107 lines
4.8 KiB
Plaintext
107 lines
4.8 KiB
Plaintext
|
Advent of Code
|
||
|
|
|
||
|
|
br0xen 46*
|
||
|
|
|
||
|
|
• [About]
|
||
|
|
• [Stats]
|
||
|
|
• [Leaderboard]
|
||
|
|
• [Settings]
|
||
|
|
• [Log out]
|
||
|
|
|
||
|
|
--- Day 25: Let It Snow ---
|
||
|
|
|
||
|
|
Merry Christmas! Santa is booting up his weather machine; looks like you might get a white
|
||
|
|
Christmas after all.
|
||
|
|
|
||
|
|
The weather machine beeps! On the console of the machine is a copy protection message asking
|
||
|
|
you to enter a code from the instruction manual. Apparently, it refuses to run unless you give
|
||
|
|
it that code. No problem; you'll just look up the code in the--
|
||
|
|
|
||
|
|
"Ho ho ho", Santa ponders aloud. "I can't seem to find the manual."
|
||
|
|
|
||
|
|
You look up the support number for the manufacturer and give them a call. Good thing, too -
|
||
|
|
that 49th star wasn't going to earn itself.
|
||
|
|
|
||
|
|
"Oh, that machine is quite old!", they tell you. "That model went out of support six minutes
|
||
|
|
ago, and we just finished shredding all of the manuals. I bet we can find you the code
|
||
|
|
generation algorithm, though."
|
||
|
|
|
||
|
|
After putting you on hold for twenty minutes (your call is very important to them, it reminded
|
||
|
|
you repeatedly), they finally find an engineer that remembers how the code system works.
|
||
|
|
|
||
|
|
The codes are printed on an infinite sheet of paper, starting in the top-left corner. The
|
||
|
|
codes are filled in by diagonals: starting with the first row with an empty first box, the
|
||
|
|
codes are filled in diagonally up and to the right. This process repeats until the infinite
|
||
|
|
paper is covered. So, the first few codes are filled in in this order:
|
||
|
|
|
||
|
|
| 1 2 3 4 5 6
|
||
|
|
---+---+---+---+---+---+---+
|
||
|
|
1 | 1 3 6 10 15 21
|
||
|
|
2 | 2 5 9 14 20
|
||
|
|
3 | 4 8 13 19
|
||
|
|
4 | 7 12 18
|
||
|
|
5 | 11 17
|
||
|
|
6 | 16
|
||
|
|
|
||
|
|
For example, the 12th code would be written to row 4, column 2; the 15th code would be written
|
||
|
|
to row 1, column 5.
|
||
|
|
|
||
|
|
The voice on the other end of the phone continues with how the codes are actually generated.
|
||
|
|
The first code is 20151125. After that, each code is generated by taking the previous one,
|
||
|
|
multiplying it by 252533, and then keeping the remainder from dividing that value by 33554393.
|
||
|
|
|
||
|
|
So, to find the second code (which ends up in row 2, column 1), start with the previous value,
|
||
|
|
20151125. Multiply it by 252533 to get 5088824049625. Then, divide that by 33554393, which
|
||
|
|
leaves a remainder of 31916031. That remainder is the second code.
|
||
|
|
|
||
|
|
"Oh!", says the voice. "It looks like we missed a scrap from one of the manuals. Let me read
|
||
|
|
it to you." You write down his numbers:
|
||
|
|
|
||
|
|
| 1 2 3 4 5 6
|
||
|
|
---+---------+---------+---------+---------+---------+---------+
|
||
|
|
1 | 20151125 18749137 17289845 30943339 10071777 33511524
|
||
|
|
2 | 31916031 21629792 16929656 7726640 15514188 4041754
|
||
|
|
3 | 16080970 8057251 1601130 7981243 11661866 16474243
|
||
|
|
4 | 24592653 32451966 21345942 9380097 10600672 31527494
|
||
|
|
5 | 77061 17552253 28094349 6899651 9250759 31663883
|
||
|
|
6 | 33071741 6796745 25397450 24659492 1534922 27995004
|
||
|
|
|
||
|
|
"Now remember", the voice continues, "that's not even all of the first few numbers; for
|
||
|
|
example, you're missing the one at 7,1 that would come before 6,2. But, it should be enough to
|
||
|
|
let your-- oh, it's time for lunch! Bye!" The call disconnects.
|
||
|
|
|
||
|
|
Santa looks nervous. Your puzzle input contains the message on the machine's console. What
|
||
|
|
code do you give the machine?
|
||
|
|
|
||
|
|
Your puzzle answer was 19980801.
|
||
|
|
|
||
|
|
The first half of this puzzle is complete! It provides one gold star: *
|
||
|
|
|
||
|
|
--- Part Two ---
|
||
|
|
|
||
|
|
The machine springs to life, then falls silent again. It beeps. "Insufficient fuel", the
|
||
|
|
console reads. "Fifty stars are required before proceeding. One star is available."
|
||
|
|
|
||
|
|
..."one star is available"? You check the fuel tank; sure enough, a lone star sits at the
|
||
|
|
bottom, awaiting its friends. Looks like you need to provide 49 yourself.
|
||
|
|
|
||
|
|
You don't have enough stars to start the machine, though. You need x more.
|
||
|
|
|
||
|
|
You can also [Shareon Twitter Google+ Reddit] this puzzle.
|
||
|
|
|
||
|
|
References
|
||
|
|
|
||
|
|
Visible links
|
||
|
|
. http://adventofcode.com/
|
||
|
|
. http://adventofcode.com/about
|
||
|
|
. http://adventofcode.com/stats
|
||
|
|
. http://adventofcode.com/leaderboard
|
||
|
|
. http://adventofcode.com/settings
|
||
|
|
. http://adventofcode.com/auth/logout
|
||
|
|
. http://adventofcode.com/day/1
|
||
|
|
. https://en.wikipedia.org/wiki/Copy_protection#Early_video_games
|
||
|
|
. https://en.wikipedia.org/wiki/Cantor's_diagonal_argument
|
||
|
|
. https://twitter.com/intent/tweet?text=I%27ve+completed+Part+One+of+%22Let+It+Snow%22+%2D+Day+25+%2D+Advent+of+Code&url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F25&related=ericwastl&hashtags=AdventOfCode
|
||
|
|
. https://plus.google.com/share?url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F25
|
||
|
|
. http://www.reddit.com/submit?url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F25&title=I%27ve+completed+Part+One+of+%22Let+It+Snow%22+%2D+Day+25+%2D+Advent+of+Code
|