112 lines
4.0 KiB
Plaintext
112 lines
4.0 KiB
Plaintext
Advent of Code
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--- Day 18: Like a Rogue ---
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As you enter this room, you hear a loud click! Some of the tiles in the
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floor here seem to be pressure plates for traps, and the trap you just
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triggered has run out of... whatever it tried to do to you. You doubt you'll
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be so lucky next time.
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Upon closer examination, the traps and safe tiles in this room seem to
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follow a pattern. The tiles are arranged into rows that are all the same
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width; you take note of the safe tiles (.) and traps (^) in the first row
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(your puzzle input).
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The type of tile (trapped or safe) in each row is based on the types of the
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tiles in the same position, and to either side of that position, in the
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previous row. (If either side is off either end of the row, it counts as
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"safe" because there isn't a trap embedded in the wall.)
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For example, suppose you know the first row (with tiles marked by letters)
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and want to determine the next row (with tiles marked by numbers):
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ABCDE
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12345
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The type of tile 2 is based on the types of tiles A, B, and C; the type of
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tile 5 is based on tiles D, E, and an imaginary "safe" tile. Let's call
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these three tiles from the previous row the left, center, and right tiles,
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respectively. Then, a new tile is a trap only in one of the following
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situations:
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• Its left and center tiles are traps, but its right tile is not.
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• Its center and right tiles are traps, but its left tile is not.
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• Only its left tile is a trap.
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• Only its right tile is a trap.
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In any other situation, the new tile is safe.
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Then, starting with the row ..^^., you can determine the next row by
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applying those rules to each new tile:
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• The leftmost character on the next row considers the left (nonexistent,
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so we assume "safe"), center (the first ., which means "safe"), and right
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(the second ., also "safe") tiles on the previous row. Because all of the
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trap rules require a trap in at least one of the previous three tiles, the
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first tile on this new row is also safe, ..
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• The second character on the next row considers its left (.), center (.),
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and right (^) tiles from the previous row. This matches the fourth rule:
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only the right tile is a trap. Therefore, the next tile in this new row is
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a trap, ^.
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• The third character considers .^^, which matches the second trap rule:
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its center and right tiles are traps, but its left tile is not. Therefore,
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this tile is also a trap, ^.
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• The last two characters in this new row match the first and third rules,
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respectively, and so they are both also traps, ^.
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After these steps, we now know the next row of tiles in the room: .^^^^.
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Then, we continue on to the next row, using the same rules, and get ^^..^.
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After determining two new rows, our map looks like this:
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..^^.
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.^^^^
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^^..^
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Here's a larger example with ten tiles per row and ten rows:
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.^^.^.^^^^
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^^^...^..^
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^.^^.^.^^.
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..^^...^^^
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.^^^^.^^.^
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^^..^.^^..
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^^^^..^^^.
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^..^^^^.^^
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.^^^..^.^^
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^^.^^^..^^
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In ten rows, this larger example has 38 safe tiles.
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Starting with the map in your puzzle input, in a total of 40 rows (including
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the starting row), how many safe tiles are there?
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Your puzzle answer 1913.
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--- Part Two ---
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How many safe tiles are there in a total of 400000 rows?
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Your puzzle answer was 19993564.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2016/about
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/sponsors
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. http://adventofcode.com/2016/sponsors
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. https://nethackwiki.com/wiki/Trap
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/day/18/input
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