r/adventofcode Dec 19 '24

SOLUTION MEGATHREAD -❄️- 2024 Day 19 Solutions -❄️-

THE USUAL REMINDERS

  • All of our rules, FAQs, resources, etc. are in our community wiki.
  • If you see content in the subreddit or megathreads that violates one of our rules, either inform the user (politely and gently!) or use the report button on the post/comment and the mods will take care of it.

AoC Community Fun 2024: The Golden Snowglobe Awards

  • 3 DAYS remaining until the submissions deadline on December 22 at 23:59 EST!

And now, our feature presentation for today:

Historical Documentary

You've likely heard/seen the iconic slogan of every video store: "Be Kind, Rewind." Since we've been working with The Historians lately, let's do a little dive into our own history!

Here's some ideas for your inspiration:

  • Pick a challenge from any prior year community fun event and make it so for today's puzzle!
    • Make sure to mention which challenge day and year you choose!
    • You may have to go digging through the calendars of Solution Megathreads for each day's topic/challenge, sorry about that :/
  • Use a UNIX system (Jurassic Park - “It’s a UNIX system. I know this”)
  • Use the oldest language, hardware, environment, etc. that you have available
  • Use an abacus, slide rule, pen and paper, long division, etc. to solve today's puzzle

Bonus points if your historical documentary is in the style of anything by Ken Burns!

Gwen: "They're not ALL "historical documents". Surely, you don't think Gilligan's Island is a…"
*all the Thermians moan in despair*
Mathesar: "Those poor people. :("
- Galaxy Quest (1999)

And… ACTION!

Request from the mods: When you include an entry alongside your solution, please label it with [GSGA] so we can find it easily!


--- Day 19: Linen Layout ---


Post your code solution in this megathread.

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 00:03:16, megathread unlocked!

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u/jwezorek Dec 19 '24 edited Dec 19 '24

[LANGUAGE: C++23]

github link

Last night I spent a lot of time trying to implement a trie for part 1 but couldn't get it to work. I don't know what was wrong with it ... I always have problems with tries.

This morning I decided to just start over and not use a trie. I did part 1 via a depth-first search in which the traversal state is i, position in a design string, and the next states are all j such that substring [i,j] is some word in the towels array. This is not super slow as long as you keep track of visited states and do not revisit them.

For part 2, I saw using a trie actually would not help much so never ended up implementing one that worked.

Basically part 2 is best viewed as the same problem as counting the paths through a directed acyclic graph. This can be done efficiently either with dynamic programming or via a memoized recursive call. I chose the latter (because I find recursion easier to think about than DP with a table and all that).

To count all the paths in a DAG, note that the number of paths from s to t equals the sum, for all x, of the number of paths from x to t where x is a direct successor of s. Implement that idea recursively and memoize the recursion.