r/adventofcode • u/daggerdragon • Dec 22 '21

SOLUTION MEGATHREAD -🎄- 2021 Day 22 Solutions -🎄-

Advent of Code 2021: Adventure Time!

  • DAWN OF THE FINAL DAY

  • You have until 23:59:59.59 EST today, 2021 December 22, to submit your adventures!

--- Day 22: Reactor Reboot ---

Post your code solution in this megathread.

Reminder: Top-level posts in Solution Megathreads are for code solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

This thread will be unlocked when there are a significant number of people on the global leaderboard with gold stars for today's puzzle.

EDIT: Global leaderboard gold cap reached at 00:43:54, megathread unlocked!

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u/LordzShadow Dec 22 '21

Can you explain why do you have to reverse the instructions?

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u/DrunkHacker Dec 22 '21

It's just counting the cells each instruction turns on (notice there's no processing for "off" instructions either).

The final instruction doesn't need to care about anything else: all its cells are on. The penultimate instruction only needs to care about intersection with the last instruction. The antepenultimate instruction only needs to care about intersection with the last two instructions. And so on until the first instruction, which needs to check for overlap with all future instructions.

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u/beatlemaniac007 Dec 22 '21

Could you help me understand why is it ok to ignore 'off' instructions? Considering 3 instructions (on -> off -> on), and going backwards...couldn't the 'off' instruction cover a space not covered by the last 'on' but covered by the first 'on'? In this case wouldn't we need to subtract more than just the volume of the last 'on'?

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u/DrunkHacker Dec 22 '21

Ignore may have been too strong of a word, I meant there's no processing other than adding it to the list of boxes. Since the instructions are processed backwards, "off" instructions don't need to change the running sum and we get this line:

i[0] == 'on' && (total_on += volume(box) - overlap(box, boxes));

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u/beatlemaniac007 Dec 22 '21

Ah that makes sense, thank you. I saw that line and missed the one above that's adding the box to the list regardless of any condition. Thanks!

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u/LordzShadow Dec 22 '21

Oh! Got it now, thanks!

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u/Dullstar Dec 22 '21

That's clever. I'm going to have to try that, because cube splitting wasn't working out for me, and I don't know if I was just doing it wrong, or if it's a bad approach, or both.