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I was told (but didn't verify it myself, nor have any sources on this) that in Half Life 2 (2004), your enemies would "learn" from your attack patterns and adapt to it. At the same time, as to make the game enjoyable/chellenging and not frustratingly difficult, the agents would adjust their difficulty level to your skill. Take it as an anecdote.

I've played an unhealthy amount of HL2 and spent a while modding it, and I don't believe this is true. People say things like this a lot about games with decent AI. For example, the old quake 3 hoax about bots "learning" to become peaceful on a server that was left running for 4 years. It's fun to imagine the AI as more than just a set of scripts.

The Valve Developer Community has some okay documents about the AI [0], moddb has some nice articles too (no links offhand). And most of the source for combine guards is available in the Source SDK [1], you can see tons of if's and hard coded sequences of actions.

I've read that the AI was changed after the Orange Box release, so maybe there was some learning features to them originally. Or maybe those features do exist within HL2's private source files, though it doesn't "feel" like it.

[0] https://developer.valvesoftware.com/wiki/AI_Learning:_Combin...

[1] https://github.com/ValveSoftware/source-sdk-2013/blob/master...

The director in L4D was had a very simple logic with inputs like number of alive special infecteds and number of zombies to control spawns. It's like seeing a health kit when your health is low.

IIRC it tried to be slightly more clever (might've been L4D2) in that it also kept track of the recent history of the game and attempted to balance the pace of the action.

If it thought players had had it easy for a while it might throw a larger waves of monsters and specifically deny spawning health for a few minutes.

There's a good presentation somewhere about how it would try to balance the rising action with periods of "stress" and "relief" over the course of a level.

And the director in L4D2 actually changed layouts of levels to suit difficulty.

Have to start somewhere, though.

I've heard a similar anecdote about a racing game (I think it might have been in Forza) where the developer had a AI where individual racers could hold grudges and that would affect how aggressively they treated you (bump someone in lap 1 and they'll be more aggressive in lap 3). Eventually they had to take it out for various reasons, but didn't advertise that fact for obvious reasons. When the game was finally released several reviewers brought up the feature as a plus, not realising that they were just reading into the actions of the standard AI.

As a side note, the writeup of F.E.A.R's AI is a fascinating read. By combining very simple goals and actions, they created AI that is still remembered as incredibly good. http://alumni.media.mit.edu/~jorkin/gdc2006_orkin_jeff_fear....

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