Turn 21 Theory

[kennyc]

hi guys, i did a quick search and couldnt find anything on it so i thought id share the only useful tip i have that hasnt been covered in this comprehensive forum!

 

when waiting outside a UFO with a squad of reaction fire goons, i noticed that an alien almost always exit at the end of turn 21. sometimes just one leaves but often several aliens leave over the following turns. i guess its not really that useful but can be some help when positioning your squad and priming nades or switching rifles and that sort of thing.

 

on another note id like to thank all of you who take the time to delve into the code and fish out interesting details regarding the game mechaincs, and the same too all of you who tirelessly repeat exercises to build up good empirical data. Ive spent a good deal of time lurking on strategy core and i appreciate all the excellent discussions that have occured here.

 

also if anyone has good guides for me to install xcom util i would greatley appreciate it, im not computer illiterate but have not been able to sucsefully patch my Alien Unknown copy downloaded from *link removed by Gimli*. thanks for the help guys.

[Gimli]

Nice tip, worth checking out.

On another note, I removed the name of that site as it's basically a link to it, and abandonware sites are not allowed here, sorry. Hope you'll enjoy your time on StrategyCore.

EDIT: Eh, I forgot to say that X-COM games are not abandonware, they are legally owned by Take 2 Interactive, who are still selling them - Enemy Unknown through Gametap and Terror From the Deep through Steam.

[kennyc]

ahh i see, sorry for that.

 

has anyone else had issues using xcom util in conjunction with the enemy unknown version avalible from a certain website?

[Zombie]

hi guys, i did a quick search and couldnt find anything on it so i thought id share the only useful tip i have that hasnt been covered in this comprehensive forum!

 

when waiting outside a UFO with a squad of reaction fire goons, i noticed that an alien almost always exit at the end of turn 21. sometimes just one leaves but often several aliens leave over the following turns. i guess its not really that useful but can be some help when positioning your squad and priming nades or switching rifles and that sort of thing.

You know, there is actually a shred of truth to this. On a Medium Scout mission on beginner skill level I corralled all the Sectoids out on the battlescape into the back of my Avenger. Three men were on overwatch at the back of the UFO. Turn 22 and a Sectoid left the craft. This is by no means a comprehensive test as it is only against one craft type and the aliens were stuck at the back of my dropship, but it does have a strategic element: on turn 20 or so, mine the exit with a proximity grenade and set up a reaction firing line.

 

also if anyone has good guides for me to install xcom util i would greatley appreciate it, im not computer illiterate but have not been able to sucsefully patch my Alien Unknown copy downloaded from *link removed by Gimli*. thanks for the help guys.

You could try this guide written by Hobbes over at the X-COM Wiki.

 

- Zombie

[kennyc]

thanks zombie! if you do conduct any in depth analysis id be most interested to hear the results. and remeber all of my games to date have been on beginner level, once i patch it im sure i will feel like an xcom newb again

[Hobbes]

Hello,

 

Rumors of the alien's behaviour being affected by the turn number aren't new. The one I've heard before concerned how the aliens essentially stopped moving after a certain number of turns. But until someone actually deciphers the AI code and confirms it's only theories.

[kennyc]

ive just been playing a new game, got quite far in the last 3 days. turn 21 consistently yeilds results!

[NKF]

The bit about when aliens stop moving seem to be more related to TFTD than UFO. I wonder if that was done on purpose, as an alien that's not moving is a deadly reaction fire menace compared to one that's been walking about.

 

I'll pipe in that 20+ turns appers to be a magic number of sorts. It needn't be spot on 20 turns or so, but if you wait that long and keep waiting, most of the aliens in a UFO (that aren't stuck to a particular point for any reason) will start heading towards towards the exits. It might be more coincidence than anything, but with lots of people getting similar results, there might just be something to it.

 

- NKF

[kennyc]

it typically takes me 7 or 8 turns to clean a small map and set up reaction fire squads. at this point i will simply pass turns untill the aliens exit.

 

i found a really nice configuration of soldiers that can make quick work out snakemen leaving a battleship.

[Bomb Bloke]

Are you sure they head towards exits, as opposed to your troops?

 

Say you station your units in a secluded spot inside the battleship itself. Do the aliens still leave the craft, or do they beeline your men?

[Hobbes]

I'll pipe in that 20+ turns appers to be a magic number of sorts. It needn't be spot on 20 turns or so, but if you wait that long and keep waiting, most of the aliens in a UFO (that aren't stuck to a particular point for any reason) will start heading towards towards the exits. It might be more coincidence than anything, but with lots of people getting similar results, there might just be something to it.

 

Depends on what you consider being stuck to a particular point. The way I see it is: the aliens on the upper levels can't get down until the aliens on the lower levels clear the route nodes that the AI uses for movement (by exiting the UFO and getting themselves killed).

Even if there's no aliens left on the upper levels it will still require several turns for the aliens to get down, depending on what the AI decides to do with each individual alien. It only takes 1 to completely block the route downwards and the others might just decide to keep hanging on the upper levels.

Whenever I ambush the aliens at the entrance of the UFOs (especially Supply Ships) the average for all aliens to clear the ship sometimes is 30 or more turns. And several times there's the elusive commander/navigator who simply refuses to get dwon.

And again it would make more sense not to have a setting on the AI for the aliens to exit the UFO after a number of turns: the aliens are dead ducks while at the open and their reaction shots are a lot more effective inside the UFOs.

[NKF]

I was thinking in particular of the alien that spawns in the middle of the abductor's bridge. The one that will wait in the room while facing the doorway.

 

There might be some way to entice it to move out on its own accord, but it usually just stays there.

 

- NKF

[Hobbes]

I was thinking in particular of the alien that spawns in the middle of the abductor's bridge. The one that will wait in the room while facing the doorway.

 

Yup, that particular alien which remains there for endless turns goes against the turn 20 theory. Figuring out why it remains there is one of those mysteries: there's nothing different on the route node it uses regarding other nodes nearby. So why does it stay there 99% of the times (I've seen it leaving once I think)

[Zombie]

Yup, that particular alien which remains there for endless turns goes against the turn 20 theory.

Not necessarily. The theory is that aliens will start to leave the UFO at turn 21, it doesn't say that all aliens will eventually leave after round 21.

 

Figuring out why it remains there is one of those mysteries: there's nothing different on the route node it uses regarding other nodes nearby. So why does it stay there 99% of the times (I've seen it leaving once I think)

Could be a number of factors. No scouts to relay the location of your troops, or perhaps the alien doesn't remember the location of your troops anymore may put the alien in some sort of "safe" mode where it ignores routes and stays put. It could also be bravery induced, but I believe I ran some tests on that scenario and it didn't matter.

 

- Zombie

[Bomb Bloke]

Is there even a route node marked as the exit of the UFO?

 

How else could you send the aliens there?

[zaimoni]

Is there even a route node marked as the exit of the UFO?

 

How else could you send the aliens there?

While there are nodes at UFO exits, they don't seem to have any special flags for "UFO exit-ness". [Either in the templates, or live -- more flags are used in live maps than in the map templates.]

[Hobbes]

Not necessarily. The theory is that aliens will start to leave the UFO at turn 21, it doesn't say that all aliens will eventually leave after round 21.

 

That's precisely my point: if not all aliens leave the UFO by turn 20 then what restrains some aliens from leaving the ship when the whole gang decides to check outside?

 

Could be a number of factors. No scouts to relay the location of your troops, or perhaps the alien doesn't remember the location of your troops anymore may put the alien in some sort of "safe" mode where it ignores routes and stays put. It could also be bravery induced, but I believe I ran some tests on that scenario and it didn't matter.

 

All of those are possible but the behaviour observed so far does not substantiate any of them. If there's no scouts to spot your troops then why do the aliens leave the ship on turn 20 or something after your troops spend 10 or more turns without seeing any of them? And there's nothing to say that the aliens coordinate their movements (i.e. one alien sees the xcom soldier, the other shoots/moves around it) other than blaster launcher firing. If the aliens used information (such as the position of soldiers spotted by the others), why then do we not see them firing heavy plasmas at soldiers on the other side of the battlescape?

 

My honest belief is that the AI is very simple but very effective: it runs every alien once (moving, checking for enemies, firing, all of this dependent on their agression stat) with the exception of blaster launchers (if an alien spots someone the AI jumps to any alien present with a blaster launcher to see if it can fire). You don't see aliens firing across the field with heavy plasmas with unseen soldiers because:

a) in most scenarios they would be blocked by terrain

b) it is better to keep them inside terrain features (than moving them out into the open to shoot) where their reaction shots can be more effective

 

This last point is what goes against any theory of them moving out into the open after a certain number of turns. If there's only a few of them left then it makes more sense if they use their advantages (reaction shots) to take down a few X-COM soldiers on the way.

[Hobbes]

While there are nodes at UFO exits, they don't seem to have any special flags for "UFO exit-ness". [Either in the templates, or live -- more flags are used in live maps than in the map templates.]

 

This sounds interesting. What sort of data have you discovered on the live maps regarding this?

[kennyc]

This last point is what goes against any theory of them moving out into the open after a certain number of turns. If there's only a few of them left then it makes more sense if they use their advantages (reaction shots) to take down a few X-COM soldiers on the way.

 

this hardly disproves the theory. it might make more sense for the aliens to stay onboard. but the ai is clearly telling them to do something else.

[zaimoni]

This sounds interesting. What sort of data have you discovered on the live maps regarding this?

In general, see my augmentations to the UFOPaedia entries on ROUTES.DAT and UNITREF.DAT. As long as *no* XCOM units have been sighted by aliens, it's possible to read off these records what patrol mode AI is likely to do.

 

Offset 19 of the routing records is important. Bits 0-2 are documented in the Daishiva map editor notes (they're in an enum that controls whether small/large/flying is restricted), and apply both to the Battlescape and to the template map data. It may be possible to use a batchfile setup to "enhance" the alien AI by carefully adjusting the nodes.

 

Bit 3 is the "did an alien start here" bit. It doesn't change on later turns, it just reflects initial placement on turn 1.

 

Bit 4 is a strong preference-bit for where an alien wants to end its turn on. It can change between turns.

 

Bits 5-7 were zero in the testing I did on a Medium Scout, but it's possible they could be used to implement this.

 

EDIT: see "Exercising the AI" UFOPaedia entry for further research topics on this.

[Bomb Bloke]

The relevant bytes I know of in UnitRef.Dat are 69 and 70. But your notes there don't make sense:

 

Byte 69 is the index of the node of the start of the planned move. Byte 70 is the index of the node of the start of the planned move.

Do you mean byte 70 is the index of the node at the end of the planned move? I'm fairly sure it looked like byte 70 was where the alien "wanted" to go. Your comment on the EtAI page suggests that the byte might have other uses as well.

 

Regarding this "stuck" alien. Are other aliens able to walk around it? Or is it not placed between other units and the doors? Does its starting node even link to other nodes so that it CAN move?

[Zombie]

Regarding this "stuck" alien. Are other aliens able to walk around it? Or is it not placed between other units and the doors? Does its starting node even link to other nodes so that it CAN move?

There aren't any spawn points in a UFO which are unconnected so it's not like the alien is "forced" to a single spot. Also recall my tests in the "How to enter a UFO..." in this forum. It doesn't matter if a node is blocked with an alien, another alien will still find its way around it.

 

- Zombie

[Bomb Bloke]

So assuming aliens will only try to travel between connected nodes, if one can't reach a given spot it'll give up and target the next one anyway?

 

Do the stuck aliens appear to be targeting a new node?

[Zombie]

So assuming aliens will only try to travel between connected nodes, if one can't reach a given spot it'll give up and target the next one anyway?

Correct (I think). It may depend on where the alien wants to end up at the end of the turn. If the destination node is occupied, the alien will probably get as close as it can.

 

On a related subject, I was playing TFTD today and ran across a couple Aquatoids standing right next to each other on round 3 outside of a Battleship. Apparently the first Aquatoid was blocking the node and the second alien parallel parked right next to it.

 

- Zombie

[zaimoni]

The relevant bytes I know of in UnitRef.Dat are 69 and 70. But your notes there don't make sense:

Do you mean byte 70 is the index of the node at the end of the planned move?

Yes. Fixed.

Your comment on the EtAI page suggests that the byte might have other uses as well.

It's much harder to analyze the AI once one of your units has been spotted, but it would be efficient to reuse it. The ranges I saw suggested some use in selecting which XCOM unit to target, but I wasn't clear enough to induct anything.

 

Regarding this "stuck" alien. Are other aliens able to walk around it? Or is it not placed between other units and the doors? Does its starting node even link to other nodes so that it CAN move?

UFO nodes are allocated fairly late in the sequence, so it's not unthinkable. I wasn't taking careful notes when, in a base mission, I was planning on a soldier being nailed by a Chrysalid that wasn't. [Ahem...blundered into seeing it from behind; couldn't reliably kill it, so took cover and prepared to deal with a planned zombie...then wondered why the Chrysalid didn't move in the alien turn. Checking the routes...that node didn't have outbound links pointing towards my soldier.

 

That said, I've never seen a Large Scout where the "ambush node" wasn't linked.