--- /dev/null
+-- import Debug.Trace
+
+import Data.Text (Text)
+import qualified Data.Text as T
+import qualified Data.Text.IO as TIO
+
+import Data.Attoparsec.Text
+-- import Data.Attoparsec.Combinator
+import Control.Applicative
+
+import qualified Data.Set as S
+import qualified Data.Vector as V
+import Data.Vector ((!), (//))
+
+data Instruction = Acc Int | Jmp Int | Nop Int
+ deriving (Show, Eq)
+
+type Program = V.Vector Instruction
+
+data Machine = Machine
+ { machineProgram :: Program
+ , machineIP :: Int
+ , machineAcc :: Int
+ }
+
+data MachineResult = Looped Int | Terminated Int | OutOfBounds Int Int
+ deriving (Show, Eq)
+
+
+main :: IO ()
+main =
+ do text <- TIO.readFile "data/advent08.txt"
+ let program = successfulParse text
+ -- print program
+ let machine = Machine program 0 0
+ print $ part1 machine
+ print $ part2 program
+
+part1 machine = executeMany S.empty machine
+
+part2 program = filter terminates $ map runProgram programs
+ where programs = altPrograms program
+ runProgram p = executeMany S.empty (Machine p 0 0)
+ terminates (Terminated _) = True
+ terminates (OutOfBounds _ _) = True
+ terminates _ = False
+
+
+executeMany visited machine
+ -- if currentIP `S.member` visited
+ -- then Looped (machineAcc machine)
+ -- else if currentIP == programSize
+ -- then Terminated (machineAcc machine)
+ -- else if currentIP > programSize
+ -- then OutOfBounds (machineAcc machine) currentIP
+ -- else executeMany visited' machine'
+ | currentIP `S.member` visited = Looped (machineAcc machine)
+ | currentIP == programSize = Terminated (machineAcc machine)
+ | currentIP > programSize = OutOfBounds (machineAcc machine) currentIP
+ | otherwise = executeMany visited' machine'
+ where machine' = executeStep machine
+ currentIP = machineIP machine
+ visited' = S.insert currentIP visited
+ programSize = V.length $ machineProgram machine
+
+
+executeStep m = execute ((machineProgram m)!(machineIP m)) m
+
+execute (Acc n) m = m { machineIP = machineIP m + 1
+ , machineAcc = machineAcc m + n
+ }
+execute (Jmp n) m = m { machineIP = machineIP m + n }
+execute (Nop n) m = m { machineIP = machineIP m + 1 }
+
+
+altPrograms program = map (mutateProgram program) [0..(V.length program - 1)]
+
+mutateProgram program i = go (program!i)
+ where go (Nop n) = program // [(i, Jmp n)]
+ go (Jmp n) = program // [(i, Nop n)]
+ go _ = program
+
+-- -- Parse the input file
+
+
+instructionP = accP <|> jmpP <|> nopP
+
+accP = Acc <$> ("acc " *> signed decimal)
+jmpP = Jmp <$> ("jmp " *> signed decimal)
+nopP = Nop <$> ("nop " *> signed decimal)
+
+programP = V.fromList <$> sepBy instructionP endOfLine
+
+successfulParse :: Text -> Program
+successfulParse input =
+ case parseOnly programP input of
+ Left _err -> V.empty -- TIO.putStr $ T.pack $ parseErrorPretty err
+ Right program -> program
--- /dev/null
+<!DOCTYPE html>
+<html lang="en-us">
+<head>
+<meta charset="utf-8"/>
+<title>Day 8 - Advent of Code 2020</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?25"/>
+<link rel="stylesheet alternate" type="text/css" href="/static/highcontrast.css?0" title="High Contrast"/>
+<link rel="shortcut icon" href="/favicon.png"/>
+</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 a massive company, 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, social media), I
+built the whole thing myself, including the design, animations, prose, and all
+of the puzzles.
+
+The puzzles are most of the work; preparing a new calendar and a new set of
+puzzles each year takes all of my free time for 4-5 months. 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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+
+
+
+-->
+<body>
+<header><div><h1 class="title-global"><a href="/">Advent of Code</a></h1><nav><ul><li><a href="/2020/about">[About]</a></li><li><a href="/2020/events">[Events]</a></li><li><a href="https://teespring.com/stores/advent-of-code" target="_blank">[Shop]</a></li><li><a href="/2020/settings">[Settings]</a></li><li><a href="/2020/auth/logout">[Log Out]</a></li></ul></nav><div class="user">Neil Smith <a href="/2020/support" class="supporter-badge" title="Advent of Code Supporter">(AoC++)</a> <span class="star-count">16*</span></div></div><div><h1 class="title-event"> <span class="title-event-wrap">0.0.0.0:</span><a href="/2020">2020</a><span class="title-event-wrap"></span></h1><nav><ul><li><a href="/2020">[Calendar]</a></li><li><a href="/2020/support">[AoC++]</a></li><li><a href="/2020/sponsors">[Sponsors]</a></li><li><a href="/2020/leaderboard">[Leaderboard]</a></li><li><a href="/2020/stats">[Stats]</a></li></ul></nav></div></header>
+
+<div id="sidebar">
+<div id="sponsor"><div class="quiet">Our <a href="/2020/sponsors">sponsors</a> help make Advent of Code possible:</div><div class="sponsor"><a href="https://www.educative.io/adventofcode" target="_blank" onclick="if(ga)ga('send','event','sponsor','sidebar',this.href);" rel="noopener">Educative.io</a> - From CSS to System Design, gain in-demand tech skills at the speed you want. Text-based courses with live coding environments help you learn without the fluff</div></div>
+</div><!--/sidebar-->
+
+<main>
+<script>window.addEventListener('click', function(e,s,r){if(e.target.nodeName==='CODE'&&e.detail===3){s=window.getSelection();s.removeAllRanges();r=document.createRange();r.selectNodeContents(e.target);s.addRange(r);}});</script>
+<article class="day-desc"><h2>--- Day 8: Handheld Halting ---</h2><p>Your flight to the major airline hub reaches cruising altitude without incident. While you consider checking the in-flight menu for one of those drinks that come with a little umbrella, you are interrupted by the kid sitting next to you.</p>
+<p>Their <a target="_blank" href="https://en.wikipedia.org/wiki/Handheld_game_console">handheld game console</a> won't turn on! They ask if you can take a look.</p>
+<p>You narrow the problem down to a strange <em>infinite loop</em> in the <span title="A trendy new line of encrypted footwear?">boot code</span> (your puzzle input) of the device. You should be able to fix it, but first you need to be able to run the code in isolation.</p>
+<p>The boot code is represented as a text file with one <em>instruction</em> per line of text. Each instruction consists of an <em>operation</em> (<code>acc</code>, <code>jmp</code>, or <code>nop</code>) and an <em>argument</em> (a signed number like <code>+4</code> or <code>-20</code>).</p>
+<ul>
+<li><code>acc</code> increases or decreases a single global value called the <em>accumulator</em> by the value given in the argument. For example, <code>acc +7</code> would increase the accumulator by 7. The accumulator starts at <code>0</code>. After an <code>acc</code> instruction, the instruction immediately below it is executed next.</li>
+<li><code>jmp</code> <em>jumps</em> to a new instruction relative to itself. The next instruction to execute is found using the argument as an <em>offset</em> from the <code>jmp</code> instruction; for example, <code>jmp +2</code> would skip the next instruction, <code>jmp +1</code> would continue to the instruction immediately below it, and <code>jmp -20</code> would cause the instruction 20 lines above to be executed next.</li>
+<li><code>nop</code> stands for <em>No OPeration</em> - it does nothing. The instruction immediately below it is executed next.</li>
+</ul>
+<p>For example, consider the following program:</p>
+<pre><code>nop +0
+acc +1
+jmp +4
+acc +3
+jmp -3
+acc -99
+acc +1
+jmp -4
+acc +6
+</code></pre>
+<p>These instructions are visited in this order:</p>
+<pre><code>nop +0 | 1
+acc +1 | 2, 8(!)
+jmp +4 | 3
+acc +3 | 6
+jmp -3 | 7
+acc -99 |
+acc +1 | 4
+jmp -4 | 5
+acc +6 |
+</code></pre>
+<p>First, the <code>nop +0</code> does nothing. Then, the accumulator is increased from 0 to 1 (<code>acc +1</code>) and <code>jmp +4</code> sets the next instruction to the other <code>acc +1</code> near the bottom. After it increases the accumulator from 1 to 2, <code>jmp -4</code> executes, setting the next instruction to the only <code>acc +3</code>. It sets the accumulator to 5, and <code>jmp -3</code> causes the program to continue back at the first <code>acc +1</code>.</p>
+<p>This is an <em>infinite loop</em>: with this sequence of jumps, the program will run forever. The moment the program tries to run any instruction a second time, you know it will never terminate.</p>
+<p>Immediately <em>before</em> the program would run an instruction a second time, the value in the accumulator is <em><code>5</code></em>.</p>
+<p>Run your copy of the boot code. Immediately before any instruction is executed a second time, <em>what value is in the accumulator?</em></p>
+</article>
+<p>Your puzzle answer was <code>1867</code>.</p><article class="day-desc"><h2 id="part2">--- Part Two ---</h2><p>After some careful analysis, you believe that <em>exactly one instruction is corrupted</em>.</p>
+<p>Somewhere in the program, <em>either</em> a <code>jmp</code> is supposed to be a <code>nop</code>, <em>or</em> a <code>nop</code> is supposed to be a <code>jmp</code>. (No <code>acc</code> instructions were harmed in the corruption of this boot code.)</p>
+<p>The program is supposed to terminate by <em>attempting to execute an instruction immediately after the last instruction in the file</em>. By changing exactly one <code>jmp</code> or <code>nop</code>, you can repair the boot code and make it terminate correctly.</p>
+<p>For example, consider the same program from above:</p>
+<pre><code>nop +0
+acc +1
+jmp +4
+acc +3
+jmp -3
+acc -99
+acc +1
+jmp -4
+acc +6
+</code></pre>
+<p>If you change the first instruction from <code>nop +0</code> to <code>jmp +0</code>, it would create a single-instruction infinite loop, never leaving that instruction. If you change almost any of the <code>jmp</code> instructions, the program will still eventually find another <code>jmp</code> instruction and loop forever.</p>
+<p>However, if you change the second-to-last instruction (from <code>jmp -4</code> to <code>nop -4</code>), the program terminates! The instructions are visited in this order:</p>
+<pre><code>nop +0 | 1
+acc +1 | 2
+jmp +4 | 3
+acc +3 |
+jmp -3 |
+acc -99 |
+acc +1 | 4
+<em>nop</em> -4 | 5
+acc +6 | 6
+</code></pre>
+<p>After the last instruction (<code>acc +6</code>), the program terminates by attempting to run the instruction below the last instruction in the file. With this change, after the program terminates, the accumulator contains the value <em><code>8</code></em> (<code>acc +1</code>, <code>acc +1</code>, <code>acc +6</code>).</p>
+<p>Fix the program so that it terminates normally by changing exactly one <code>jmp</code> (to <code>nop</code>) or <code>nop</code> (to <code>jmp</code>). <em>What is the value of the accumulator after the program terminates?</em></p>
+</article>
+<p>Your puzzle answer was <code>1303</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="/2020">return to your Advent calendar</a> and try another puzzle.</p>
+<p>If you still want to see it, you can <a href="8/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+%22Handheld+Halting%22+%2D+Day+8+%2D+Advent+of+Code+2020&url=https%3A%2F%2Fadventofcode%2Ecom%2F2020%2Fday%2F8&related=ericwastl&hashtags=AdventOfCode" target="_blank">Twitter</a>
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