Advent of Code F# – Day 23

ps. look out for all my other solutions for Advent of Code challenges here.


Day 23

See details of the challenge here.

Today’s input looks like this:

cpy a b
dec b
cpy a d
cpy 0 a
cpy b c
inc a


Today’s challenge is an extension to Day 12, where we have introduced a new instruction for toggle (tgl). To make toggling logic simpler let’s introduce a union type to represent the different instructions we can receive and write a function parse them.

Next, let’s add a toggle function to toggle a given instruction and modify the execute function from Day 12 to work with the union type and support the new tgl instruction as well.

Now we can solve part 1.

let part1 = (execute [ “a”, 7 ] inputs).[“a”]


Part 2

The safe doesn’t open, but it does make several angry noises to express its frustration.

You’re quite sure your logic is working correctly, so the only other thing is… you check the painting again. As it turns out, colored eggs are still eggs. Now you count 12.

As you run the program with this new input, the prototype computer begins to overheat. You wonder what’s taking so long, and whether the lack of any instruction more powerful than “add one” has anything to do with it. Don’t bunnies usually multiply?

Anyway, what value should actually be sent to the safe?

As the description eludes to, if we use 12 as initial input it might take a while to run.. and the naive approach of just executing the logic as before, ie

let part2 = (execute [ “a”, 12 ] inputs).[“a”]

took a good 4 minutes to run on my 2015 MBP.

If you add a few printfn statements and you’ll a few blocks of instructions that repeat for many cycles, eg.

cpy b c
inc a
dec c
jnz c -2
dec d
jnz d -5

you can add a short-circuit when you see this happen and replace it with the equivalent of a = b * d (as the description eluded to).



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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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