57 lines
2.3 KiB
Org Mode
57 lines
2.3 KiB
Org Mode
** --- Day 8: Treetop Tree House ---
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The expedition comes across a peculiar patch of tall trees all planted
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carefully in a grid. The Elves explain that a previous expedition
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planted these trees as a reforestation effort. Now, they're curious if
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this would be a good location for a
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[[https://en.wikipedia.org/wiki/Tree_house][tree house]].
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First, determine whether there is enough tree cover here to keep a tree
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house /hidden/. To do this, you need to count the number of trees that
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are /visible from outside the grid/ when looking directly along a row or
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column.
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The Elves have already launched a
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[[https://en.wikipedia.org/wiki/Quadcopter][quadcopter]] to generate a
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map with the height of each tree (your puzzle input). For example:
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#+begin_example
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30373
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25512
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65332
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33549
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35390
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#+end_example
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Each tree is represented as a single digit whose value is its height,
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where =0= is the shortest and =9= is the tallest.
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A tree is /visible/ if all of the other trees between it and an edge of
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the grid are /shorter/ than it. Only consider trees in the same row or
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column; that is, only look up, down, left, or right from any given tree.
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All of the trees around the edge of the grid are /visible/ - since they
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are already on the edge, there are no trees to block the view. In this
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example, that only leaves the /interior nine trees/ to consider:
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- The top-left =5= is /visible/ from the left and top. (It isn't visible
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from the right or bottom since other trees of height =5= are in the
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way.)
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- The top-middle =5= is /visible/ from the top and right.
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- The top-right =1= is not visible from any direction; for it to be
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visible, there would need to only be trees of height /0/ between it
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and an edge.
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- The left-middle =5= is /visible/, but only from the right.
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- The center =3= is not visible from any direction; for it to be
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visible, there would need to be only trees of at most height =2=
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between it and an edge.
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- The right-middle =3= is /visible/ from the right.
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- In the bottom row, the middle =5= is /visible/, but the =3= and =4=
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are not.
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With 16 trees visible on the edge and another 5 visible in the interior,
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a total of =21= trees are visible in this arrangement.
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Consider your map; /how many trees are visible from outside the grid?/
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To begin, [[file:8/input][get your puzzle input]].
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