It’s that time of year again!

Advent of Code is a series of daily programming puzzles from December 1st to 25th, becoming progressively more difficult each day. The puzzles can be completed in any programming language- inputs are short text files and the solutions are integers. Many people use these puzzles to learn a new language or try new techniques.

Last year, we had lively daily discussions and solutions in both Ada and SPARK on the forum. This year, I want to open the conversation up to any language with a focus on safety or reliability- Ada, SPARK, MISRA-C, or any other metallic compounds.

I think we can reuse last year’s leaderboard code 1708445-6a8f7730, which you can join on the Private Leaderboards page while logged into your AoC account.

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I’m not sure why he didn’t call this one SNAFU, but it was easy and fun, which I think is typical on Day 25.

Anyone add up the numbers using SNAFU arithmetic, or did everyone convert to decimal, add, then convert back?

PS If I may be allowed a personal remark: This is my third time doing AoC, but the first one I’ve completed it in time: for the 2020 competition I finished up around New Year’s instead of Christmas Day; then I went back and completed the 2019 competition, and then I took a break until this year. Having thsi forum to discuss things with, even to vent frustration once or twice, was helpful. I really enjoyed learning from everyone. Maybe I’ll go back and complete some other years… after getting some first.

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Part 1 is another spoiler. I wasted too much time getting the blizzard logic wrong. I knew what to do; I just kept getting the math wrong! I did at least have the presence of mind spoiler.

Part 2 was also pretty straightforward, once Part 1 is done. Interesting that the result is not three times the value of Part 1, but not especially surprising, given how the blizzards move around.

Glancing over at the Reddit Solutions Megathread, some commenters have noticed that spoiler (It certainly would be in my case.)

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Another one that I thought was fun! The input proved helpful, and was not so different from the example today as it was yesterday. Then again, it would be hard to diverge as greatly from the example to the input as yesterday did…

My solution spoiler To determine the direction an elf considers, spoiler

Is there a way to determine how many variants an enumerated type has?

I have an idea for a visualization, and if I find time I’ll do it later.

Added later: Now that I review the discussion on Reddit, I notice that a lot of people missed something that I almost missed myself: spoiler Yeah, watch out for that.

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This was fun. Even the debugging was fun. Even though the example was once again naughty, at least for Part 2, this was fun. (If anyone’s input creates a cube in the same way as the example, I’d like to know!)

Only one hint / comment from me: in part 2, spoiler.

I thought about drawing the paths onto the map, but it’s pretty late where I am, so maybe tomorrow. In lieu of that, here’s an aid I made to help with Part 2.

3 days left!

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I really enjoyed today’s puzzle! Observations / hints / spoilers:

• The example does not cover all the cases you need to check in Part 2. Interestingly, the input is relatively small if you look at it right, so even though I got Part 2 wrong the first time, it didn’t take me long to work through the example and find my mistake.
• In part 1, spoiler
• In part 2, spoiler

4 days left!!!

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(edited after remembering another bad idea, whoops)

I had one very good idea, one good idea, and quite a few bad ideas, including one very bad idea that held me up a long time.

• The very good idea was spoiler As a result, the solution to both parts takes only less than two seconds (using full optimization and inlining).
• The good idea I don’t quite remember, sorry, but I had it in mind at one point.
• One bad idea was spoiler. Fortunately, I caught that one quickly.
• Another bad idea was spoiler Rather naughty of the example not to have a value that would illustrate that.
• The very bad idea came while testing my code on the example. spoiler I wasted over an hour on that stupid mistake…

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Nope. I’ve been at this for hours.

• I wasted hours just trying to get my code to run the example in a reasonable time, and when I gave up I discovered that the input takes not much longer to run than the example.

  So there’s a hint that doesn’t require a spoiler: don’t bother trying to get your program to work on the example in seconds; the input isn’t that much harder! Just try to get the program to work on the example, period!

• Unfortunately, that’s not enough. The example doesn’t really illuminate the problem. Your code can run correctly on the example and run incorrectly on the problem input. This happens from time to time, but on a problem like today it’s ruinous.

• To help me debug my program, I downloaded and ran several people’s solutions posted online (in other languages). I haven’t tried any of the solutions, but there’s a morbid satisfaction in the fact that different “solutions” produce different results on my input.

• Some authors admit that they guessed at optimizations and got lucky; the optimizations probably don’t work in general. I’ve tried a few, and nope, they don’t work on my input (unless I’m implementing them wrong).

• Some authors used parallelism. I may yet try that, since the problem is parallelizable, and otherwise I have to wait 5-10 minutes per attempt.

In short, I haven’t finished it yet. IMHO the puzzle is terribly flawed, for the reasons given above. And, yeah, sour grapes, too.

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It was nice to get an easy one after the past two days. The only hard thing was that gnatprove was either hanging or crashing, and I guessed that it didn’t like all the nested-nested-nested loops. I rewrote it so I generated an array of all coordinates and looped through that instead of the nested loops and I was able to get gnatprove to finish.

In case anyone needs help with part B:

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Part 1 is pretty straightforward, I think.

Part 2 requires some mathematics, or at least that’s what got me to solve it quite quickly I did one other thing, too.

1. spoiler
2. spoiler
3. spoiler

The solution I uploaded for part 2 will probably not work on the example program, because the example’s sequences aren’t quite as well-behaved as the actual puzzle, which made the problem a little harder. But the same technique really does work!

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I taught my elephant to change places with me if his room name is bigger than mine and that cut the number of search options in half But this isn’t enough to get the answer fast anyway…

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Only 10 days left!

My solution to Part 1 was quite slow, perhaps because I used an OrderedSet. It might be worth retrying with a HashedSet at some point.

So I was really worried that Part 2 would be one of those problems whose obvious solution is to apply Part 1, but whoses complexity explodes on you, and as I first began to read it, my heart filled with dread. But then I realized that spoiler It still takes a few moments, but it worked! So I will be interested in seeing if anyone manages to solve it in a fraction of a second, and if so, then how.

I’m also amazed that Part 2 can even be a problem; the fact that he pulled off such a puzzle impresses me. Maybe it won’t impress me if I think about it enough, but even after a couple of glances, I’m still impressed. Maybe I just need some

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Playing around with visualizing the output…

Test Data part 1: First and Final states:

Animated version:

Part 2:

For the real data it gets very large, so I didn’t output the frames, but only first and final states of part 1:

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We’re past halfway!

(The downside is that the problems have suddenly stepped up in difficulty!)

For the second day in a row, writing a program that solves the example wasn’t enough. Twice I ran it and got solutions that were rejected as too large. I had to comb through some of the pairs that my program claimed were legitimate. In both cases, it was the case where I had to compare a number and a list. The instructions are to create a new list that includes the number, then compare the lists, but it seemed equivalent to me simply to compare the number with the first element of the list. This produced false positives. I haven’t quite figured out why, though.

Curiously, in part 2 I was able to make it work simply by spoiler

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Big-time spoilers that would have helped me finish a lot sooner. Most are available elsewhere; e.g., the subreddit.

• spoiler
• spoiler
• spoiler

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Spent a huge amount of time to understand why Ada.Integer_Text_IO.Get is unable to read a value from 0: string . As always I forgot about deprecated integer format, where # could b replaced by :. Isn’t time to get rid of this in 2022?

http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-J-2.html

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I’m having fun building a visualization, so I’ve only finished part 1. It’s not about the leaderboard, right?

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I think the problem statement was intentionally difficult to parse this time. Those conditionals are a lot simpler than it makes them sound.

I kept the visited positions in a Hashed_Set and coming up with a good Hash function for a pair of Integers was new for me. I ended up making a lot of assumptions (Integer'Size = 32 and Hash_Type'Size = 64) and just shifted one of the integers left 32 bits. A cursory Google search turned up the Cantor Pairing function, but I wasn’t able to make this work with negative integers in a short amount of time. Clearly I need to do more reading.

Part 2 caught me off guard, I hadn’t anticipated I’d need to deal with more elements.

Note: I’m going to stop using the spoiler blur on the daily posts. If you’ve gotten this far, I assume you know that these posts contain spoilers.

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