Now that weapons and entities in general are covered, we may explore another kind of entities: monsters.

Monsters, or NPCs (acronym for "Non-Player Character"), are some 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 the 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: 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 use 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 spawns through its Spawn() method, a call to CBaseMonster::MonsterInit() is made which is responsible for checking if it is allowed to do so using the returned value of bool CGameRules::FAllowMonsters(). The latter returns true or false depending on the game rules implementation used in the CGameRules* g_pGameRules global pointer.

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.

You may ask after reading this: "but why Snarks can be used in multiplayer then?". The answer to that is that they aren't really "monsters" but more like "grenades", and this fact is reinforced that CSqueakGrenade is a child of CGrenade and CBaseMonster is not within the hierarchy.

The CBaseMonster::MonsterInit() method make a call to CBaseMonster::StartMonster() which is more interesting because it initialize the monster's "thinking" to CBaseMonster::MonsterThink() creating a loop of calls to CBaseMonster::RunAI() until the monster dies (bool CBaseEntity::IsAlive() returns false).

CBaseMonster::RunAI() has several responsibilities such as:

• Calling CBaseMonster::Look() to gather visual information like a monster has spotted an enemy, friend, nemesis.
• Calling CBaseMonster::Listen() to gather audio information like footsteps, grenade explosions, shooting firearms.
• Seeking enemies with bool CBaseMonster::GetEnemy() and determine the best one to attack based on relationship and other factors.
• Check ammo if needed with CBaseMonster::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 condition bits. They are used as a way to give information to the AI like "this monster has taken light damage".

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 CBaseMonster::Look() and bool CBaseMonster::GetEnemy() toggle on and/or off these condition bits. CBaseMonster::Look() handles all condition bits that involve vision, such as bits_COND_SEE_FEAR which means "I see an enemy that I'm afraid of".

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

Conditions bits are reset when the monster is being "restored" (like loading a saved game) if and only if it doesn't have an enemy.

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

Constant	Value	What it means to 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 includes all attack bits ((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 purpose of the CBaseMonster::Listen() method (called by CBaseMonster::RunAI()) is to make the monster hear sounds it 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 way as condition bits. Here is a table of common sound bits (and "smell bits") and what they mean (soundent.h):

Constant	Value	What it means to 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 CBaseMonster::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 acts 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 it means to the monster
bits_MEMORY_PROVOKED	(1 << 0)	A friendly (likely the player) provoked me (likely through intended friendly fire).
bits_MEMORY_INCOVER	(1 << 1)	I am in a spot for cover.
bits_MEMORY_SUSPICIOUS	(1 << 2)	A friendly (likely the player) harmed me but it was an accident, I hope it doesn't happen again.
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 "accidentally" 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 "iMemory", false otherwise
inline bool HasMemory(int iMemory)
{
    if (m_afMemory & iMemory)
        return true;

    return false;
}

// Returns true if this monster remember everything in "iMemory", false otherwise
inline bool HasAllMemories(int iMemory)
{
    if ((m_afMemory & iMemory) == iMemory)
        return true;

    return false;
}

Capabilities

A capability basically describes 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 it means to 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.

When the monster is being "initialized" , the "common" monsters code will automatically set the attack capabilities by checking if the monster's model has the activities setup (more on that later). In other words, if the model has an ACT_MELEE_ATTACK1 activity

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.

One important thing when setting up capabilities, you don't need to set the "can do X attack" bits, the common monsters code will handle that for you when the monster itself is being "initialized" (CBaseMonster::InitMonster() called at the end of the monster's Spawn() method). It achieves this by querying the monster's model for activities (more on that in a little bit). In other words, if the model has at least one sequence tied to the ACT_MELEE_ATTACK1 activity, then bits_CAP_MELEE_ATTACK1 will automatically be set, etc...

However, for the other bits (use, fly, doors, turn head...), you have to set these ones manually in the monster's Spawn() method in the m_afCapability attribute.

There are no methods to manipulate capabilities. 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 monsters have a monster state stored in m_MonsterState generally describing their current state.

That state impacts several things such as task and schedule selection, some generic behavior, behavior when performing scripted events (aiscripted_sequence/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 repelling down a rope (human grunt).
MONSTERSTATE_SCRIPT	6	Performing a script (aiscripted_sequence/scripted_sequence).
MONSTERSTATE_PLAYDEAD	7	Play dead (doesn't seems to be used).
MONSTERSTATE_DEAD	8	Dying/dead.

Activities

An activity allows tying sequences/animations in the model (MDL) to 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 Asset Manager (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 CBaseAnimating::LookupSequence( const char *label ) instead of int CBaseAnimating::LookupActivity( int activity ). Sequences/animations not tied to an activity are also likely used for scripting purposes (aiscripted_sequence/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 it means to 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 for choosing a sequence (simplified) is:

• For each sequence in the model...
• ...ignore it if it's not tied to the requested activity.
• Add it's weight to a counter (total weight).
• If that counter (total weight) is 0 or a randomly generated number between 0 and that total weight is less than the sequence's weight, then that sequence is remembered to be chosen (unless a next iteration of the loop overrides it).

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 CBaseMonster::SetActivity(Activity newActivity) and CBaseMonster::Stop() that you might find useful.

Animation events

A model can contain events allowing the entity using the model to execute specific things, such as showing a muzzle flash, playing 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. CBaseMonster::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 an example, it will call CBaseMonster::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.

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 aiscripted_sequence/scripted_sequence, this is the condition to trigger something (half health, see a player...)
string_t m_iszTriggerTarget	For aiscripted_sequence/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 to impact their behavior ("I see a new enemy", "I can perform that kind of attack", "I took light or heavy damage"...)
• Sound bits (and smell by the way) may be used by monsters to hear sounds (and smell stuff) and classify them (danger, world...)
• Memory bits allow monsters to remember things (Did the player betray me? Did I already flinch?)
• Capability bits tell the monster what it can and cannot do (open doors, swim, fly, climb ropes and ladders...)
• Condition, sound, memory, capability bits as well as tasks and schedules can be common to all monsters or specific to a monster.
• Activities allow to group together sequences/animations by actions (idling, walking...) Changes are exceptional and "fragile".
• Animation events allow the monster's model to communicate with the game code (actually shoot a firearm, throw a grenade...)

Here is also a simplified flow chart of the lifecycle of a monster: