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Advent of Code 2024 Week 2: this time it's all grids, all the time (awful.systems)

submitted 1 week ago by [email protected] to c/[email protected]

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[–] [email protected] 3 points 1 week ago* (last edited 1 week ago) (3 children)

Day 13, day 13 of shirking other responsibilities.

Ok. So, I overthought this a little. Ultimately, this problem boils down to solving a system of 2 linear equations aka inverting a matrix.

Of course, anyone who has done undergraduate linear algebra knows to look to the determinant in case some shit goes down. For the general problem space, a zero determinant means that one equation is just a multiple of the other. A solution could still exist in this case. Consider:

a => x+1, y+1
b => x+2, y+2
x = 4, y = 4 (answer: 2 tokens)

The following has no solution:

a => x+2, y+2
b => x+4, y+4
x = 3, y = 3

I thought of all this, and instead of coding the solution, I just checked if any such cases were in my input. There weren't, and I was home free.

No real changes to the solution to p1 aside from the new targets. I wasn't sure if 64 bit ints were big enough to fit the numbers so I changed my code to use big ints.

I'm looking at my code again and I'm pretty sure that was all unnecessary.

[–] [email protected] 3 points 1 week ago* (last edited 1 week ago)

13 commentary

Solved p1 by graph search before looking a bit closer on the examples and going, oh...

In pt2 I had some floating point weirdness when solving for keypress count, I was checking if the key presses where integers (can't press button A five and half times after all) by checking if A = floor(A) and sometimes A would drop to the number below when floored, i.e. it was in reality (A-1).999999999999999999999999999999999999999999999. Whatever, I rounded it away but I did spend a stupid amount of time on it because it didn't happen in the example set.

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