. Other examples includes shooting firearms, shooting projectiles, throwing grenades.

Animation events may be common to several entities or specific to a particular one.

Classification & relationship

All entities even non-monsters have a Classify() method that returns its classification, here is the table of them (cbase.h):

Constant	Value	What does it mean for the entity
CLASS_NONE	0	This entity does not need a classification.
CLASS_MACHINE	1	This entity is a machine (sentry, turret).
CLASS_PLAYER	2	This entity is a player.
CLASS_HUMAN_PASSIVE	3	This entity is a passive human (scientist).
CLASS_HUMAN_MILITARY	4	This entity is a human with military knowledge (Barney).
CLASS_ALIEN_MILITARY	5	This entity is an alien with military knowledge (alien grunt).
CLASS_ALIEN_PASSIVE	6	This entity is a passive alien (not used by any existing monster).
CLASS_ALIEN_MONSTER	7	This entity is a not so friendly alien monster (Big Momma, Gargantua).
CLASS_ALIEN_PREY	8	This entity is an alien prey for predators (headcrab).
CLASS_ALIEN_PREDATOR	9	This entity is an alien predator on the hunt for preys (bullsquid).
CLASS_INSECT	10	This entity is an insect (cockroach).
CLASS_PLAYER_ALLY	11	This entity is an ally for the player (Barney & scientist).
CLASS_PLAYER_BIOWEAPON	12	This entity is an alien projectile fired by a player (player hornets).
CLASS_ALIEN_BIOWEAPON	13	This entity is an alien projectile fired by an alien monster (alien grunt hornets).
CLASS_BARNACLE	99	Special classification for barnacles.

Whenever a relationship needs to be tested, a call to the method IRelationship is made and returns one of the following values (monsters.h):

Constant	Value	What does it mean for the monster
R_AL	-2	Ally - This monster is my ally and a friend.
R_FR	-1	Fear - I am afraid of this monster, I need to get away from him.
R_NO	0	No relationship - Not my friend but not my enemy either.
R_DL	1	Dislike - This is an enemy I need to attack him.
R_HT	2	Hate - I hate this enemy and I will attack him in priority.
R_NM	3	Nemesis - I really hate this enemy and I want him dead no matter what.

A matrix is built to determine the relationship between two entities based on their classification, that matrix is (static int iEnemy[14][14] defined in int CBaseMonster::IRelationship( CBaseEntity *pTarget )):

Matrix	CLASS_NONE	CLASS_MACHINE	CLASS_PLAYER	CLASS_HUMAN_PASSIVE	CLASS_HUMAN_MILITARY	CLASS_ALIEN_MILITARY	CLASS_ALIEN_PASSIVE	CLASS_ALIEN_MONSTER	CLASS_ALIEN_PREY	CLASS_ALIEN_PREDATOR	CLASS_INSECT	CLASS_PLAYER_ALLY	CLASS_PLAYER_BIOWEAPON	CLASS_ALIEN_BIOWEAPON
CLASS_NONE	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)
CLASS_MACHINE	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)
CLASS_PLAYER	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)
CLASS_HUMAN_PASSIVE	R_NO (ignore)	R_NO (ignore)	R_AL (ally)	R_AL (ally)	R_HT (hate)	R_FR (fear)	R_NO (ignore)	R_HT (hate)	R_DL (dislike)	R_FR (fear)	R_NO (ignore)	R_AL (ally)	R_NO (ignore)	R_NO (ignore)
CLASS_HUMAN_MILITARY	R_NO (ignore)	R_NO (ignore)	R_HT (hate)	R_DL (dislike)	R_NO (ignore)	R_HT (hate)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_HT (hate)	R_NO (ignore)	R_NO (ignore)
CLASS_ALIEN_MILITARY	R_NO (ignore)	R_DL (dislike)	R_HT (hate)	R_DL (dislike)	R_HT (hate)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)
CLASS_ALIEN_PASSIVE	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)
CLASS_ALIEN_MONSTER	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)
CLASS_ALIEN_PREY	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_FR (fear)	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)
CLASS_ALIEN_PREDATOR	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_HT (hate)	R_DL (dislike)	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)
CLASS_INSECT	R_FR (fear)	R_FR (fear)	R_FR (fear)	R_FR (fear)	R_FR (fear)	R_NO (ignore)	R_FR (fear)	R_FR (fear)	R_FR (fear)	R_FR (fear)	R_NO (ignore)	R_FR (fear)	R_NO (ignore)	R_NO (ignore)
CLASS_PLAYER_ALLY	R_NO (ignore)	R_DL (dislike)	R_AL (ally)	R_AL (ally)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)	R_NO (ignore)
CLASS_PLAYER_BIOWEAPON	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_DL (dislike)	R_NO (ignore)	R_DL (dislike)
CLASS_ALIEN_BIOWEAPON	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_DL (dislike)	R_AL (ally)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)	R_NO (ignore)	R_DL (dislike)	R_DL (dislike)	R_NO (ignore)

The way to read this matrix is simple, the correct line is the entity's classification determining the relationship and the column is the target's classification.

Keep in mind that entities can override int IRelationship( CBaseEntity *pTarget ) and thus have a specific relationship with one or more specific entities in specific cases.

Let's take a look at the human grunt (int CHGrunt::IRelationship( CBaseEntity *pTarget )) relationship code for an example:

int CHGrunt::IRelationship( CBaseEntity *pTarget )
{
    if ( FClassnameIs( pTarget->pev, "monster_alien_grunt" ) || ( FClassnameIs( pTarget->pev,  "monster_gargantua" ) ) )
    {
        return R_NM;
    }

    return CSquadMonster::IRelationship( pTarget );
}

We can determine that human grunts are really pissed off against alien grunts and gargantuas that they want them dead at all costs because they treat them as "nemesis".

There is no override of int IRelationship( CBaseEntity *pTarget ) in CSquadMonster (the human grunt's base class) so we fall back to the matrix shown above defined in CBaseMonster instead.

From there, we have multiple cases depending on the target:

• They hate players, alien military and player allies like Barney (R_HT on CLASS_PLAYER, CLASS_ALIEN_MILITARY and CLASS_PLAYER_ALLY).
• They dislike passive humans like scientists, passive aliens, alien monsters like Vortigaunts, alien preys like headcrabs, alien predators like bullsquids (R_DL on CLASS_HUMAN_PASSIVE, CLASS_ALIEN_PASSIVE, CLASS_ALIEN_MONSTER, CLASS_ALIEN_PREY and CLASS_ALIEN_PREDATOR).
• They ignore the rest (R_NO on classifications not mentioned in this list).

Other attributes

There are other attributes common to all monsters that are not mentioned (yet for some of them) in this page. However, they do deserve to be known but they don't need a dedicated section to explain what they actually do.

Here is a table of some of them (basemonster.h):

Attribute	Description
EHANDLE m_hEnemy	Current enemy.
EHANDLE m_hTargetEnt	Current target (to follow or to move to).
EHANDLE m_hOldEnemy[MAX_OLD_ENEMIES]	Old enemies (MAX_OLD_ENEMIES = 4).
Vector m_vecOldEnemy[MAX_OLD_ENEMIES]	Position of old enemies.
float m_flFieldOfView	Field of view of the monster (not in degrees, 0.5f is 180 degrees to give an idea of the scale)
float m_flWaitFinished	Game time to tell the monster that the wait is over (defined by waiting tasks).
float m_flMoveWaitFinished	Game time to tell the monster that the wait before doing any kind of movement is over.
int m_LastHitGroup	Last hitgroup being hit (head, chest, stomach...)
Schedule_t *m_pSchedule	Current schedule.
Vector m_vecEnemyLKP	Last known position of the current enemy.
int m_cAmmoLoaded	How much ammo the firearm has.
float m_flNextAttack	Time before making a new attack.
int m_bloodColor	Color of the blood when being hurt (red, yellow...)
int m_failSchedule	Schedule to use in case of a task failure.
float m_flHungryTime	Time before eating something again.
float m_flDistTooFar	Range in units to determine that the monster is too far from its enemy (see condition bit bits_COND_ENEMY_TOO_FAR).
float m_flDistLook	Range in units to acquire enemies.
int m_iTriggerCondition	For (ai)scripted_sequence, this is the condition to trigger something (half health, see a player...)
string_t m_iszTriggerTarget	For (ai)scripted_sequence, this is the name of the entity to trigger.
Vector m_HackedGunPos	The position of the firearm relative to the monster's origin.

Recap

This page has a lot of information to digest so you might want a recap of everything we have seen so far:

• There are multiple base classes for monsters providing different functionality but they all share a common base.
• A monster does not always involve an AI (player).
• Condition bits are used by monsters