day08.html

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   5 <title>Day 8 - Advent of Code 2016</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.  The best surprises don't
  19 even appear in the source until you unlock them for real.
  20
  21 Please be careful with automated requests; I'm not Google, and I can only take
  22 so much traffic.  Please be considerate so that everyone gets to play.
  23
  24 If you're curious about how Advent of Code works, it's running on some custom
  25 Perl code. Other than a few integrations (auth, analytics, ads, social media),
  26 I built the whole thing myself, including the design, animations, prose, and
  27 all of the puzzles.
  28
  29 The puzzles probably took the longest; the easiest ones were around 45 minutes
  30 each, but the harder ones took 2-3 hours, and a few even longer than that. A
  31 lot of effort went into building this thing - I hope you're enjoying playing it
  32 as much as I enjoyed making it for you!
  33
  34 If you'd like to hang out, I'm @ericwastl on Twitter.
  35
  36 - Eric Wastl
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  88 <body>
  89 <header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2016/about">[About]</a></li><li><a href="/2016/support">[AoC++]</a></li><li><a href="/2016/events">[Events]</a></li><li><a href="/2016/settings">[Settings]</a></li><li><a href="/2016/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <span class="supporter">(AoC++)</span> <span class="star-count">16*</span></div></div><div><h1 class="title-event">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="title-event-wrap">/^</span><a href="/2016">2016</a><span class="title-event-wrap">$/</span></h1><nav><ul><li><a href="/2016">[Calendar]</a></li><li><a href="/2016/leaderboard">[Leaderboard]</a></li><li><a href="/2016/stats">[Stats]</a></li><li><a href="/2016/sponsors">[Sponsors]</a></li></ul></nav></div></header>
  90
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  92 <div id="sponsor"><div class="quiet">Our <a href="/2016/sponsors">sponsors</a> help make AoC possible:</div><p><a href="https://www.detroitlabs.com/careers" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);">Detroit Labs</a> - Build beautiful mobile apps.</p></div>
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 107 <article class="day-desc"><h2>--- Day 8: Two-Factor Authentication ---</h2><p>You come across a door implementing what you can only assume is an implementation of <a href="https://en.wikipedia.org/wiki/Multi-factor_authentication">two-factor authentication</a> after a long game of <a href="https://en.wikipedia.org/wiki/Requirement">requirements</a> <a href="https://en.wikipedia.org/wiki/Chinese_whispers">telephone</a>.</p>
 108 <p>To get past the door, you first swipe a keycard (no problem; there was one on a nearby desk). Then, it displays a code on a <a href="https://www.google.com/search?q=tiny+lcd&tbm=isch">little screen</a>, and you type that code on a keypad. Then, presumably, the door unlocks.</p>
 109 <p>Unfortunately, the screen has been <span title="BUT BY WHOM?!">smashed</span>. After a few minutes, you've taken everything apart and figured out how it works. Now you just have to work out what the screen <em>would</em> have displayed.</p>
 110 <p>The magnetic strip on the card you swiped encodes a series of instructions for the screen; these instructions are your puzzle input. The screen is <em><code>50</code> pixels wide and <code>6</code> pixels tall</em>, all of which start <em>off</em>, and is capable of three somewhat peculiar operations:</p>
 111 <ul>
 112 <li><code>rect AxB</code> turns <em>on</em> all of the pixels in a rectangle at the top-left of the screen which is <code>A</code> wide and <code>B</code> tall.</li>
 113 <li><code>rotate row y=A by B</code> shifts all of the pixels in row <code>A</code> (0 is the top row) <em>right</em> by <code>B</code> pixels. Pixels that would fall off the right end appear at the left end of the row.</li>
 114 <li><code>rotate column x=A by B</code> shifts all of the pixels in column <code>A</code> (0 is the left column) <em>down</em> by <code>B</code> pixels. Pixels that would fall off the bottom appear at the top of the column.</li>
 115 </ul>
 116 <p>For example, here is a simple sequence on a smaller screen:</p>
 117 <ul>
 118 <li><p><code>rect 3x2</code> creates a small rectangle in the top-left corner:</p><pre><code>###....
 119 ###....
 120 .......</code></pre></li>
 121 <li><p><code>rotate column x=1 by 1</code> rotates the second column down by one pixel:</p><pre><code>#.#....
 122 ###....
 123 .#.....</code></pre></li>
 124 <li><p><code>rotate row y=0 by 4</code> rotates the top row right by four pixels:</p><pre><code>....#.#
 125 ###....
 126 .#.....</code></pre></li>
 127 <li><p><code>rotate column x=1 by 1</code> again rotates the second column down by one pixel, causing the bottom pixel to wrap back to the top:</p><pre><code>.#..#.#
 128 #.#....
 129 .#.....</code></pre></li>
 130 </ul>
 131 <p>As you can see, this display technology is extremely powerful, and will soon dominate the tiny-code-displaying-screen market.  That's what the advertisement on the back of the display tries to convince you, anyway.</p>
 132 <p>There seems to be an intermediate check of the voltage used by the display: after you swipe your card, if the screen did work, <em>how many pixels should be lit?</em></p>
 133 </article>
 134 <p>Your puzzle answer was <code>119</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p>You notice that the screen is only capable of displaying capital letters; in the font it uses, each letter is <code>5</code> pixels wide and <code>6</code> tall.</p>
 135 <p>After you swipe your card, <em>what code is the screen trying to display?</em></p>
 136 </article>
 137 <p>Your puzzle answer was <code>ZFHFSFOGPO</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
 138 <p>At this point, you should <a href="/2016">return to your advent calendar</a> and try another puzzle.</p>
 139 <p>If you still want to see it, you can <a href="8/input" target="_blank">get your puzzle input</a>.</p>
 140 <p>You can also <span class="share">[Share<span class="share-content">on
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 143   <a href="http://www.reddit.com/submit?url=http%3A%2F%2Fadventofcode%2Ecom%2F2016%2Fday%2F8&amp;title=I%27ve+completed+%22Two%2DFactor+Authentication%22+%2D+Day+8+%2D+Advent+of+Code+2016" target="_blank">Reddit</a
 144 ></span>]</span> this puzzle.</p>
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