AC enhancement bonuses aren't implemented correctly

[dolio]

While messing around a few days back, I realized that various AC enhancement bonuses aren't actually implemented properly. Here's the way it works right now:

• All AC bonuses are put in a bonus list with their respective types
• Base armor bonuses from worn armor have a different type than enhancement bonuses to that armor, which allows them to stack
• All the bonuses are added up using the usual 3.5 rules of bonuses of the same type failing to stack

This might seem like it would give the correct result, but it actually doesn't. You can observe the results this way:

• Wear a chain shirt +3, which should give 7 AC
• Wear bracers of defense +6 which give 6 AC

Tabletop rules say this should give you +7 AC from armor. The enhancement modifies the chain's base 4 AC to 7, which is greater than the 6 from the bracers. However, in ToEE you get +9 AC, because you get the +6 base armor bonus from the bracers (which is grater than the +4 from the shirt) and the +3 enhancement bonus from the shirt still applies.

I suspect this also happens with shields. So if you wear a +3 buckler, casting shield probably gives you +7 shield AC instead of +4. It would also happen for natural armor, but I don't think there are ways to get multiple sources of natural armor; all the spells and items give enhancements, and the base just comes from the object fields.

I'm not sure how to fix this best. I'll think it over.

[dolio]

I think this is actually pretty easy to fix.

There's already a d20 query Q_Armor_Get_AC_Bonus for calculating the AC of a specific piece of worn armor. You can call it from the console with:

critter.d20_query_with_object(Q_Armor_Get_AC_Bonus, armor)

This will give you 7 for the chain shirt example. It works for shields, too.

So, instead of just having both the Armor Bonus and Armor Enhancement Bonus respond directly to the GetAC event, I think only Armor Bonus should, and it should figure out what value to add by running that d20 query to incorporate the enhancement bonus. That way you get just armor bonuses from each source, and they won't stack. Same with shields.

[doug1234]

That seems like it would work well. Have you given it a try?

[dolio]

Not yet. Maybe I'll just do it, though, because I have to mess around with the shield bonus stuff anyway for shield bashing, and once I get that working it's not much more work to do for armor as well.

[dolio]

Okay, so, there's actually a problem with my strategy above. The d20 query isn't correct, either. If you get multiple enhancement bonuses on an item they stack, because a d20 query only has a return value, not a bonus list. You can actually see this in game, because the query is used for the AC value reported in the UI. If you cast e.g. Magic Vestment on an already enchanted piece of armor, the UI will say it provides more AC than it actually does.

I could fix the query by doing some packing, and fix up the result where it's called. Like, put the base AC bonus in the low byte, and the enhancement bonus in the second byte. The queries would manually take the max of the relevant byte with their value. Then you add the two bytes up at the end.

I only see two places where it's called. One is the monk AC bonus, which just checks if it's non-zero, and doesn't require a change. The other is the UI function, which is already partially replaced for the armor UI, so it's not a big deal to tweak that, either. Then it would fix the AC reported in the UI as well.

Does that sound all right?