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+--- Day 21: Fractal Art ---
+
+You find a program trying to generate some art. It uses a strange
+process that involves repeatedly enhancing the detail of an image
+through a set of rules.
+
+The image consists of a two-dimensional square grid of pixels that are
+either on (#) or off (.). The program always begins with this pattern:
+
+.#.
+..#
+###
+
+Because the pattern is both 3 pixels wide and 3 pixels tall, it is
+said to have a size of 3.
+
+Then, the program repeats the following process:
+
+    If the size is evenly divisible by 2, break the pixels up into 2x2
+    squares, and convert each 2x2 square into a 3x3 square by
+    following the corresponding enhancement rule.
+
+    Otherwise, the size is evenly divisible by 3; break the pixels up
+    into 3x3 squares, and convert each 3x3 square into a 4x4 square by
+    following the corresponding enhancement rule.
+
+
+Because each square of pixels is replaced by a larger one, the image
+gains pixels and so its size increases.
+
+The artist's book of enhancement rules is nearby (your puzzle input);
+however, it seems to be missing rules. The artist explains that
+sometimes, one must rotate or flip the input pattern to find a match.
+(Never rotate or flip the output pattern, though.) Each pattern is
+written concisely: rows are listed as single units, ordered top-down,
+and separated by slashes. For example, the following rules correspond
+to the adjacent patterns:
+
+../.#  =  ..
+          .#
+
+                .#.
+.#./..#/###  =  ..#
+                ###
+
+                        #..#
+#..#/..../#..#/.##.  =  ....
+                        #..#
+                        .##.
+
+When searching for a rule to use, rotate and flip the pattern as
+necessary. For example, all of the following patterns match the same
+rule:
+
+.#.   .#.   #..   ###
+..#   #..   #.#   ..#
+###   ###   ##.   .#.
+
+Suppose the book contained the following two rules:
+
+../.# => ##./#../...
+.#./..#/### => #..#/..../..../#..#
+
+As before, the program begins with this pattern:
+
+.#.
+..#
+###
+
+The size of the grid (3) is not divisible by 2, but it is divisible by
+3. It divides evenly into a single square; the square matches the
+second rule, which produces:
+
+#..#
+....
+....
+#..#
+
+The size of this enhanced grid (4) is evenly divisible by 2, so that
+rule is used. It divides evenly into four squares:
+
+#.|.#
+..|..
+--+--
+..|..
+#.|.#
+
+Each of these squares matches the same rule (../.# => ##./#../...),
+three of which require some flipping and rotation to line up with the
+rule. The output for the rule is the same in all four cases:
+
+##.|##.
+#..|#..
+...|...
+---+---
+##.|##.
+#..|#..
+...|...
+
+Finally, the squares are joined into a new grid:
+
+##.##.
+#..#..
+......
+##.##.
+#..#..
+......
+
+Thus, after 2 iterations, the grid contains 12 pixels that are on.
+
+How many pixels stay on after 5 iterations?
+
+Your puzzle answer was 176.
+
+--- Part Two ---
+
+How many pixels stay on after 18 iterations?
+
+Your puzzle answer was 2368161.
+
+Both parts of this puzzle are complete! They provide two gold stars: **