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cant you take the ai off of another game
Most of the Games I know have protection, and I mean a legal one, on their source codes, those are the main programmed blocks where all the structures of code the game uses are located, and in most cases, the A.I. as well... This means that first, you need to know at least C# language to understand what you will examine, and second, to have access to their Source Code you need an "explicit permisson" from them.
Tough, there are open source projects (I know only one) that offer free access to their main codes. The one I know is Glest: www.glest.org
In short words, its very complex to be done, but not impossible.
if you make one it needs to have a easy. medium and hard modes to it. then the map editor needs to be able to make maps for it.
That is possible, but keep in mind that a skirmish mode, if made, would be added as a "Game-mode" in Multiplayer, or as "Bot" blocks, for each Computer side. Normally, these modes are available for Single Player, but I guess it could be possible to configure the level of the Bots or the "mode" one.
and to come to think of it you might aswell do a total conversion of the game to make it viable.
A total conversion means purely hardcoding... The only capable of that here, and also authorized, is Stu, the forum admin and main collaborator. And about collaborating, he is at the moment very busy working on the patches and improving the online play, as some often-ocurring synch-losts were encountered time ago. So in conclusion, its not viable to make a hardcoding of the game to make the skirmish; in the other hand, Stu may provide improvements by creating useful functions for tracing important data, for example a function to create an area in the map, and then checking it for enemy units, and another for deleting that area when it becomes useless. The coding for the multiplayer isn't very hard as far as I know, so in that part it might be viable.
In final words, an "AI" should be possible, but two things are needed: time and knowledge. These two things don't come from one day to another, but there is some people who have them, or part of. If someone wants to collaborate with this and knows something about, or has a lot of interest (AND TIME) to participate, I invite him/she to do so as a Mip mod collab., feel free to PM me and I will tell you how to become a member and what you could start doing.
Have fun! Visit us often! And sorry for the so long post!