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<title>Day 15 - Advent of Code 2021</title>
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</head><div id="readability-page-1" class="page"><div>
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<article><h2>--- Day 15: Chiton ---</h2><p>You've almost reached the exit of the cave, but the walls are getting closer together. Your submarine can barely still fit, though; the main problem is that the walls of the cave are covered in <a href="https://en.wikipedia.org/wiki/Chiton" target="_blank">chitons</a>, and it would be best not to bump any of them.</p>
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<p>The cavern is large, but has a very low ceiling, restricting your motion to two dimensions. The shape of the cavern resembles a square; a quick scan of chiton density produces a map of <em>risk level</em> throughout the cave (your puzzle input). For example:</p>
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<pre><code>1163751742
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1381373672
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2136511328
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3694931569
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7463417111
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1319128137
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1359912421
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3125421639
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1293138521
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2311944581
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</code></pre>
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<p>You start in the top left position, your destination is the bottom right position, and you <span title="Can't go diagonal until we can repair the caterpillar unit. Could be the liquid helium or the superconductors.">cannot move diagonally</span>. The number at each position is its <em>risk level</em>; to determine the total risk of an entire path, add up the risk levels of each position you <em>enter</em> (that is, don't count the risk level of your starting position unless you enter it; leaving it adds no risk to your total).</p>
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<p>Your goal is to find a path with the <em>lowest total risk</em>. In this example, a path with the lowest total risk is highlighted here:</p>
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<pre><code><em>1</em>163751742
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<em>1</em>381373672
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<em>2136511</em>328
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369493<em>15</em>69
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7463417<em>1</em>11
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1319128<em>13</em>7
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13599124<em>2</em>1
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31254216<em>3</em>9
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12931385<em>21</em>
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231194458<em>1</em>
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</code></pre>
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<p>The total risk of this path is <code><em>40</em></code> (the starting position is never entered, so its risk is not counted).</p>
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<p><em>What is the lowest total risk of any path from the top left to the bottom right?</em></p>
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</article>
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<p>Your puzzle answer was <code>410</code>.</p><article><h2 id="part2">--- Part Two ---</h2><p>Now that you know how to find low-risk paths in the cave, you can try to find your way out.</p>
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<p>The entire cave is actually <em>five times larger in both dimensions</em> than you thought; the area you originally scanned is just one tile in a 5x5 tile area that forms the full map. Your original map tile repeats to the right and downward; each time the tile repeats to the right or downward, all of its risk levels <em>are 1 higher</em> than the tile immediately up or left of it. However, risk levels above <code>9</code> wrap back around to <code>1</code>. So, if your original map had some position with a risk level of <code>8</code>, then that same position on each of the 25 total tiles would be as follows:</p>
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<pre><code>8 9 1 2 3
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9 1 2 3 4
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1 2 3 4 5
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2 3 4 5 6
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3 4 5 6 7
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</code></pre>
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<p>Each single digit above corresponds to the example position with a value of <code>8</code> on the top-left tile. Because the full map is actually five times larger in both dimensions, that position appears a total of 25 times, once in each duplicated tile, with the values shown above.</p>
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<p>Here is the full five-times-as-large version of the first example above, with the original map in the top left corner highlighted:</p>
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<pre><code><em>1163751742</em>2274862853338597396444961841755517295286
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<em>1381373672</em>2492484783351359589446246169155735727126
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<em>2136511328</em>3247622439435873354154698446526571955763
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<em>3694931569</em>4715142671582625378269373648937148475914
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<em>7463417111</em>8574528222968563933317967414442817852555
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<em>1319128137</em>2421239248353234135946434524615754563572
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<em>1359912421</em>2461123532357223464346833457545794456865
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<em>3125421639</em>4236532741534764385264587549637569865174
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<em>1293138521</em>2314249632342535174345364628545647573965
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<em>2311944581</em>3422155692453326671356443778246755488935
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22748628533385973964449618417555172952866628316397
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24924847833513595894462461691557357271266846838237
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32476224394358733541546984465265719557637682166874
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47151426715826253782693736489371484759148259586125
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85745282229685639333179674144428178525553928963666
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24212392483532341359464345246157545635726865674683
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24611235323572234643468334575457944568656815567976
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42365327415347643852645875496375698651748671976285
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23142496323425351743453646285456475739656758684176
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34221556924533266713564437782467554889357866599146
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33859739644496184175551729528666283163977739427418
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35135958944624616915573572712668468382377957949348
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43587335415469844652657195576376821668748793277985
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58262537826937364893714847591482595861259361697236
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96856393331796741444281785255539289636664139174777
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35323413594643452461575456357268656746837976785794
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35722346434683345754579445686568155679767926678187
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53476438526458754963756986517486719762859782187396
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34253517434536462854564757396567586841767869795287
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45332667135644377824675548893578665991468977611257
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44961841755517295286662831639777394274188841538529
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46246169155735727126684683823779579493488168151459
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54698446526571955763768216687487932779859814388196
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69373648937148475914825958612593616972361472718347
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17967414442817852555392896366641391747775241285888
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46434524615754563572686567468379767857948187896815
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46833457545794456865681556797679266781878137789298
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64587549637569865174867197628597821873961893298417
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45364628545647573965675868417678697952878971816398
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56443778246755488935786659914689776112579188722368
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55172952866628316397773942741888415385299952649631
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57357271266846838237795794934881681514599279262561
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65719557637682166874879327798598143881961925499217
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71484759148259586125936169723614727183472583829458
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28178525553928963666413917477752412858886352396999
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57545635726865674683797678579481878968159298917926
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57944568656815567976792667818781377892989248891319
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75698651748671976285978218739618932984172914319528
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56475739656758684176786979528789718163989182927419
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67554889357866599146897761125791887223681299833479
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</code></pre>
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<p>Equipped with the full map, you can now find a path from the top left corner to the bottom right corner with the lowest total risk:</p>
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<pre><code><em>1</em>1637517422274862853338597396444961841755517295286
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<em>1</em>3813736722492484783351359589446246169155735727126
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<em>2</em>1365113283247622439435873354154698446526571955763
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<em>3</em>6949315694715142671582625378269373648937148475914
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<em>7</em>4634171118574528222968563933317967414442817852555
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<em>1</em>3191281372421239248353234135946434524615754563572
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<em>1</em>3599124212461123532357223464346833457545794456865
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<em>3</em>1254216394236532741534764385264587549637569865174
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<em>1</em>2931385212314249632342535174345364628545647573965
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<em>2</em>3119445813422155692453326671356443778246755488935
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<em>2</em>2748628533385973964449618417555172952866628316397
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<em>2</em>4924847833513595894462461691557357271266846838237
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<em>324</em>76224394358733541546984465265719557637682166874
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47<em>15</em>1426715826253782693736489371484759148259586125
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857<em>4</em>5282229685639333179674144428178525553928963666
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242<em>1</em>2392483532341359464345246157545635726865674683
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246<em>1123532</em>3572234643468334575457944568656815567976
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423653274<em>1</em>5347643852645875496375698651748671976285
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231424963<em>2342</em>5351743453646285456475739656758684176
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342215569245<em>332</em>66713564437782467554889357866599146
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33859739644496<em>1</em>84175551729528666283163977739427418
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35135958944624<em>61</em>6915573572712668468382377957949348
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435873354154698<em>44</em>652657195576376821668748793277985
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5826253782693736<em>4</em>893714847591482595861259361697236
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9685639333179674<em>1</em>444281785255539289636664139174777
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3532341359464345<em>2461</em>575456357268656746837976785794
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3572234643468334575<em>4</em>579445686568155679767926678187
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5347643852645875496<em>3</em>756986517486719762859782187396
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3425351743453646285<em>4564</em>757396567586841767869795287
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4533266713564437782467<em>554</em>8893578665991468977611257
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449618417555172952866628<em>3163</em>9777394274188841538529
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462461691557357271266846838<em>2</em>3779579493488168151459
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546984465265719557637682166<em>8</em>7487932779859814388196
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693736489371484759148259586<em>125</em>93616972361472718347
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17967414442817852555392896366<em>6413</em>91747775241285888
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46434524615754563572686567468379<em>7</em>67857948187896815
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46833457545794456865681556797679<em>26</em>6781878137789298
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645875496375698651748671976285978<em>21</em>873961893298417
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4536462854564757396567586841767869<em>7</em>952878971816398
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5644377824675548893578665991468977<em>6112</em>579188722368
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5517295286662831639777394274188841538<em>5</em>299952649631
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5735727126684683823779579493488168151<em>4</em>599279262561
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6571955763768216687487932779859814388<em>1</em>961925499217
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7148475914825958612593616972361472718<em>34725</em>83829458
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28178525553928963666413917477752412858886<em>3</em>52396999
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57545635726865674683797678579481878968159<em>2</em>98917926
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57944568656815567976792667818781377892989<em>24</em>8891319
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756986517486719762859782187396189329841729<em>1431</em>9528
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564757396567586841767869795287897181639891829<em>2</em>7419
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675548893578665991468977611257918872236812998<em>33479</em>
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</code></pre>
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<p>The total risk of this path is <code><em>315</em></code> (the starting position is still never entered, so its risk is not counted).</p>
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<p>Using the full map, <em>what is the lowest total risk of any path from the top left to the bottom right?</em></p>
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</article>
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<p>Your puzzle answer was <code>2809</code>.</p><p>Both parts of this puzzle are complete! They provide two gold stars: **</p>
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<p>At this point, you should <a href="https://adventofcode.com/2021">return to your Advent calendar</a> and try another puzzle.</p>
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<p>If you still want to see it, you can <a href="https://adventofcode.com/2021/day/15/input" target="_blank">get your puzzle input</a>.</p>
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<p>You can also this puzzle.</p>
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</div></div> |