pub fn y15d18(input: &str, steps: u32, part: u32) -> u32Expand description
The solution for the day eighteen challenge.
We take the input as a string as usual, and then accept the number of steps
to process and the part of the problem that we’re solving (in part 2 the
corners are always on). We start by parsing the input to build a grid of
the lights and their state. If we’re in part 2 then we turn on the lights
in the corner regardless of the initial input. Then for each step until we
reach the total number of steps we build a new grid. We process all of the
current lights and check their neighbors to see if they are on or not (if
we’re in part two and on a corner we just skip as we always leave the
corner lights on). After we’ve counted the number of lights that are on we
turn the light on or off based on the constraints from the prompt: if the
light is currently on and it has two or three neighbors that are on then it
stays on otherwise it turns off, and if it’s currently off and it has three
neighbors that are on then it turns on otherwise it stays off. If we’re in
part two we turn on the corner lights before replacing the current grid
with the new grid that we’ve just built and moving on to the next step.
Finally, we count the number of lights that are on and return the result.
§Example
// probably read this from the input file...
let input = ".#.#.#\n...##.\n#....#\n..#...\n#.#..#\n####..";
assert_eq!(y15d18(input, 1, 1), 11);
assert_eq!(y15d18(input, 3, 2), 18);