advent-of-code-25.html

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   5 <title>Day 25 - Advent of Code</title>
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  15 Oh, hello!  Funny seeing you here.
  16
  17 I appreciate your enthusiasm, but you aren't going to find much down here.
  18 There certainly aren't clues to any of the puzzles.  You do risk spoiling a few
  19 surprises for yourself, though.  Best to play the normal way and discover
  20 everything as it was intended, I think.  The best surprises don't even appear
  21 in the source until you unlock them for real.
  22
  23 Please be careful with automated requests; I'm not Google, and I can only take
  24 so much traffic.  Please be considerate so that everyone gets to play.
  25
  26 If you're curious about how Advent of Code works, it's running on some custom
  27 Perl code. Other than a few integrations (auth, analytics, ads, social media),
  28 I built the whole thing myself, including the design, animations, prose, and
  29 all of the puzzles.
  30
  31 The puzzles probably took the longest; the easiest ones were around 45 minutes
  32 each, but the harder ones took 2-3 hours, some even longer than that. A lot of
  33 effort went into building this thing - I hope you're enjoying playing it as
  34 much as I enjoyed making it for you!
  35
  36 If you'd like to hang out, I'm @ericwastl on Twitter.
  37
  38 - Eric Wastl
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  89 -->
  90 <body>
  91 <header><h1><a href="/">Advent of Code</a></h1><div class="user">Neil Smith <span class="star-count">50*</span></div><nav><ul><li><a href="/about">[About]</a></li><li><a href="/stats">[Stats]</a></li><li><a href="/leaderboard">[Leaderboard]</a></li><li><a href="/settings">[Settings]</a></li><li><a href="/auth/logout">[Log out]</a></li></ul></nav></header>
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 106 <style>article *[title]{border-bottom:1px dotted #ffff66;</style><article class="day-desc"><h2>--- Day 25: Let It Snow ---</h2><p>Merry Christmas!  Santa is booting up his weather machine; looks like you might get a <a href="/day/1">white Christmas</a> after all.</p>
 107 <p>The weather machine beeps!  On the console of the machine is a copy protection message asking you to <a href="https://en.wikipedia.org/wiki/Copy_protection#Early_video_games">enter a code from the instruction manual</a>.  Apparently, it refuses to run unless you give it that code.  No problem; you'll just look up the code in the--</p>
 108 <p>"Ho ho ho", Santa ponders aloud.  "I can't seem to find the manual."</p>
 109 <p>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.</p>
 110 <p>"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."</p>
 111 <p>After putting you on hold for twenty minutes (your call is <em>very</em> important to them, it reminded you repeatedly), they finally find an engineer that remembers how the code system works.</p>
 112 <p>The codes are printed on an <span title="The paper is very thin so it can be folded up neatly into the manual.">infinite sheet of paper</span>, 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 <a href="https://en.wikipedia.org/wiki/Cantor's_diagonal_argument">infinite paper is covered</a>.  So, the first few codes are filled in in this order:</p>
 113 <pre><code>   | 1   2   3   4   5   6
 114 ---+---+---+---+---+---+---+
 115  1 |  1   3   6  10  15  21
 116  2 |  2   5   9  14  20
 117  3 |  4   8  13  19
 118  4 |  7  12  18
 119  5 | 11  17
 120  6 | 16
 121 </code></pre>
 122 <p>For example, the 12th code would be written to row <code>4</code>, column <code>2</code>; the 15th code would be written to row <code>1</code>, column <code>5</code>.</p>
 123 <p>The voice on the other end of the phone continues with how the codes are actually generated.  The first code is <code>20151125</code>.  After that, each code is generated by taking the previous one, multiplying it by <code>252533</code>, and then keeping the remainder from dividing that value by <code>33554393</code>.</p>
 124 <p>So, to find the second code (which ends up in row <code>2</code>, column <code>1</code>), start with the previous value, <code>20151125</code>.  Multiply it by <code>252533</code> to get <code>5088824049625</code>.  Then, divide that by <code>33554393</code>, which leaves a remainder of <code>31916031</code>.  That remainder is the second code.</p>
 125 <p>"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:</p>
 126 <pre><code>   |    1         2         3         4         5         6
 127 ---+---------+---------+---------+---------+---------+---------+
 128  1 | 20151125  18749137  17289845  30943339  10071777  33511524
 129  2 | 31916031  21629792  16929656   7726640  15514188   4041754
 130  3 | 16080970   8057251   1601130   7981243  11661866  16474243
 131  4 | 24592653  32451966  21345942   9380097  10600672  31527494
 132  5 |    77061  17552253  28094349   6899651   9250759  31663883
 133  6 | 33071741   6796745  25397450  24659492   1534922  27995004
 134 </code></pre>
 135 <p>"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.</p>
 136 <p>Santa looks nervous.  Your puzzle input contains the message on the machine's console.  <em>What code do you give the machine?</em></p>
 137 </article>
 138 <p>Your puzzle answer was <code>9132360</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p>The machine springs to life, then falls silent again.  It beeps.  "Insufficient fuel", the console reads.  "<em class="star">Fifty stars</em> are required before proceeding.  <em class="star">One star</em> is available."</p>
 139 <p>..."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.</p>
 140 </article>
 141 <form method="post" action="/day/25/answer"><input type="hidden" name="level" value="2"/><input type="hidden" name="answer" value="0"/><p>
 142 If you like, you can <input type="submit" value="[Turn it off and on again]"/>.
 143 </p></form>
 144 <p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
 145 <p>At this point, all that is left is for you to <a href="/">admire your advent calendar</a>.</p>
 146 <p>If you still want to see it, you can <a href="/day/25/input" target="_blank">get your puzzle input</a>.</p>
 147 <p>You can also <span class="share">[Share<span class="share-content">on
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 152  this puzzle.</p>
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