adventofcode/2022/day23/problem

Advent of Code
br0xen (AoC++) 39*

--- Day 23: Unstable Diffusion ---

   You enter a large crater of gray dirt where the grove is supposed to be. All around
   you, plants you imagine were expected to be full of fruit are instead withered and
   broken. A large group of Elves has formed in the middle of the grove.

   "...but this volcano has been dormant for months. Without ash, the fruit can't grow!"

   You look up to see a massive, snow-capped mountain towering above you.

   "It's not like there are other active volcanoes here; we've looked everywhere."

   "But our scanners show active magma flows; clearly it's going somewhere."

   They finally notice you at the edge of the grove, your pack almost overflowing from
   the random star fruit you've been collecting. Behind you, elephants and monkeys
   explore the grove, looking concerned. Then, the Elves recognize the ash cloud slowly
   spreading above your recent detour.

   "Why do you--" "How is--" "Did you just--"

   Before any of them can form a complete question, another Elf speaks up: "Okay, new
   plan. We have almost enough fruit already, and ash from the plume should spread here
   eventually. If we quickly plant new seedlings now, we can still make it to the
   extraction point. Spread out!"

   The Elves each reach into their pack and pull out a tiny plant. The plants rely on
   important nutrients from the ash, so they can't be planted too close together.

   There isn't enough time to let the Elves figure out where to plant the seedlings
   themselves; you quickly scan the grove (your puzzle input) and note their positions.

   For example:

 ....#..
 ..###.#
 #...#.#
 .#...##
 #.###..
 ##.#.##
 .#..#..

   The scan shows Elves # and empty ground .; outside your scan, more empty ground
   extends a long way in every direction. The scan is oriented so that north is up;
   orthogonal directions are written N (north), S (south), W (west), and E (east), while
   diagonal directions are written NE, NW, SE, SW.

   The Elves follow a time-consuming process to figure out where they should each go; you
   can speed up this process considerably. The process consists of some number of rounds
   during which Elves alternate between considering where to move and actually moving.

   During the first half of each round, each Elf considers the eight positions adjacent
   to themself. If no other Elves are in one of those eight positions, the Elf does not
   do anything during this round. Otherwise, the Elf looks in each of four directions in
   the following order and proposes moving one step in the first valid direction:

     • If there is no Elf in the N, NE, or NW adjacent positions, the Elf proposes moving
       north one step.
     • If there is no Elf in the S, SE, or SW adjacent positions, the Elf proposes moving
       south one step.
     • If there is no Elf in the W, NW, or SW adjacent positions, the Elf proposes moving
       west one step.
     • If there is no Elf in the E, NE, or SE adjacent positions, the Elf proposes moving
       east one step.

   After each Elf has had a chance to propose a move, the second half of the round can
   begin. Simultaneously, each Elf moves to their proposed destination tile if they were
   the only Elf to propose moving to that position. If two or more Elves propose moving
   to the same position, none of those Elves move.

   Finally, at the end of the round, the first direction the Elves considered is moved to
   the end of the list of directions. For example, during the second round, the Elves
   would try proposing a move to the south first, then west, then east, then north. On
   the third round, the Elves would first consider west, then east, then north, then
   south.

   As a smaller example, consider just these five Elves:

 .....
 ..##.
 ..#..
 .....
 ..##.
 .....

   The northernmost two Elves and southernmost two Elves all propose moving north, while
   the middle Elf cannot move north and proposes moving south. The middle Elf proposes
   the same destination as the southwest Elf, so neither of them move, but the other
   three do:

 ..##.
 .....
 ..#..
 ...#.
 ..#..
 .....

   Next, the northernmost two Elves and the southernmost Elf all propose moving south. Of
   the remaining middle two Elves, the west one cannot move south and proposes moving
   west, while the east one cannot move south or west and proposes moving east. All five
   Elves succeed in moving to their proposed positions:

 .....
 ..##.
 .#...
 ....#
 .....
 ..#..

   Finally, the southernmost two Elves choose not to move at all. Of the remaining three
   Elves, the west one proposes moving west, the east one proposes moving east, and the
   middle one proposes moving north; all three succeed in moving:

 ..#..
 ....#
 #....
 ....#
 .....
 ..#..

   At this point, no Elves need to move, and so the process ends.

   The larger example above proceeds as follows:

 == Initial State ==
 ..............
 ..............
 .......#......
 .....###.#....
 ...#...#.#....
 ....#...##....
 ...#.###......
 ...##.#.##....
 ....#..#......
 ..............
 ..............
 ..............

 == End of Round 1 ==
 ..............
 .......#......
 .....#...#....
 ...#..#.#.....
 .......#..#...
 ....#.#.##....
 ..#..#.#......
 ..#.#.#.##....
 ..............
 ....#..#......
 ..............
 ..............

 == End of Round 2 ==
 ..............
 .......#......
 ....#.....#...
 ...#..#.#.....
 .......#...#..
 ...#..#.#.....
 .#...#.#.#....
 ..............
 ..#.#.#.##....
 ....#..#......
 ..............
 ..............

 == End of Round 3 ==
 ..............
 .......#......
 .....#....#...
 ..#..#...#....
 .......#...#..
 ...#..#.#.....
 .#..#.....#...
 .......##.....
 ..##.#....#...
 ...#..........
 .......#......
 ..............

 == End of Round 4 ==
 ..............
 .......#......
 ......#....#..
 ..#...##......
 ...#.....#.#..
 .........#....
 .#...###..#...
 ..#......#....
 ....##....#...
 ....#.........
 .......#......
 ..............

 == End of Round 5 ==
 .......#......
 ..............
 ..#..#.....#..
 .........#....
 ......##...#..
 .#.#.####.....
 ...........#..
 ....##..#.....
 ..#...........
 ..........#...
 ....#..#......
 ..............

   After a few more rounds...

 == End of Round 10 ==
 .......#......
 ...........#..
 ..#.#..#......
 ......#.......
 ...#.....#..#.
 .#......##....
 .....##.......
 ..#........#..
 ....#.#..#....
 ..............
 ....#..#..#...
 ..............

   To make sure they're on the right track, the Elves like to check after round 10 that
   they're making good progress toward covering enough ground. To do this, count the
   number of empty ground tiles contained by the smallest rectangle that contains every
   Elf. (The edges of the rectangle should be aligned to the N/S/E/W directions; the
   Elves do not have the patience to calculate arbitrary rectangles.) In the above
   example, that rectangle is:

 ......#.....
 ..........#.
 .#.#..#.....
 .....#......
 ..#.....#..#
 #......##...
 ....##......
 .#........#.
 ...#.#..#...
 ............
 ...#..#..#..

   In this region, the number of empty ground tiles is 110.

   Simulate the Elves' process and find the smallest rectangle that contains the Elves
   after 10 rounds. How many empty ground tiles does that rectangle contain?

   Your puzzle answer was 4000.

--- Part Two ---

   It seems you're on the right track. Finish simulating the process and figure out where
   the Elves need to go. How many rounds did you save them?

   In the example above, the first round where no Elf moved was round 20:

 .......#......
 ....#......#..
 ..#.....#.....
 ......#.......
 ...#....#.#..#
 #.............
 ....#.....#...
 ..#.....#.....
 ....#.#....#..
 .........#....
 ....#......#..
 .......#......

   Figure out where the Elves need to go. What is the number of the first round where no
   Elf moves?

   Your puzzle answer was 1040.

   Both parts of this puzzle are complete! They provide two gold stars: **

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