This is more of a community wiki answer, aggregating the various modelling tools suggested by other answers, I've just added additional links to resources.

I don't think there's any need to restate that you should be using a different approach to thousands of hard-coded if/else statements.

* [DSL Programming (Domain Specific Language)](
* [Markov Model]([Markov Decision process](
* [Rete net / Rete algorithm]( 
* [Expert System](
* [Matlab](, [(Wikipedia)](
* [R](