\--- 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 `179`.

\--- Part Two ---
----------

*How many pixels stay on* after `18` iterations?

Your puzzle answer was `2766750`.

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](/2017).

If you still want to see it, you can [get your puzzle input](21/input).