124 lines
5.7 KiB
Plaintext
124 lines
5.7 KiB
Plaintext
Advent of Code
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--- Day 12: Subterranean Sustainability ---
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The year 518 is significantly more underground than your history books implied. Either that, or you've arrived in a vast
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cavern network under the North Pole.
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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
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and right. A few of them contain plants - someone is trying to grow things in these geothermally-heated caves.
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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,
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3.... Your puzzle input contains a list of pots from 0 to the right and whether they do (#) or do not (.) currently contain
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a plant, the initial state. (No other pots currently contain plants.) For example, an initial state of #..##.... indicates
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that pots 0, 3, and 4 currently contain plants.
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Your puzzle input also contains some notes you find on a nearby table: someone has been trying to figure out how these
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plants spread to nearby pots. Based on the notes, for each generation of plants, a given pot has or does not have a plant
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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
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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
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whether the current pot will have a plant in the next generation. For example:
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• A note like ..#.. => . means that a pot that contains a plant but with no plants within two pots of it will not have a
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plant in it during the next generation.
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• A note like ##.## => . means that an empty pot with two plants on each side of it will remain empty in the next
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generation.
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• A note like .##.# => # means that a pot has a plant in a given generation if, in the previous generation, there were
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plants in that pot, the one immediately to the left, and the one two pots to the right, but not in the ones immediately
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to the right and two to the left.
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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
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configuration of plants is sustainable by determining what will happen after 20 generations.
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For example, given the following input:
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initial state: #..#.#..##......###...###
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...## => #
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..#.. => #
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.#... => #
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.#.#. => #
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.#.## => #
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.##.. => #
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.#### => #
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#.#.# => #
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#.### => #
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##.#. => #
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##.## => #
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###.. => #
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###.# => #
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####. => #
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For brevity, in this example, only the combinations which do produce a plant are listed. (Your input includes all possible
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combinations.) Then, the next 20 generations will look like this:
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1 2 3
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0 0 0 0
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0: ...#..#.#..##......###...###...........
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1: ...#...#....#.....#..#..#..#...........
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2: ...##..##...##....#..#..#..##..........
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3: ..#.#...#..#.#....#..#..#...#..........
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4: ...#.#..#...#.#...#..#..##..##.........
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5: ....#...##...#.#..#..#...#...#.........
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6: ....##.#.#....#...#..##..##..##........
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7: ...#..###.#...##..#...#...#...#........
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8: ...#....##.#.#.#..##..##..##..##.......
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9: ...##..#..#####....#...#...#...#.......
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10: ..#.#..#...#.##....##..##..##..##......
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11: ...#...##...#.#...#.#...#...#...#......
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12: ...##.#.#....#.#...#.#..##..##..##.....
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13: ..#..###.#....#.#...#....#...#...#.....
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14: ..#....##.#....#.#..##...##..##..##....
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15: ..##..#..#.#....#....#..#.#...#...#....
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16: .#.#..#...#.#...##...#...#.#..##..##...
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17: ..#...##...#.#.#.#...##...#....#...#...
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18: ..##.#.#....#####.#.#.#...##...##..##..
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19: .#..###.#..#.#.#######.#.#.#..#.#...#..
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20: .#....##....#####...#######....#.#..##.
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The generation is shown along the left, where 0 is the initial state. The pot numbers are shown along the top, where 0
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labels the center pot, negative-numbered pots extend to the left, and positive pots extend toward the right. Remember, the
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initial state begins at pot 0, which is not the leftmost pot used in this example.
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After one generation, only seven plants remain. The one in pot 0 matched the rule looking for ..#.., the one in pot 4
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matched the rule looking for .#.#., pot 9 matched .##.., and so on.
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In this example, after 20 generations, the pots shown as # contain plants, the furthest left of which is pot -2, and the
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furthest right of which is pot 34. Adding up all the numbers of plant-containing pots after the 20th generation produces
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325.
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After 20 generations, what is the sum of the numbers of all pots which contain a plant?
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Your puzzle answer was 2281.
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--- Part Two ---
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You realize that 20 generations aren't enough. After all, these plants will need to last another 1500 years to even reach
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your timeline, not to mention your future.
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After fifty billion (50000000000) generations, what is the sum of the numbers of all pots which contain a plant?
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Your puzzle answer was 2250000000120.
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Both parts of this puzzle are complete! They provide two gold stars: **
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References
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Visible links
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. https://adventofcode.com/
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. https://adventofcode.com/2018/about
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. https://adventofcode.com/2018/events
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. https://adventofcode.com/2018/settings
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. https://adventofcode.com/2018/auth/logout
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. Advent of Code Supporter
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https://adventofcode.com/2018/support
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018/support
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. https://adventofcode.com/2018/sponsors
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. https://adventofcode.com/2018/leaderboard
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. https://adventofcode.com/2018/stats
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. https://adventofcode.com/2018/sponsors
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. https://adventofcode.com/2018
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. https://adventofcode.com/2018/day/12/input
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