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Comment by tyingq | Hacker News Reader

Link to last year's Day 19: https://adventofcode.com/2020/day/19

I didn't read it in depth, but it feels like you could build regexes on the fly to solve it. Perhaps I missed something crucial.

Edit: Ahh, day 13: https://adventofcode.com/2020/day/13

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Part 2 complicated it, but yes. I just built regexes on the fly for Part 1. I wonder if they mean Day 13. The underlying math is based on the Chinese Remainder Theorem, but it is solvable without knowing it (or, in my case, "knowing" it, once I saw others mention it it came back but my math degree was a long time ago). A solution could be discovered incrementally without knowing the math by trying to optimize the naive version.

I did mean Day 13, thank you -- I updated my post accordingly. Sorry about the confusion!

Apologies, I updated my post. It was Day 13 where this happened. Not sure why I stopped at Day 19, tbh.

Day 13 you really needed to know of Chinese remainder theorem; I would say this was probably the only day in the last couple of years so dependent on non-trivial maths. I would still recommend AoC though.

I loved day 19 because I managed to use some Automata theory I studied at uni. Something like, I cannot simply use a regex because it's a context-free grammar. Then I learned about recursive regexes and how they could be used to parse a cfg. Aaah fascinating, really.

yep, that's what I did. simply converted the input into a regex and matched it. part 2 had to add a little special casing but ultimately not that difficult.

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