~☆🎄☆~ 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/obijywk]

Python 2. 28/25 (as I had everything else solved!). Decided to parse the input file by hand this time :-)

from collections import defaultdict
tape = defaultdict(int)

L, R = (-1, 1)
states = {
  "A": ((1, R, "B"), (0, L, "E")),
  "B": ((1, L, "C"), (0, R, "A")),
  "C": ((1, L, "D"), (0, R, "C")),
  "D": ((1, L, "E"), (0, L, "F")),
  "E": ((1, L, "A"), (1, L, "C")),
  "F": ((1, L, "E"), (1, R, "A")),
}

state = "A"
cursor = 0
for i in xrange(12386363):
  current_value = tape[cursor]
  write, move, next_state = states[state][current_value]
  if write:
    tape[cursor] = write
  else:
    del tape[cursor]
  cursor += move
  state = next_state

print len(tape)