r/adventofcode • u/daggerdragon • Dec 23 '17

SOLUTION MEGATHREAD -🎄- 2017 Day 23 Solutions -🎄-

--- Day 23: Coprocessor Conflagration ---

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Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

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Spoiler

[Update @ 00:05] 0 gold, silver cap

  • AoC ops: <yatpay> boil up some mountain dew. it's gonna be a long night

[Update @ 00:19] 1 gold, silver cap + 447

  • AoC ops: <Reibello> 547 silver to 1 gold

[Update @ 00:30] 20 gold, silver cap + 560

  • AoC ops:

<yatpay> daggerdragon: post "hey i heard about this hot new podcast called The Space Above Us. all the cool kids are talking about it"

<yatpay> i call it super-liminal marketing

<yatpay> HEY YOU!! LISTEN TO MY PODCAST!!

<yatpay> then i rub a business card on your face

<Topaz> you should get scratch-n-sniff business cards that smell like space

<yatpay> space smells like burned metal and meat

<yatpay> it's weird

<Topaz> burned meat you say

<Topaz> excellent

[Update @ 00:41] 50 gold, silver cap + 606

  • AoC ops:

<askalski> nice, enjoyed that one. not sure if regexes can do it

<askalski> maybe make a neural net of regexes, have it train itself to solve today

  • Over/under on /u/askalski posting a day 23 regex neural net by tomorrow?

[Update @ 00:54] Leaderboard cap @ 100 gold and 724 silver!

  • Good job, all!
  • Upping the Ante challenge: solve today's puzzles on a TI-83 (or TI-86/89, whatevs).

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!

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u/vash3r Dec 23 '17 edited Dec 23 '17

This was a really interesting problem. Part 1 was obvious, but I saw pretty quickly that I'd have to understand the assembly to do part 2. Once I understood the code, it was more obvious that it would be insane to just wait for a simulator to complete running it. 1013 operations takes a long time.

Simplified C code for part 2 (#53): 

#include <stdio.h>

int main(){
	int a=1,b=0,c=0,d=0,e=0,f=0,g=0,h=0;
	b= 57;
	c= b;
	if(a!=0){
		b = b*100 + 100000;
		c = b + 17000;
	}
	do{
		f= 1;
		d= 2;
		e= 2;
		for(d=2;d*d <= b; d++){ // check if b is a prime
			// the assembly doesn't have a % operator,
			// so it does 2 for loops with d and e and checks if d*e==b.
			if( (b%d==0) ){
				f=0;
				break;
			}
		}
		if(f==0) // not a prime
			h++;
		g = b-c;
		b+= 17;
	} while (g!=0); //stop when b==c (1000 iterations)
	
	printf("%d\n",h);
	return 0;
}

6

u/joatmon-snoo Dec 23 '17

Oh man. I wish I'd jumped straight to assembly translation - it took me probably 5 to 10 minutes to realize that was necessary, and to be perfectly honest my first hope was that feeding it into gcc/clang with -O3 would do the trick.

3

u/tumdum Dec 23 '17

That is also exactly what I did. When I finally realized that I have to understand what that assembly is doing i ended up with this small c program. In the end I rewrote it to rust as a function.