Function advent_solutions::advent2017::day21::solve [−] [src]

pub fn solve(input: &HashMap<Grid, Grid>, iterations: usize) -> usize

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.

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.

let input = parse_input("\
../.# => ##./#../...
.#./..#/### => #..#/..../..../#..#
");

assert_eq!(solve(&input, 2), 12);