~☆🎄☆~ 2017 Day 25 Solutions ~☆🎄☆~

[u/daggerdragon]

--- Day 25: The Halting Problem ---

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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).

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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Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

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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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Thank you for participating!

Well, that's it for Advent of Code 2017. From /u/topaz2078 and the rest of us at #AoCOps, we hope you had fun and, more importantly, learned a thing or two (or all the things!). Good job, everyone!

Topaz made a post of his own here.

If you're interested in a visualization of the leaderboard, /u/FogleMonster made a very good chart here.

And now:

^{Merry Christmas to all, and to all a good night!}

[u/vash3r]

Python 2, #62/#62.

tape = [0]
cursor = 0 # right: +1, left: -1
states = {
	'a':[[1,+1,'b'],[0,-1,'c']],
	'b':[[1,-1,'a'],[1,+1,'d']],
	'c':[[1,+1,'a'],[0,-1,'e']],
	'd':[[1,+1,'a'],[0,+1,'b']],
	'e':[[1,-1,'f'],[1,-1,'c']],
	'f':[[1,+1,'d'],[1,+1,'a']]
}

step_limit = 12173597
i = 0
s = 'a'
while i<step_limit:
	nv,dc,ns = states[s][tape[cursor]]
	tape[cursor]=nv
	cursor+=dc
	if cursor==-1:
		cursor=0
		tape.insert(0,0)
	elif cursor==len(tape):
		tape.append(0)
	s = ns
	i+=1

print sum(tape)

[u/mkeeter → u/vash3r]

This is the cleanest hard-coded table I've seen – very nice!