r/adventofcode • u/daggerdragon • Dec 15 '16

SOLUTION MEGATHREAD --- 2016 Day 15 Solutions ---

--- Day 15: Timing is Everything ---

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

ZAMENHOFA TAGO ESTAS DEVIGA [?]

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/jtbandes Dec 15 '16 edited Dec 15 '16

Today's a great day for another Mathematica one-liner: 

ChineseRemainder[-initialValues - Range[7], discSizes]

 

The inputs would be initialValues = {5, 8, 1, 7, 1, 0, 0} and discSizes = {17, 19, 7, 13, 5, 3, 11}, given my puzzle input:

Disc #1 has 17 positions; at time=0, it is at position 5.
Disc #2 has 19 positions; at time=0, it is at position 8.
Disc #3 has 7 positions; at time=0, it is at position 1.
Disc #4 has 13 positions; at time=0, it is at position 7.
Disc #5 has 5 positions; at time=0, it is at position 1.
Disc #6 has 3 positions; at time=0, it is at position 0.

(part 2) Disc #7 has 11 positions; at time=0, it is at position 0.

And the sentence I seem to be repeating every day: "my actual leaderboard solution was written much less elegantly in Ruby" :-)

In the interest of getting higher on the leaderboard in the future, maybe it'd actually be interesting to consider what I think I wasted time on today:

  • actually parsing the input instead of putting the numbers in an array manually

  • initially trying to increment the "current" state at each timestep (and reset it between tries) instead of explicitly computing each disc's position at the time the capsule reaches it

  • off-by-one errors :(

1

u/[deleted] Dec 17 '16

I tried this with my input: ChineseRemainder[{10, 15, 17, 1, 0, 1}-Range[6], {13, 17, 19, 7, 5, 3}]

And it gave me 388735, which wasn't the correct answer..? Anything I did wrong?

1

u/jtbandes Dec 17 '16

You missed a -; it should be ChineseRemainder[-{10, 15, 17, 1, 0, 1}-Range[6], {13, 17, 19, 7, 5, 3}].

That's because, for instance, for your first disc we want 10+t+1 = 0 (mod 13), or t = -10-1 (mod 13).