Some say strategy games, simulators, and RPG's - generally speaking I think it comes down to individual mechanics. For the space strategy game I'm working on, I'm in the finishing stages for a dynamic logistics system. After 7 months of having to re-design everything in my game, I get to witness supply lines that form themselves between planetary colonies, with minimal player input.

You want to build a cruiser at Shipyard A? One click, done! Based on the available ships in the system, exactly 11 mining freighters assign themselves to extract iron ore from Mars, and deliver it to a Steel Ingot Factory. As soon as there's enough steel for a decent pickup (for efficiency's sake), any kind of available ship with a cargo hold can deliver it to Shipyard A.

I can keep going for hours, but It's starting to feel like bragging, which I don't feel I have the right to do yet. I just found this mechanic very challenging to code, yet I enjoyed the struggle the whole way through because I learned a ton.

So, what's the most challenging thing you've coded, and if you managed to complete it, how did you feel?