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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[–] hades@programming.dev 3 points 2 weeks ago

Just needs a bit of formal languages theory, but otherwise the algorithm is fairly simple. Here’s my code if you decide to look at it at some point: https://github.com/hades/aoc2015/blob/master/dec19.cc