Advent Of Code

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An unofficial home for the advent of code community on programming.dev! Other challenges are also welcome!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

Everybody Codes is another collection of programming puzzles with seasonal events.

AoC 2025

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[2015 Day 19] Help with an old one... (adventofcode.com)

submitted 2 weeks ago by owenfromcanada@lemmy.ca to c/advent_of_code@programming.dev

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Okay, so I'm almost a decade behind on this...

But I only found out about AoC this year. So I'm gradually going through the backlog. And this is the first one that has left me clueless about a reasonable approach.

https://adventofcode.com/2015/day/19

I'm on part 2 of this problem (reconstructing the molecule in the least number of steps). So far, I've figured...

I need to work "backwards" (going from the target molecule down to e in the fewest steps possible) in order to bound the problem
Tried brute forcing it this way, way too inefficient
So I tried limiting the maximum number of steps--even at 10 steps max, I only make it through a small fraction in several minutes
I tried rewriting my recursive function as a while loop, didn't help

Things I could try:

use more efficient data types (I'm using Python and doing slicing, not sure if changing that would make a difference)
represent the formulae in some other format (not sure what would be better)
doing something less "brute-force-y", not sure what that would be

Anyone willing to offer any pointers?

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[–] lwhjp@piefed.blahaj.zone 2 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

Oh gosh, I remember this one. Working backwards is a good idea. In addition, you can just look at the start of the string when trying substitutions. I don't think that's valid in general, but it worked for me in this case.

There's another trick you can do if you look carefully at the input data. I didn't implement it in my solution because I didn't spot it myself, but it essentially makes the problem trivial.

[–] owenfromcanada@lemmy.ca 2 points 2 weeks ago (1 children)

Thanks for your thoughts! For the input data, I did notice that every substitution goes from 1 element to 2-8 elements. I suppose I could take that into account, and order them from "most efficient" to "least efficient", and that way I might be able to stop iterating earlier?

I also haven't looked into whether there are any combinations that are impossible to build from (or inversely to break down into e).

Those two are probably good avenues to try.

Thanks again!

[–] lwhjp@piefed.blahaj.zone 2 points 2 weeks ago* (last edited 2 weeks ago)

That's not quite the key observation...

spoiler

Many of the productions end in an element which does not appear on the left-hand side. That acts as a flag which tells you where to look for substitutions.