Doing some C learnin'.

Also, reformatted all the problems.

2017/day21/problem

@@ -0,0 +1,142 @@
+Advent of Code
+
+--- 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 164.
+
+--- Part Two ---
+
+   How many pixels stay on after 18 iterations?
+
+   Your puzzle answer was 2355110.
+
+   Both parts of this puzzle are complete! They provide two gold stars: **
+
+   At this point, all that is left is for you to admire your advent calendar.
+
+   If you still want to see it, you can get your puzzle input.
+
+References
+
+   Visible links
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+   . http://adventofcode.com/2017
+   . http://adventofcode.com/2017/day/21/input