Advent of Code

--- Day 12: Subterranean Sustainability ---

   The year 518 is significantly more underground than your history books
   implied. Either that, or you've arrived in a vast cavern network under the
   North Pole.

   After exploring a little, you discover a long tunnel that contains a row of
   small pots as far as you can see to your left and right. A few of them
   contain plants - someone is trying to grow things in these
   geothermally-heated caves.

   The pots are numbered, with 0 in front of you. To the left, the pots are
   numbered -1, -2, -3, and so on; to the right, 1, 2, 3.... Your puzzle input
   contains a list of pots from 0 to the right and whether they do (#) or do
   not (.) currently contain a plant, the initial state. (No other pots
   currently contain plants.) For example, an initial state of #..##....
   indicates that pots 0, 3, and 4 currently contain plants.

   Your puzzle input also contains some notes you find on a nearby table:
   someone has been trying to figure out how these plants spread to nearby
   pots. Based on the notes, for each generation of plants, a given pot has or
   does not have a plant based on whether that pot (and the two pots on either
   side of it) had a plant in the last generation. These are written as LLCRR
   => N, where L are pots to the left, C is the current pot being considered, R
   are the pots to the right, and N is whether the current pot will have a
   plant in the next generation. For example:

     • A note like ..#.. => . means that a pot that contains a plant but with
     no plants within two pots of it will not have a plant in it during the
     next generation. • A note like ##.## => . means that an empty pot with two
     plants on each side of it will remain empty in the next generation. • A
     note like .##.# => # means that a pot has a plant in a given generation
     if, in the previous generation, there were plants in that pot, the one
     immediately to the left, and the one two pots to the right, but not in the
     ones immediately to the right and two to the left.

   It's not clear what these plants are for, but you're sure it's important, so
   you'd like to make sure the current configuration of plants is sustainable
   by determining what will happen after 20 generations.

   For example, given the following input:

 initial state: #..#.#..##......###...###

 ...## => #
 ..#.. => #
 .#... => #
 .#.#. => #
 .#.## => #
 .##.. => #
 .#### => #
 #.#.# => #
 #.### => #
 ##.#. => #
 ##.## => #
 ###.. => #
 ###.# => #
 ####. => #

   For brevity, in this example, only the combinations which do produce a plant
   are listed. (Your input includes all possible combinations.) Then, the next
   20 generations will look like this:

                  1         2         3
        0         0         0         0
  0: ...#..#.#..##......###...###...........
  1: ...#...#....#.....#..#..#..#...........
  2: ...##..##...##....#..#..#..##..........
  3: ..#.#...#..#.#....#..#..#...#..........
  4: ...#.#..#...#.#...#..#..##..##.........
  5: ....#...##...#.#..#..#...#...#.........
  6: ....##.#.#....#...#..##..##..##........
  7: ...#..###.#...##..#...#...#...#........
  8: ...#....##.#.#.#..##..##..##..##.......
  9: ...##..#..#####....#...#...#...#.......
 10: ..#.#..#...#.##....##..##..##..##......
 11: ...#...##...#.#...#.#...#...#...#......
 12: ...##.#.#....#.#...#.#..##..##..##.....
 13: ..#..###.#....#.#...#....#...#...#.....
 14: ..#....##.#....#.#..##...##..##..##....
 15: ..##..#..#.#....#....#..#.#...#...#....
 16: .#.#..#...#.#...##...#...#.#..##..##...
 17: ..#...##...#.#.#.#...##...#....#...#...
 18: ..##.#.#....#####.#.#.#...##...##..##..
 19: .#..###.#..#.#.#######.#.#.#..#.#...#..
 20: .#....##....#####...#######....#.#..##.

   The generation is shown along the left, where 0 is the initial state. The
   pot numbers are shown along the top, where 0 labels the center pot,
   negative-numbered pots extend to the left, and positive pots extend toward
   the right. Remember, the initial state begins at pot 0, which is not the
   leftmost pot used in this example.

   After one generation, only seven plants remain. The one in pot 0 matched the
   rule looking for ..#.., the one in pot 4 matched the rule looking for .#.#.,
   pot 9 matched .##.., and so on.

   In this example, after 20 generations, the pots shown as # contain plants,
   the furthest left of which is pot -2, and the furthest right of which is pot
   34. Adding up all the numbers of plant-containing pots after the 20th
   generation produces 325.

   After 20 generations, what is the sum of the numbers of all pots which
   contain a plant?

   Your puzzle answer was 2281.

--- Part Two ---

   You realize that 20 generations aren't enough. After all, these plants will
   need to last another 1500 years to even reach your timeline, not to mention
   your future.

   After fifty billion (50000000000) generations, what is the sum of the
   numbers of all pots which contain a plant?

   Your puzzle answer was 2250000000120.

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

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