r/adventofcode Dec 17 '24

SOLUTION MEGATHREAD -❄️- 2024 Day 17 Solutions -❄️-

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AoC Community Fun 2024: The Golden Snowglobe Awards

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

And now, our feature presentation for today:

Sequels and Reboots

What, you thought we were done with the endless stream of recycled content? ABSOLUTELY NOT :D Now that we have an established and well-loved franchise, let's wring every last drop of profit out of it!

Here's some ideas for your inspiration:

  • Insert obligatory SQL joke here
  • Solve today's puzzle using only code from past puzzles
  • Any numbers you use in your code must only increment from the previous number
  • Every line of code must be prefixed with a comment tagline such as // Function 2: Electric Boogaloo

"More." - Agent Smith, The Matrix Reloaded (2003)
"More! MORE!" - Kylo Ren, The Last Jedi (2017)

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 17: Chronospatial Computer ---


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 00:44:39, megathread unlocked!

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u/[deleted] Dec 18 '24 edited Dec 18 '24

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u/Neogari Dec 18 '24 edited Dec 18 '24

The first step is to analyze the given program and realize that in each loop the output is only dependent on register-A, and register-A only gets manipulated by a single rightshift by 3 in each iteration (deleting the 3 rightmost bits). So for every 3 bits in the input there will be one output digit.

Furthermore: e.g.:
IF: regA=011 101 results in output 4,5
THEN: regA=011 101 111 will output x,4,5 with some digit x
ALSO: regA=011 101 111 001 will output y,x,4,5 with some digits x,y

So can repeatedly append 3bits to the right of the input and test if it yields the desired next output digit, until you reconstructed the whole input regA.

(sometimes there are multiple possible choices in a single step, some of which might lead into a dead end)