Advent of Code

--- Day 18: Like a GIF For Your Yard ---

   After the million lights incident, the fire code has gotten stricter: now,
   at most ten thousand lights are allowed. You arrange them in a 100x100 grid.

   Never one to let you down, Santa again mails you instructions on the ideal
   lighting configuration. With so few lights, he says, you'll have to resort
   to animation.

   Start by setting your lights to the included initial configuration (your
   puzzle input). A # means "on", and a . means "off".

   Then, animate your grid in steps, where each step decides the next
   configuration based on the current one. Each light's next state (either on
   or off) depends on its current state and the current states of the eight
   lights adjacent to it (including diagonals). Lights on the edge of the grid
   might have fewer than eight neighbors; the missing ones always count as
   "off".

   For example, in a simplified 6x6 grid, the light marked A has the neighbors
   numbered 1 through 8, and the light marked B, which is on an edge, only has
   the neighbors marked 1 through 5:

 1B5...
 234...
 ......
 ..123.
 ..8A4.
 ..765.

   The state a light should have next is based on its current state (on or off)
   plus the number of neighbors that are on:

     • A light which is on stays on when 2 or 3 neighbors are on, and turns off
       otherwise.

     • A light which is off turns on if exactly 3 neighbors are on, and stays
       off otherwise.

   All of the lights update simultaneously; they all consider the same current
   state before moving to the next.

   Here's a few steps from an example configuration of another 6x6 grid:

 Initial state:
 .#.#.#
 ...##.
 #....#
 ..#...
 #.#..#
 ####..

 After 1 step:
 ..##..
 ..##.#
 ...##.
 ......
 #.....
 #.##..

 After 2 steps:
 ..###.
 ......
 ..###.
 ......
 .#....
 .#....

 After 3 steps:
 ...#..
 ......
 ...#..
 ..##..
 ......
 ......

 After 4 steps:
 ......
 ......
 ..##..
 ..##..
 ......
 ......

   After 4 steps, this example has four lights on.

   In your grid of 100x100 lights, given your initial configuration, how many
   lights are on after 100 steps?

   Your puzzle answer was 814.

--- Part Two ---

   You flip the instructions over; Santa goes on to point out that this is all
   just an implementation of Conway's Game of Life. At least, it was, until you
   notice that something's wrong with the grid of lights you bought: four
   lights, one in each corner, are stuck on and can't be turned off. The
   example above will actually run like this:

 Initial state:
 ##.#.#
 ...##.
 #....#
 ..#...
 #.#..#
 ####.#

 After 1 step:
 #.##.#
 ####.#
 ...##.
 ......
 #...#.
 #.####

 After 2 steps:
 #..#.#
 #....#
 .#.##.
 ...##.
 .#..##
 ##.###

 After 3 steps:
 #...##
 ####.#
 ..##.#
 ......
 ##....
 ####.#

 After 4 steps:
 #.####
 #....#
 ...#..
 .##...
 #.....
 #.#..#

 After 5 steps:
 ##.###
 .##..#
 .##...
 .##...
 #.#...
 ##...#

   After 5 steps, this example now has 17 lights on.

   In your grid of 100x100 lights, given your initial configuration, but with
   the four corners always in the on state, how many lights are on after 100
   steps?

   Your puzzle answer was 924.

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

   At this point, you should return to your advent calendar and try another
   puzzle.

   If you still want to see it, you can get your puzzle input.

References

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   . http://adventofcode.com/day/6
   . https://en.wikipedia.org/wiki/Conway's_Game_of_Life
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   . http://adventofcode.com/day/18/input
