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@@ -0,0 +1,195 @@
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+--- Day 15: Chiton ---
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+----------------------
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+
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+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 [chitons](https://en.wikipedia.org/wiki/Chiton), and it would be best not to bump any of them.
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+
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+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 *risk level* throughout the cave (your puzzle input). For example:
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+
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+
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+
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+```
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+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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+
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+```
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+
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+You start in the top left position, your destination is the bottom right position, and you cannot move diagonally. The number at each position is its *risk level*; to determine the total risk of an entire path, add up the risk levels of each position you *enter* (that is, don't count the risk level of your starting position unless you enter it; leaving it adds no risk to your total).
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+
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+
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+Your goal is to find a path with the *lowest total risk*. In this example, a path with the lowest total risk is highlighted here:
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+
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+
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+
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+```
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+*1*163751742
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+*1*381373672
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+*2136511*328
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+369493*15*69
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+7463417*1*11
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+1319128*13*7
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+13599124*2*1
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+31254216*3*9
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+12931385*21*
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+231194458*1*
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+
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+```
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+
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+The total risk of this path is `*40*` (the starting position is never entered, so its risk is not counted).
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+
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+
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+*What is the lowest total risk of any path from the top left to the bottom right?*
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+
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+
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+--- Part Two ---
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+----------------
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+
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+Now that you know how to find low-risk paths in the cave, you can try to find your way out.
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+
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+
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+The entire cave is actually *five times larger in both dimensions* 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 *are 1 higher* than the tile immediately up or left of it. However, risk levels above `9` wrap back around to `1`. So, if your original map had some position with a risk level of `8`, then that same position on each of the 25 total tiles would be as follows:
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+
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+
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+
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+```
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+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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+
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+```
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+
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+Each single digit above corresponds to the example position with a value of `8` 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.
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+
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+
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+Here is the full five-times-as-large version of the first example above, with the original map in the top left corner highlighted:
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+
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+
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+
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+```
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+*1163751742*2274862853338597396444961841755517295286
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+*1381373672*2492484783351359589446246169155735727126
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+
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+```
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+
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+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:
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+
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+
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+
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+```
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+*1*1637517422274862853338597396444961841755517295286
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+*1*3813736722492484783351359589446246169155735727126
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+675548893578665991468977611257918872236812998*33479*
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+
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+```
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+
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+The total risk of this path is `*315*` (the starting position is still never entered, so its risk is not counted).
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+
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+
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+Using the full map, *what is the lowest total risk of any path from the top left to the bottom right?*
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+
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+
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