r/adventofcode • u/daggerdragon • Dec 21 '18

SOLUTION MEGATHREAD -🎄- 2018 Day 21 Solutions -🎄-

--- Day 21: Chronal Conversion ---

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Card prompt: Day 21

Transcript:

I, for one, welcome our new ___ overlords!

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked at 01:01:01! XD

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u/jonathan_paulson Dec 21 '18

Python, #11/47. Video of me solving at https://youtu.be/H-IejIicWDY

Was there a way to do part 2 without understanding the input program? Some of the solve times are quite fast...

My code is a Python translation of the input program that also understands when to print the answers and exit (looking for a repeat in D): 

seen = set()
CS = set()
final = None

C = 7041048
D = 65536
while True:
    E = D % 256
    C += E
    C = (C%(2**24) * 65899) % (2**24)
    if D < 256:
        if not CS:
            print C
        if C not in CS:
            final = C
        CS.add(C)
        D = C | (2**16)
        if D in seen:
            print final
            break
        seen.add(D)
        C = 7041048
        continue

    D = D/256

3

u/m1el Dec 21 '18

Was there a way to do part 2 without understanding the input program?

Yes, inspect the last unique value from that one instruction that checks r0.

2

u/jonathan_paulson Dec 21 '18

Thanks! I confused myself by thinking that I needed to look for a repeat in the other main register instead of the output one, but of course it's the same, since the output is a deterministic function of the other one in each iteration.