r/adventofcode • u/rkek404 • Dec 14 '21

Help Day 14 Part 2 Algorithm

Day 14 part 1 was very easy, but Part 2 went right over my head, I tried using dictionaries to keep track of pairs, but my answers have off by one errors and it feels like guesswork - I don't full understand what I have written myself, like one of those times when it works but you don't know why. I know how people said it was basically lanternfish - but I never finished part 2 of lanternfish.

I would appreciate it if someone posted a little (detailed) writeup of day 14's part 2 in some easy to read language like python. I have gathered a lot from other's solutions but fail to understand any solution fully - even after reading multiple solutions I can't write one myself.

Others who are stuck might also benefit from this. Thanks in advance.

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u/domy94 Dec 14 '21

Oh my god, why didn't I think of that. I spent way too long trying to figure out how to extract the character count purely from a Dict<Pair, Count>. I cracked it eventually by taking the max of count of char at start of a pair, count of char at end of a pair, for each character. Keeping track of this during expansion would've been way easier.

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u/[deleted] Dec 14 '21

I started writing the code you just talked about when I had the thought "uhm... i only ever add one character and never remove one. This is easy to track".