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+<title>Day 25 - Advent of Code 2017</title>
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+Oh, hello! Funny seeing you here.
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+I appreciate your enthusiasm, but you aren't going to find much down here.
+There certainly aren't clues to any of the puzzles. The best surprises don't
+even appear in the source until you unlock them for real.
+
+Please be careful with automated requests; I'm not Google, and I can only take
+so much traffic. Please be considerate so that everyone gets to play.
+
+If you're curious about how Advent of Code works, it's running on some custom
+Perl code. Other than a few integrations (auth, analytics, ads, social media),
+I built the whole thing myself, including the design, animations, prose, and
+all of the puzzles.
+
+The puzzles probably took the longest; the easiest ones took an hour or two
+each, but the harder ones took 4-5 hours, and a few even longer than that. A
+lot of effort went into building this thing - I hope you're enjoying playing it
+as much as I enjoyed making it for you!
+
+If you'd like to hang out, I'm @ericwastl on Twitter.
+
+- Eric Wastl
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+<header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2017/about">[About]</a></li><li><a href="/2017/support">[AoC++]</a></li><li><a href="/2017/events">[Events]</a></li><li><a href="/2017/settings">[Settings]</a></li><li><a href="/2017/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <span class="supporter">(AoC++)</span> <span class="star-count">49*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">0.0.0.0:</span><a href="/2017">2017</a><span class="title-event-wrap"></span></h1><nav><ul><li><a href="/2017">[Calendar]</a></li><li><a href="/2017/leaderboard">[Leaderboard]</a></li><li><a href="/2017/stats">[Stats]</a></li><li><a href="/2017/sponsors">[Sponsors]</a></li></ul></nav></div></header>
+
+<div id="sidebar">
+<div id="sponsor"><div class="quiet">Our <a href="/2017/sponsors">sponsors</a> help make Advent of Code possible:</div><p><a href="http://xebia.com/community/advent-of-code" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);" rel="noopener">Xebia</a> - Passionate consultants taking up IT challenges all year round</p></div>
+<p class="quiet">By popular demand, there are now AoC-themed objects available (until Jan. 3rd)! Get them shipped <a href="https://teespring.com/advent-of-code" target="_blank">from the US</a> or <a href="https://teespring.com/advent-of-code-eu" target="_blank">from Europe</a>.</p>
+
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+
+<main>
+<article class="day-desc"><h2>--- Day 25: The Halting Problem ---</h2><p>Following the twisty passageways deeper and deeper into the CPU, you finally reach the <span title="Get it? CPU core?">core</span> of the computer. Here, in the expansive central chamber, you find a grand apparatus that fills the entire room, suspended nanometers above your head.</p>
+<p>You had always imagined CPUs to be noisy, chaotic places, bustling with activity. Instead, the room is quiet, motionless, and dark.</p>
+<p>Suddenly, you and the CPU's <em>garbage collector</em> startle each other. "It's not often we get many visitors here!", he says. You inquire about the stopped machinery.</p>
+<p>"It stopped milliseconds ago; not sure why. I'm a garbage collector, not a doctor." You ask what the machine is for.</p>
+<p>"Programs these days, don't know their origins. That's the <em>Turing machine</em>! It's what makes the whole computer work." You try to explain that Turing machines are merely models of computation, but he cuts you off. "No, see, that's just what they <em>want</em> you to think. Ultimately, inside every CPU, there's a Turing machine driving the whole thing! Too bad this one's broken. <a href="https://www.youtube.com/watch?v=cTwZZz0HV8I">We're doomed!</a>"</p>
+<p>You ask how you can help. "Well, unfortunately, the only way to get the computer running again would be to create a whole new Turing machine from scratch, but there's no <em>way</em> you can-" He notices the look on your face, gives you a curious glance, shrugs, and goes back to sweeping the floor.</p>
+<p>You find the <em>Turing machine blueprints</em> (your puzzle input) on a tablet in a nearby pile of debris. Looking back up at the broken Turing machine above, you can start to identify its parts:</p>
+<ul>
+<li>A <em>tape</em> which contains <code>0</code> repeated infinitely to the left and right.</li>
+<li>A <em>cursor</em>, which can move left or right along the tape and read or write values at its current position.</li>
+<li>A set of <em>states</em>, each containing rules about what to do based on the current value under the cursor.</li>
+</ul>
+<p>Each slot on the tape has two possible values: <code>0</code> (the starting value for all slots) and <code>1</code>. Based on whether the cursor is pointing at a <code>0</code> or a <code>1</code>, the current state says <em>what value to write</em> at the current position of the cursor, whether to <em>move the cursor</em> left or right one slot, and <em>which state to use next</em>.</p>
+<p>For example, suppose you found the following blueprint:</p>
+<pre><code>Begin in state A.
+Perform a diagnostic checksum after 6 steps.
+
+In state A:
+ If the current value is 0:
+ - Write the value 1.
+ - Move one slot to the right.
+ - Continue with state B.
+ If the current value is 1:
+ - Write the value 0.
+ - Move one slot to the left.
+ - Continue with state B.
+
+In state B:
+ If the current value is 0:
+ - Write the value 1.
+ - Move one slot to the left.
+ - Continue with state A.
+ If the current value is 1:
+ - Write the value 1.
+ - Move one slot to the right.
+ - Continue with state A.
+</code></pre>
+<p>Running it until the number of steps required to take the listed <em>diagnostic checksum</em> would result in the following tape configurations (with the <em>cursor</em> marked in square brackets):</p>
+<pre><code>... 0 0 0 [0] 0 0 ... (before any steps; about to run state A)
+... 0 0 0 1 [0] 0 ... (after 1 step; about to run state B)
+... 0 0 0 [1] 1 0 ... (after 2 steps; about to run state A)
+... 0 0 [0] 0 1 0 ... (after 3 steps; about to run state B)
+... 0 [0] 1 0 1 0 ... (after 4 steps; about to run state A)
+... 0 1 [1] 0 1 0 ... (after 5 steps; about to run state B)
+... 0 1 1 [0] 1 0 ... (after 6 steps; about to run state A)
+</code></pre>
+<p>The CPU can confirm that the Turing machine is working by taking a <em>diagnostic checksum</em> after a specific number of steps (given in the blueprint). Once the specified number of steps have been executed, the Turing machine should pause; once it does, count the number of times <code>1</code> appears on the tape. In the above example, the <em>diagnostic checksum</em> is <em><code>3</code></em>.</p>
+<p>Recreate the Turing machine and save the computer! <em>What is the diagnostic checksum</em> it produces once it's working again?</p>
+</article>
+<p>Your puzzle answer was <code>2846</code>.</p><p class="day-success">The first half of this puzzle is complete! It provides one gold star: *</p>
+<article class="day-desc"><h2>--- Part Two ---</h2><p>The Turing machine, and soon the entire computer, springs back to life. A console glows dimly nearby, awaiting your command.</p>
+<pre><code>> reboot printer
+Error: That command requires <em>priority 50</em>. You currently have <em>priority 0</em>.
+You must deposit <em class="star">50 stars</em> to increase your priority to the required level.
+</code></pre>
+<p>The console flickers for a moment, and then prints another message:</p>
+<pre><code><em class="star">Star</em> accepted.
+You must deposit <em class="star">49 stars</em> to increase your priority to the required level.
+</code></pre>
+<p>The <em>garbage collector</em> winks at you, then continues sweeping.</p>
+</article>
+<form method="post" action="25/answer"><input type="hidden" name="level" value="2"/><input type="hidden" name="answer" value="0"/><p>You have enough stars to <input type="submit" value="[Reboot the Printer]"/>.</p></form>
+<p>You can also <span class="share">[Share<span class="share-content">on
+ <a href="https://twitter.com/intent/tweet?text=I%27ve+completed+Part+One+of+%22The+Halting+Problem%22+%2D+Day+25+%2D+Advent+of+Code+2017&url=http%3A%2F%2Fadventofcode%2Ecom%2F2017%2Fday%2F25&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
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+></span>]</span> this puzzle.</p>
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