How do the opponent characters think and act in games?

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AI games
When I play computer games that have AI opponents, I always had the questions and doubts lingering in mind about the game engines.
In the earliest games that I have been involved with like Doom or Quake, the game play was much like a storyboard and the meanies were programmed to use the linear sight of view to shoot at you. However, in the past decade the games have evolved much and the straightforward kind of intelligence has been overtaken by better and smarter engines. Even though, there have been instances in games where you might suddenly jump to conclusions that the opponents are really smart or really stupid.
In this quest I found this nice article that explains how the game engines work and how they have to follow the computational constraint to enable uninterrupted game play. Do give this a read.
Link: http://www.extremetech.com/article2/0,1697,1856568,00.asp