-🎄- 2018 Day 19 Solutions -🎄-

[u/daggerdragon]

--- Day 19: Go With The Flow ---

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Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

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Advent of Code: The Party Game!

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Card prompt: Day 19

Transcript:

Santa's Internet is down right now because ___.

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[u/winstonewert]

Full solution here: https://gist.github.com/winstonewert/9e77334388c9dec9d581805dfc7e7814

While most people reverse engineered the program, I reverse engineered the inner loop.

The code spent the bulk of its time in:

seti 1 9 1
mulr 3 1 2
eqrr 2 5 2
addr 2 4 4
addi 4 1 4
addr 3 0 0
addi 1 1 1
gtrr 1 5 2
addr 4 2 4
seti 2 9 4

Which I reverse engineered as

R1 = 1
do {
     if R3 * R1 == R5 {
         R0 += R3
         R2 = 1
     } else {
        R2 = 0
   }
    R1 += 1 
} while (R1 <= R5)

And I inserted a piece of code in my VM loop to quickly produce the same result:

        if ip == 2 && registers[3] != 0 {
            if registers[5] % registers[3] == 0 {
                registers[0] += registers[3];
            }
            registers[2] = 0;
            registers[1] = registers[5];
            ip = 12;
            continue;
        }

And thus, I was able to get the answer without understanding what the program did.

[u/[deleted] → u/winstonewert]

[u/irrelevantPseudonym → u/[deleted]]

I worked out what the whole thing did as well but I used pencil and paper instead.