Announcing Advent of Code Golf 2021! (+ Event challenge sandbox)

Originally from caird coinheringaahing's idea, I thought about how we could do it this year with such a short time before December, and I decided to gather some ideas from previous AoC events. (There's an explicit permission from Eric Wastl, the one who runs the AoC event.)

As I found 25 potentially good challenge ideas from previous AoC events (2015-2020), I'm running the Advent of Code Golf (AoCG) this year. On each day between Dec 1st and Dec 25th (inclusive), a new challenge will be posted at UTC midnight. It is a free-for-all and just-have-fun-by-participation event, no leaderboards and no prizes for solving them fast or solving them in the shortest code. But, if the event goes well, I will hand out a few bounties to the most active participants, based on various measures.

For the posting schedule: I expect myself to be unavailable on weekends, and I might have problems on other days too. I'll post the challenges when I can at around UTC midnight, but if a new challenge doesn't go live for an hour, or on weekends, anyone else is free to move that day's challenge to main instead. Please don't post it as Community Wiki though (it causes all answers to be CW too). You can take some extra rep as a bonus for helping me run the event :D

Event challenges

This post serves as the announcement of AoCG 2021, but it is also a public sandbox of challenges that will be used for the event. I will write up the 25 challenges for feedback here. If you have another idea (specifically related to previous AoC problems), feel free to post it; we will refine it together and line up the posting schedule accordingly, in case we have to throw away some existing ideas.

Note to those who are opposed to the AoCG being AoC-G instead of Ao-CG: I don't have any intention to make the event an AoC-G. I simply saw AoC as a good source of challenges to quickly draw ideas from (I'd totally agree with using sandbox gems if we had much more time than ~2 weeks before December), and the majority of challenges will be quite different from what you see on the AoC.

AoCG2021 Day 1: Automated delivery frenzy

• Can we instead output the total number of santas? Nov 19 '21 at 8:52
• It's worth mentioning that each instruction moves them exactly 1 square, since I was confused by that at first.
– Wheat Wizard Mod
Nov 19 '21 at 13:56
• I can has more test cases? Nov 23 '21 at 1:29
• Problem changed to total number of santas. Added a mention of moving to "neighboring house". Test cases added. Nov 23 '21 at 2:16

AoCG2021 Day 16: Interpret Duet

• The 34th line in the second example is jgz 1 3. But X should be a register in jgz X Y. Nov 28 '21 at 12:45
• @alephalpha Changed the description to allow constants in the first argument of jgz. Nov 29 '21 at 1:02
• Tempted to answer this in Go or something... lol Dec 2 '21 at 17:57
• "Y and Z can be a register or a constant" - constants are only integers, and can be negative, I assume? Could you specify that? Dec 15 '21 at 2:37
• @pxeger Fixed, thanks. Dec 15 '21 at 2:42

AoCG2021 Day 25: Stitching maps together

• Not this again... Nov 30 '21 at 8:30

AoCG2021 Day 3: Say-Look-Say

• "Be sure to handle multi-digit numbers that appear since iteration 7." - I believe this means we need to output [..., 1, 1, 3, 1, 3, 4, 1, ...] not [..., 1, 13, 1, 34, 1, ...] (for iteration 8), but it took me a while to work that out based on the test-cases. Do you think there's any way you could clarify that? Dec 2 '21 at 16:15

AoCG2021 Day 21: Blinking through the forest

• Can we take the rational number as [numerator, denominator]? Can we assume the rational will always be in reduced form? Nov 29 '21 at 10:29
• @emanresuA Yes, as indicated in the last line of the Input. Nov 29 '21 at 23:22

AoCG2021 Day 22: Hyperbolic rescue

• Thanks for your challenge ideas. I decided to discard the remaining three in my list and use all of yours instead. What is your opinion about the challenge order? Is it OK to shuffle the challenges around so that consecutive challenges look less repetitive? Nov 30 '21 at 7:49
• @Bubbler Sure. However I still had had one or two more ideas to write up as part 2s, I was thinking I would still have a little bit of time, so I'm not sure when exactly those might get posted.
– Wheat Wizard Mod
Nov 30 '21 at 9:07
• If you can write up more challenges, please go ahead. I think it would be better if we can avoid three copies of day 1s for a more interesting one. You can do whatever you want for the leftover(s). Nov 30 '21 at 10:10

AoCG2021 Day 7: Bathroom security goes wild

• It's not clear without looking at the example that you start at 0. Also, while this is interesting as a programming problem, I'm not sure how good it will be for golf because the lookup table is just going to be copied from whoever finds the optimal one first, and the rest of the challenge just applies several layers of boring loops. Dec 5 '21 at 20:56
• @pxeger Fixed the description. I understand your concern, but trying to replace it with another challenge idea is IMO risky at this point. Dec 5 '21 at 23:25

AoCG2021 Day 11: Garbageful streams

• The outmost group in the last example is not closed. The first { does not have a corresponding }. Nov 26 '21 at 2:10
• @alephalpha Thanks, fixed. Nov 26 '21 at 6:46
• What is the meaning of the {{!{ at the start of the last test-case? As far as I can tell, the rules should prohibit a ! from being inside a group without being inside garbage. Nov 30 '21 at 12:33
• @pxeger Oh, I misread the AoC spec myself and thought ! can cancel anything anywhere. Fixed now. Nov 30 '21 at 23:20