102 lines
4.0 KiB
Plaintext
102 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 floor here seem to be
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pressure plates for traps, and the trap you just triggered has run out of... whatever it tried
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to do to you. You doubt you'll be so lucky next time.
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Upon closer examination, the traps and safe tiles in this room seem to follow a pattern. The
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tiles are arranged into rows that are all the same width; you take note of the safe tiles (.)
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and traps (^) in the first row (your puzzle input).
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The type of tile (trapped or safe) in each row is based on the types of the tiles in the same
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position, and to either side of that position, in the previous row. (If either side is off
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either end of the row, it counts as "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) and want to
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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 tile 5 is based on
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tiles D, E, and an imaginary "safe" tile. Let's call these three tiles from the previous row
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the left, center, and right tiles, respectively. Then, a new tile is a trap only in one of the
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following 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 applying those rules to
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each new tile:
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• The leftmost character on the next row considers the left (nonexistent, so we assume
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"safe"), center (the first ., which means "safe"), and right (the second ., also "safe")
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tiles on the previous row. Because all of the trap rules require a trap in at least one of
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the previous three tiles, the first tile on this new row is also safe, ..
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• The second character on the next row considers its left (.), center (.), and right (^)
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tiles from the previous row. This matches the fourth rule: only the right tile is a trap.
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Therefore, the next tile in this new row is a trap, ^.
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• The third character considers .^^, which matches the second trap rule: its center and
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right tiles are traps, but its left tile is not. Therefore, this tile is also a trap, ^.
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• The last two characters in this new row match the first and third rules, respectively, and
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so they are both also traps, ^.
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After these steps, we now know the next row of tiles in the room: .^^^^. Then, we continue on
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to the next row, using the same rules, and get ^^..^. After determining two new rows, our map
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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 the starting
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row), how many safe tiles are there?
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Your puzzle answer ________.
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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 ________.
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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/support
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. http://adventofcode.com/2016/events
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. http://adventofcode.com/2016/settings
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. http://adventofcode.com/2016/auth/logout
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/leaderboard
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. http://adventofcode.com/2016/stats
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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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