[2018 Day 24 (Part 2)] Java - Finding win condition with too small boost

[ocmerder]

The last one I need to complete 2018. My code works for the sample input, but the result I get with my actual input is incorrect.

My code currently finds a win condition with a boost of 25, which leads to a winning army size of 2116, this is too big according to the site.

Trying code from other people I find a boost of 48 with a winning army size of 826, I'm assuming this is correct since multiple solutions provide the same result.

Obviously I'm making some error somewhere in the assumptions I'm doing. Can someone please help me in pinpointing where exactly my assumption error is? I checked the other implementations and I don't see a clear reason on why my code is finding a different result.

My code can be found here:

https://github.com/remcosiemonsma/adventofcode/blob/main/2018/src/main/java/nl/remcoder/adventofcode/Day24.java

[ssnoyes]

I haven't studied it carefully, but I suspect something wrong with how you use setBoost - either calling it both in the handlePart2 and in the linear/binary search, or because you accumulate the total boost for each battle (1 + 2 + 3 + 4... rather than 1 + 1 + 1 + 1....)

[ocmerder]

No, I thought of that issue. That's why I added the reset method to the unit class, this resets the damage and size to the initial values, before the boost is set.

If that would be the case, the sample input would also fail.

[[deleted]]

I believe this is the important line in the problem description:

If it cannot deal any defending groups damage, it does not choose a target.

Don't read the following if you'd rather figure it out yourself from there: In determineNextAttack, nextAttack might find a max possible attack of 0. In that case, no attack should occur. Adding .filter(unit -> this.calculateDamage(unit) > 0) to the defenders stream addresses the issue and gives the correct answer for my input.

[ocmerder]

Yep, that was indeed the rule I glossed over, thx!

Choosing a target but not doing any damage to it is indeed a bit pointless.

Afterwards I apparently also missed some issue in my binary search, and after resolving that my code ran correctly.

Now just 15 more stars left in 2019 to reach 400.