181 lines
4.4 KiB
Plaintext
181 lines
4.4 KiB
Plaintext
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Advent of Code
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br0xen 40*
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• [About]
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• [Stats]
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• [Leaderboard]
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• [Settings]
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• [Log out]
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--- Day 18: Like a GIF For Your Yard ---
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After the million lights incident, the fire code has gotten stricter: now, at most ten thousand
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lights are allowed. You arrange them in a 100x100 grid.
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Never one to let you down, Santa again mails you instructions on the ideal lighting
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configuration. With so few lights, he says, you'll have to resort to animation.
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Start by setting your lights to the included initial configuration (your puzzle input). A #
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means "on", and a . means "off".
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Then, animate your grid in steps, where each step decides the next configuration based on the
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current one. Each light's next state (either on or off) depends on its current state and the
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current states of the eight lights adjacent to it (including diagonals). Lights on the edge of
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the grid might have fewer than eight neighbors; the missing ones always count as "off".
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For example, in a simplified 6x6 grid, the light marked A has the neighbors numbered 1 through
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8, and the light marked B, which is on an edge, only has the neighbors marked 1 through 5:
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1B5...
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234...
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......
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..123.
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..8A4.
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..765.
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The state a light should have next is based on its current state (on or off) plus the number of
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neighbors that are on:
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• A light which is on stays on when 2 or 3 neighbors are on, and turns off otherwise.
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• A light which is off turns on if exactly 3 neighbors are on, and stays off otherwise.
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All of the lights update simultaneously; they all consider the same current state before moving
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to the next.
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Here's a few steps from an example configuration of another 6x6 grid:
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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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After 1 step:
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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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After 2 steps:
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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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After 3 steps:
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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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After 4 steps:
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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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After 4 steps, this example has four lights on.
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In your grid of 100x100 lights, given your initial configuration, how many lights are on after
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100 steps?
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Your puzzle answer was 814.
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--- Part Two ---
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You flip the instructions over; Santa goes on to point out that this is all just an
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implementation of Conway's Game of Life. At least, it was, until you notice that something's
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wrong with the grid of lights you bought: four lights, one in each corner, are stuck on and
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can't be turned off. The example above will actually run like this:
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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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After 1 step:
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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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After 2 steps:
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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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After 3 steps:
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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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After 4 steps:
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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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After 5 steps:
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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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After 5 steps, this example now has 17 lights on.
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In your grid of 100x100 lights, given your initial configuration, but with the four corners
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always in the on state, how many lights are on after 100 steps?
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Your puzzle answer was 924.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your advent calendar and try another puzzle.
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If you still want to see it, you can get your puzzle input.
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You can also [Shareon Twitter Google+ Reddit] this puzzle.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/about
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. http://adventofcode.com/stats
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. http://adventofcode.com/leaderboard
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. http://adventofcode.com/settings
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. http://adventofcode.com/auth/logout
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. http://adventofcode.com/day/6
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. https://en.wikipedia.org/wiki/Conway's_Game_of_Life
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. http://adventofcode.com/
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. http://adventofcode.com/day/18/input
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. https://twitter.com/intent/tweet?text=I%27ve+completed+%22Like+a+GIF+For+Your+Yard%22+%2D+Day+18+%2D+Advent+of+Code&url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F18&related=ericwastl&hashtags=AdventOfCode
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. https://plus.google.com/share?url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F18
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. http://www.reddit.com/submit?url=http%3A%2F%2Fadventofcode%2Ecom%2Fday%2F18&title=I%27ve+completed+%22Like+a+GIF+For+Your+Yard%22+%2D+Day+18+%2D+Advent+of+Code
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