Gun Roamer

Gun Patrol

Gun Stationary

Melee Patrol / Fatso

Melee Stationary

Dog

Idle

Neutral

State 0

speed: 0 or 1 u/ fr

random load direction

RandomMove 60-120
load offset: 60-120

speed: 1 u/fr

point right
turn left on collision

speed: 0 or 2 u/fr

random load direction

create path home

speed: 1 u/fr

point right
turn left on collision

speed: 0 or 2 u/fr

random load direction

create path home

speed: 1 u/fr

point right
turn right on no collision

see Tab. 2

(Idle State 0)

If hear sound:
walk along path

switch to State 2

AlertWait 10

load offset: -1

CheckReload 30

load offset: 0-30

If direct LoS:

AlertWait = 15

If indirect LoS:
switch to State 2

If direct LoS:

switch to State 1

If indirect or no LoS:

AlertWait = -1

switch to State 2

If hear sound:

switch to State 2

AlertWait 0

load offset: -1

CheckReload 30

load offset: 0-30

If direct LoS:

AlertWait = 0

If indirect LoS:
switch to State 2

If direct LoS:

switch to State 1

If indirect or no LoS:

AlertWait = -1

switch to State 2

Charging

State 1

(attacking w/ direct LoS)

speed: 3 u/fr

turn speed: 10°/fr

shoot + run directly at player

If distance < 80 units:

Turn to player 10°/fr
decelerate by 0.25 u/fr

Else

face player

Accelerate by .25 u/fr (max 3)

speed: 3 u/fr

turn speed: 10°/fr

run directly at player

If distance < 8 units:
kill player

speed: 5 u/fr

turn speed: n/a

Accelerate towards player
by 0.5 u/fr (max 5)

If touching player
and speed > 1:
kill player

CheckReload 0

If indirect LoS:
switch to State 2

If no LoS:
switch to State 3

Aiming

State 2

(attacking w/ indirect LoS)

If distance > 280 units:
switch to State 3

Else shoot

If no path:

stand still

If distance < 80 units:
Turn to player 10°/fr

Else

face player

CheckReload 0

If direct LoS:

switch to State 1

If no LoS:
switch to State 3

switch to State 3

Chasing

State 3

(following path)

speed: 2 u/fr

If not on a path

generate path to player

CheckReload 20

If direct LoS:

switch to State 1 (unless Dog/Detective)

If indirect LoS:

update path (unless Dog/Detective/Policeman)

If at end of path:

play animation

stand still for 50 fr (unless Melee Patrol)
switch to State 0

Knocked Down

hitbox: 16x16    After 115-235 frames: switch to Unarmed

Unarmed

RandomMove 60-120          If LoS to weapon: grab weapon and switch to Melee Patrol/Gun Roamer State 0
load offset: 60-120

Peeing

Smoking

Sitting

Phone

Guard

State 0

CheckReload 20
load offset 1-10

If LoS:

randomly create

Melee Stationary

or

Gun Roamer (AR)

face player

If direct LoS:

switch to State 1

If indirect LoS:

switch to State 2

CheckReload 15
load offset 1-10

If LoS:

randomly create

Melee Stationary

or

Gun Roamer (AR)

face player

If direct LoS:

switch to State 1

If indirect LoS:

switch to State 2

CheckReload 30
load offset 1-10

If LoS:

get up animation

randomly create

Melee Stationary

or

Gun Roamer (AR)

face player

If direct LoS:

switch to State 1

If indirect LoS:

switch to State 2

CheckReload 15
load offset 1-10

If LoS:

create

Gun Roamer (AR)

face player

If direct LoS:

switch to State 1

If indirect LoS:

switch to State 2

CheckReload 15
load offset 1-10

If LoS:

create

Gun Roamer (AR)

face player

If direct LoS:

switch to State 1

If indirect LoS:

switch to State 2

On death

drop gun / melee

drop gun

drop gun / melee

drop gun

drop M16

On Knockdown

drop gun / melee

1 u/fr

 2 u/fr

3 u/fr

3.5 u/fr

4 u/fr

5 u/fr

State 0

Unarmed

Patrols / Roamers / Dogs

Roaming

State 0 

State 3

Hostage
Stairs

Unarmed

Stationaries
all AI

on controller

only player

getting weapon

Richard
Biker

State 1

without Cleaver

all but Dogs

Graham
Biker
Jacket

maskless

Brandon

Hostage Bug

State 1

Dogs

12 u/fr

10-16 u/fr

12-16 u/fr

16 u/fr

Throwing

incl Biker

Double Barrel

Pumpgun

every other gun's bullets

0°

4*

6°

10°

12°

20°

Magnum
Biker's throwing knives

Throwing weapons

M16

Scorpion

Pistol

Uzi (silenced)

MP5
thrown melee or guns

Pumpgun

Double Barrel

method

Dropping

Standing Execution

Melee Kill

Gun Kill

Knockdown

0° direction

player direction

AI direction

straight up (cardinal direction)

direction of bullet impact

AI direction

drop direction

dir+70+random(40)

direction-115+random(50)

-70+random(40)

bullet.direction-70+random(40)

direction-70+random(40)

speed

1-3 u/fr

2-3 u/fr

2-4 u/fr

2-4 u/fr

2-4 u/fr

visual

size

16x16

16x8

6x6

objects

     Jacket / Biker

     Gun Roamers

     Gun Stationaries

     Knocked Down

     Unarmed

     Gun Patrols

     Melee Patrols

     Melee Stationaries

     Dropped Weapons

     Thrown Weapons

visual

   Melee Weapons

     Ranged Weapons

     Executions

14   +6

Fists

+4

M16

12 +13

Bat

+30

Pumpgun

12 +10

Pipe

+10

DB

12   +7

Golf Club

+8

Magnum

8   +2

Knife

+2

Skorpion

18   +2

Suitcase

+2

(Silenced) Uzi

14 +11

Pot

+3

MP5

12   +8

Pool Cue

+6

Silenced Pistol

10 +10

Broken Pool Cue

+20

Throwing weapons

16   +4

Crowbar

+30

Biker's Knife

16   +4

Pan

20 +10

Axe

12   +2

Baton

16   +4

Trophy

18   +2

Drill

7   +8

Katana

  16   

Machete

37   

Sledge Hammer

16    

Biker's Cleaver

        Fig. 1 Distances

 Fig. 2 Player Hitbox and Melee Hurtbox

LoS / Line of Sight

There are two types of LoS:
Direct LoS is established if there are no objects between you and the AI and your distance is below 280 units.
Indirect LoS is established when there are only transparent objects (i.e. glass, tables, etc.) between you and the AI. This can happen over any distance.
State 0 Dogs cannot see through unbroken glass.

CheckReload

This variable decrements by 1 every frame until it reaches 0, at which point it gets reset to whatever value is specified for the respective state the AI is in.
Lastly all the colour matched checks will be done.
Together with RandomMove this is the single biggest (intended) source of randomness in the game.

AlertWait

Like CheckReload this variable decrements by 1 per frame. However it only does so while in State 0. When it reaches 0 it gets reset to the specified value (-1) and all the colour matched checks are done.

This timer is essentially a reaction delay and has the effect of making the AI seemingly see you through walls, since it does not check for LoS.

RandomMove

This variable also decrements by 1 every frame. When it reaches 0 it gets reset to a value from the specified range. The AI is then rolled a new direction and a new speed (0 or 1 u/fr).

load offset

When a new floor is loaded, every AI is individually rolled a starting value for their respective timers. The load offset is the range from which a starting value can be chosen.
Offsets and movement are saved when you load into a screen, so when respawning on the same screen the AI's initial attributes (reaction and roamer movement) will always be the same.

random  /  roll

The terms random and roll will always refer to a uniform distribution within the specified ranges.

units  /  u/fr

A unit corresponds to 1 pixel at a vertical resolution of 256px. That means in 1080 resolution it corresponds to around 4.22 pixels.
Accordingly u/fr (units per frame) describes the speed of an AI or the player. Naturally the game is locked at 60 fps.

Path

In State 3 (rarely 2) the AIs walk along paths. These paths are generated along the nav mesh which means that they are very angular and include weird movements like hugging the sides of doorways and such. A good rule of thumb is that both you and the AI should stand away from furniture, as well as the bottom and right side of walls, all of which casts large dead areas in the nav mesh.
Due to the crude nav mesh and likely some engine level issues path generation is prone to fail, in which case the AI will go back to State 0.
An AI will keep their sound reaction path even in State 2 and only prematurely end any path when going into State 1.

shoot

When a gun AI is in state 1 or 2, they will shoot at you. They will hold down the trigger and shoot as fast as possible. For this the game has a variable Reload which decrements by 1 per frame, but only in State 1 and 2. When it reaches 0, the AI will shoot and the timer is reset to the gun's fire rate. Importantly every AI starts at a Reload value of 13 when a floor is loaded or after picking up a gun from the Unarmed state. If you decrement an AI's Reload timer and break LoS, they will have different (potentially even close to 0) Reload values, the next time they see you.
Here are the different AI fire rates: M16: 5fr, Pump: 60fr, DB: 15fr, Pistol: 10fr

hear sound

The AI's reaction to sound is the highest perceived source of RNG in the game. Ironically most of that perceived RNG comes from the bad path generation.
First every State 0, non-idle AI within a distance of 300 units (except Mafia Stationary Gunmen) has a path generated to a random point within a 128x128 square around the player. Out of the created paths (those that did not fail to be created) the three shortest ones are alarmed. All the others remain in State 0.
The idle AI on Clean Hit will always hear you and turn into a State 0 AR roaming gunman if path generation fails.

collision mask

Collision masks (CMs) are the blue rectangles from Tab. 6 and Fig. 2. They determine if an object is touching another object or not. As such they are the ones bumping into objects and walls, not your or the AI's sprite. CMs also determine whether or not you can execute an enemy, as your CM has to touch theirs for it to be the case. Lastly a thrown weapon will hit its target if their CMs overlap.
Importantly CMs do not rotate with the object they're attached to. Both you and an AI may turn around, but a 16x8 CM will always double as wide in east/west direction than north/south.

hitbox

The hitbox is the area of effect of the player's melee weapon. It can be seen in Fig. 2 as the green box around Jacket. It is the same for all weapons. Yes, that indeed means knife and sledgehammer have the same range.

hurtbox

Hurtboxes are the regions of the player and the AI where they are vulnerable to damage. In the case of bullets, the hurtbox is simply the CM. That means some AI, like melee AI, are easier to hit in a north/south direction. When you perform a melee swing, the AI will actually have two hurtboxes. The first being the CM again and the second being the green boy around the Ai in Fig. 2. Hitting either of these with your hitbox will kill the AI.

AlertWait / Reaction Delay

A State 0 AI will see you within 1-30 frames (CheckReload), then Alertwait is set to 15fr, after which they will either be set to State 1 or 2 depending on whether you are still in dLoS or not. So a State 0 AI's reaction time is anywhere between 15fr and 45fr. Two weird effects arise from AlertWait:
If you break LoS while AlertWait is decrementing from 15, the AI will see you through walls and switch to State 2 (then 3).
More impressively, when switching from State 0 to State 1 the AI's Alertwait is set to 10, but does not decrement until it is back in State 0. That means you can have the AI switch from 2 to 3 and back as often as you like, and once they reach the end of their path in State 3 (so they've completely lost track of you), they go back to State 0, AlertWait decrements from 10 and they will see you anywhere on the level, through any amount of walls. You will see them walk to the end of their opath, wait for a moment for CheckReload to reach 0, then go into State 0, wait 10 frames for AlertWait to go to 0, then see you through a wall.
These effects are fairly apparent on Metro 2 or during Neighbors skip.

 Other Reaction Times

In State 3 CheckReload is between 0-20 and there is no AlertWait, so AI's reaction time can go as low as 0 frames. However the third timer Reload can add an additional buffer to gunmen reaction times. Every gunman spawns with a Reload offset of 13fr. So the first time you see a gunman, they will not shoot until 13fr after they see you, so their total reaction time is 28-58fr. Reload only decrements in State 0 and is only reset when the AI shoots, so you could be walking past an AI, lowering their Reload timer and the next time you come across them their reaction time can again be as low as 15fr in State 0 and 0fr in State 3. (Caution: Idle AI will mostly have lower State 0 CheckReloads than 30)
Additionally, State 1 and 2 gun AI will snap directly onto you, if they see you from further than 80 units away. Closer AI will turn around toward you at a rate of 10°/fr, giving you up to 18fr of extra reaction delay.

indirect LoS

Indirect LoS (iLoS) also leads to some sketchy behaviour.
As most will know, State 3 AI has no check for iLoS, which means that you can hide behind transparent objects from them, until they reach the end of their path and go back to State 0. Non-State 0 AI will also not react to sound, so you can shoot at them without changing their behaviour.
State 2 AI (AI with iLoS) will usually stand in place and snap onto you (because you are unlikely to be seen by an AI from less than 80 units over furniture or glass.
If a gunman hears you and is in iLoS at the same time, he will have a path generated and then be sent to State 2, where he remains for however long he has iLoS. That means he will move along the path and shoot at the same time, resulting in those weird dudes shooting sideways.