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.

EC 2025

AoC 2025

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[2024 Day 6] What is the optimal algorithm for Part 2? (programming.dev)

submitted 1 year ago by Pyro@programming.dev to c/advent_of_code@programming.dev

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I know that we can brute force it by placing an obstacle at every valid position in the path, but is there a more elegant / efficient solution?

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[–] lwhjp@lemmy.sdf.org 1 points 1 year ago

You gave me an idea!

Give each obstacle in the initial map an ID, and build three indexes by ID, by row, and by column.
Use the list of obstacles to build up a map from (ID, direction) to (next ID, direction) (if any) -- that is, for each obstacle, update the map entry for any preceding obstacle that could reach it.
This map can be used to compute the path very cheaply.
And crucially, you can easily update the map for new obstacles using the same method above.

I think this would give a pretty good speed up, and you might not need to worry about only checking intersections.