Declaration of war mechanics

I've read in the forum you can avoid the AI declaring war on you by "keeping them busy". Also, perhaps having a strong army might help.

However, this seems all qualitative, I'd like to see some more solid numbers. Can anyone point out the code or explain the way that the AI declaration (or preparing for war) works? Perhaps a formula which one can plug in the AI attitudes and any other factors, or if it's not a simple formula, a commented sample of the code.