--- /dev/null
+import Data.List (iterate, tails)
+
+-- input = "..^^."
+-- input = ".^^.^.^^^^"
+input = "^.^^^.^..^....^^....^^^^.^^.^...^^.^.^^.^^.^^..^.^...^.^..^.^^.^..^.....^^^.^.^^^..^^...^^^...^...^."
+
+
+main :: IO ()
+main = do
+ part1
+ part2
+
+
+part1 :: IO ()
+-- part1 = putStrLn $ unlines $ map (showRow) $ take 10 $ iterate nextRow $ readRow input
+part1 = print $ length $ filter (not) $ concat $ take 40 $ iterate nextRow $ readRow input
+
+part2 :: IO ()
+part2 = print $ length $ filter (not) $ concat $ take 400000 $ iterate nextRow $ readRow input
+
+readRow :: String -> [Bool]
+readRow = map (=='^')
+
+showRow :: [Bool] -> String
+showRow = map (\c -> if c then '^' else '.')
+
+extended :: [Bool] -> [Bool]
+extended row = [False] ++ row ++ [False]
+
+nextRow :: [Bool] -> [Bool]
+nextRow = map (isTrap) . segments . extended
+
+segments :: [a] -> [[a]]
+segments = filter ((==3) . length) . map (take 3) . tails
+
+isTrap :: [Bool] -> Bool
+isTrap segment
+ | segment == [True, True, False] = True
+ | segment == [False, True, True] = True
+ | segment == [True, False, False] = True
+ | segment == [False, False, True] = True
+ | otherwise = False
--- /dev/null
+<!DOCTYPE html>
+<html lang="en-us">
+<head>
+<meta charset="utf-8"/>
+<title>Day 18 - Advent of Code 2016</title>
+<!--[if lt IE 9]><script src="/static/html5.js"></script><![endif]-->
+<link href='//fonts.googleapis.com/css?family=Source+Code+Pro:300&subset=latin,latin-ext' rel='stylesheet' type='text/css'>
+<link rel="stylesheet" type="text/css" href="/static/style.css?9"/>
+<link rel="shortcut icon" href="/favicon.ico?2"/>
+</head><!--
+
+
+
+
+Oh, hello! Funny seeing you here.
+
+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 were around 45 minutes
+each, but the harder ones took 2-3 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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+
+
+
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+
+
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+
+
+
+
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+
+
+
+
+
+
+-->
+<body>
+<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">36*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap"><y></span><a href="/2016">2016</a><span class="title-event-wrap"></y></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>
+
+<div id="sidebar">
+<div id="sponsor"><div class="quiet">Our <a href="/2016/sponsors">sponsors</a> help make AoC possible:</div><p><a href="https://infi.nl/vacature/infi-zoekt-een-ontwikkelaar/" target="_blank" onclick="if(ga)ga('send','event','sponsor','click',this.href);">Infi</a> - Fvzcry gbpu? Xbz arkg-yriry glcra va Hgerpug bc baf areqxjnegvre!</p></div>
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+
+<main>
+<article class="day-desc"><h2>--- Day 18: Like a Rogue ---</h2><p>As you enter this room, you hear a loud click! Some of the tiles in the floor here seem to be pressure plates for <a href="https://nethackwiki.com/wiki/Trap">traps</a>, and the trap you just triggered has run out of... whatever it tried to do to you. You doubt you'll be so lucky next time.</p>
+<p>Upon closer examination, the traps and safe tiles in this room seem to follow a pattern. The tiles are arranged into rows that are all the same width; you take note of the safe tiles (<code>.</code>) and traps (<code>^</code>) in the first row (your puzzle input).</p>
+<p>The type of tile (trapped or safe) in each row is based on the types of the tiles in the same position, and to either side of that position, in the previous row. (If either side is off either end of the row, it counts as "safe" because there isn't a trap embedded in the wall.)</p>
+<p>For example, suppose you know the first row (with tiles marked by letters) and want to determine the next row (with tiles marked by numbers):</p>
+<pre><code>ABCDE
+12345
+</code></pre>
+<p>The type of tile <code>2</code> is based on the types of tiles <code>A</code>, <code>B</code>, and <code>C</code>; the type of tile <code>5</code> is based on tiles <code>D</code>, <code>E</code>, and an imaginary "safe" tile. Let's call these three tiles from the previous row the <em>left</em>, <em>center</em>, and <em>right</em> tiles, respectively. Then, a new tile is a <em>trap</em> only in one of the following situations:</p>
+<ul>
+<li>Its <em>left</em> and <em>center</em> tiles are traps, but its <em>right</em> tile is not.</li>
+<li>Its <em>center</em> and <em>right</em> tiles are traps, but its <em>left</em> tile is not.</li>
+<li>Only its <em>left</em> tile is a trap.</li>
+<li>Only its <em>right</em> tile is a trap.</li>
+</ul>
+<p>In any other situation, the new tile is safe.</p>
+<p>Then, starting with the row <code>..^^.</code>, you can determine the next row by applying those rules to each new tile:</p>
+<ul>
+<li>The leftmost character on the next row considers the left (nonexistent, so we assume "safe"), center (the first <code>.</code>, which means "safe"), and right (the second <code>.</code>, also "safe") tiles on the previous row. Because all of the trap rules require a trap in at least one of the previous three tiles, the first tile on this new row is also safe, <code>.</code>.</li>
+<li>The second character on the next row considers its left (<code>.</code>), center (<code>.</code>), and right (<code>^</code>) tiles from the previous row. This matches the fourth rule: only the right tile is a trap. Therefore, the next tile in this new row is a trap, <code>^</code>.</li>
+<li>The third character considers <code>.^^</code>, which matches the second trap rule: its center and right tiles are traps, but its left tile is not. Therefore, this tile is also a trap, <code>^</code>.</li>
+<li>The last two characters in this new row match the first and third rules, respectively, and so they are both also traps, <code>^</code>.</li>
+</ul>
+<p>After these steps, we now know the next row of tiles in the room: <code>.^^^^</code>. Then, we continue on to the next row, using the same rules, and get <code>^^..^</code>. After determining two new rows, our map looks like this:</p>
+<pre><code>..^^.
+.^^^^
+^^..^
+</code></pre>
+<p>Here's a larger example with ten tiles per row and ten rows:</p>
+<pre><code>.^^.^.^^^^
+^^^...^..^
+^.^^.^.^^.
+..^^...^^^
+.^^^^.^^.^
+^^..^.^^..
+^^^^..^^^.
+^..^^^^.^^
+.^^^..^.^^
+^^.^^^..^^
+</code></pre>
+<p>In ten rows, this larger example has <code>38</code> safe tiles.</p>
+<p>Starting with the map in your puzzle input, in a total of <code>40</code> rows (including the starting row), <em>how many safe tiles</em> are there?</p>
+</article>
+<p>Your puzzle answer was <code>1926</code>.</p><article class="day-desc"><h2>--- Part Two ---</h2><p><em>How many safe tiles</em> are there in a total of <span title="You feel a wrenching sensation."><code>400000</code> rows</span>?</p>
+</article>
+<p>Your puzzle answer was <code>19986699</code>.</p><p class="day-success">Both parts of this puzzle are complete! They provide two gold stars: **</p>
+<p>At this point, you should <a href="/2016">return to your advent calendar</a> and try another puzzle.</p>
+<p>If you still want to see it, you can <a href="18/input" target="_blank">get your puzzle input</a>.</p>
+<p>You can also <span class="share">[Share<span class="share-content">on
+ <a href="https://twitter.com/intent/tweet?text=I%27ve+completed+%22Like+a+Rogue%22+%2D+Day+18+%2D+Advent+of+Code+2016&url=http%3A%2F%2Fadventofcode%2Ecom%2F2016%2Fday%2F18&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
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+></span>]</span> this puzzle.</p>
+</main>
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