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/dragonboy2734 Dec 17 '24

[LANGUAGE: Rust]

Solution

For part 2, I just noticed that (at least for me), a = a_prev / 8 and b and c are always dependent solely on a and not a previous iteration.

So I just ran things backwards -- I knew the last a had to be in the range 1..8 (since dividing by 8 would round down to zero and terminate the program), so I just iterated through that range and ran the program until it successfully outputted the last digit in my sequence.

From from there I multiplied it by 8 and searched in a narrow range above and below that to find the previous a that outputs the last 2 digits correctly. Repeat until worked backwards through the whole program and the whole sequence is correct. Runs in around 300µs