143 lines
3.8 KiB
Plaintext
143 lines
3.8 KiB
Plaintext
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Advent of Code
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--- Day 21: Fractal Art ---
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You find a program trying to generate some art. It uses a strange process
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that involves repeatedly enhancing the detail of an image through a set of
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rules.
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The image consists of a two-dimensional square grid of pixels that are
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either on (#) or off (.). The program always begins with this pattern:
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.#.
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..#
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###
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Because the pattern is both 3 pixels wide and 3 pixels tall, it is said to
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have a size of 3.
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Then, the program repeats the following process:
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• If the size is evenly divisible by 2, break the pixels up into 2x2
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squares, and convert each 2x2 square into a 3x3 square by following the
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corresponding enhancement rule.
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• Otherwise, the size is evenly divisible by 3; break the pixels up into
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3x3 squares, and convert each 3x3 square into a 4x4 square by following
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the corresponding enhancement rule.
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Because each square of pixels is replaced by a larger one, the image gains
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pixels and so its size increases.
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The artist's book of enhancement rules is nearby (your puzzle input);
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however, it seems to be missing rules. The artist explains that sometimes,
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one must rotate or flip the input pattern to find a match. (Never rotate or
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flip the output pattern, though.) Each pattern is written concisely: rows
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are listed as single units, ordered top-down, and separated by slashes. For
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example, the following rules correspond to the adjacent patterns:
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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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When searching for a rule to use, rotate and flip the pattern as necessary.
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For example, all of the following patterns match the same rule:
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.#. .#. #.. ###
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..# #.. #.# ..#
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### ### ##. .#.
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Suppose the book contained the following two rules:
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../.# => ##./#../...
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.#./..#/### => #..#/..../..../#..#
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As before, the program begins with this pattern:
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.#.
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..#
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###
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The size of the grid (3) is not divisible by 2, but it is divisible by 3. It
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divides evenly into a single square; the square matches the second rule,
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which produces:
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#..#
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....
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....
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#..#
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The size of this enhanced grid (4) is evenly divisible by 2, so that rule is
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used. It divides evenly into four squares:
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#.|.#
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..|..
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--+--
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..|..
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#.|.#
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Each of these squares matches the same rule (../.# => ##./#../...), three of
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which require some flipping and rotation to line up with the rule. The
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output for the rule is the same in all four cases:
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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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Finally, the squares are joined into a new grid:
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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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Thus, after 2 iterations, the grid contains 12 pixels that are on.
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How many pixels stay on after 5 iterations?
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Your puzzle answer was 164.
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--- Part Two ---
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How many pixels stay on after 18 iterations?
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Your puzzle answer was 2355110.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, all that is left is for you to admire your advent calendar.
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If you still want to see it, you can get your puzzle input.
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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/2017/about
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. http://adventofcode.com/2017/support
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. http://adventofcode.com/2017/events
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. http://adventofcode.com/2017/settings
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. http://adventofcode.com/2017/auth/logout
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/leaderboard
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. http://adventofcode.com/2017/stats
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. http://adventofcode.com/2017/sponsors
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. http://adventofcode.com/2017/sponsors
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/day/21/input
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