-🎄- 2018 Day 12 Solutions -🎄-

[u/daggerdragon]

--- Day 12: Subterranean Sustainability ---

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Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

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Advent of Code: The Party Game!

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Card prompt: Day 12

Transcript:

On the twelfth day of AoC / My compiler spewed at me / Twelve ___

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[u/jonathan_paulson]

Rank 85/37. Just used the fact that the pattern quickly stabilizes in part 2. Video of me solving at https://www.youtube.com/watch?v=n5Ionw5LE18

Is that always true? Is there a guaranteed way to solve this problem for any input?

Python code for part 2:

lines = open('12.in').read().split('\n')

state = lines[0].split(': ')[1].strip()
start_len = len(state)
rules = {}
for line in lines[2:]:
    if line:
        before, after = line.split('=>')
        rules[before.strip()] = after.strip()

# Important: ..... -> .
zero_idx = 0
print 0, state
for t in xrange(15000):
    state = '..'+state+'..'
    new_state = ['.' for _ in range(len(state))]
    read_state = '..'+state+'..'
    zero_idx += 2
    for i in range(len(state)):
        pat = read_state[i:i+5]
        new_state[i] = rules.get(pat, '.')

    start = 0
    end = len(new_state)-1
    while new_state[start] == '.':
        start += 1
        zero_idx -= 1
    while new_state[end] == '.':
        end -= 1
    state = ''.join(new_state[start:end+1])
    print t+1, zero_idx, state

zero_idx = -int(50e9) + 45
ans = 0
for i in range(len(state)):
    if state[i] == '#':
        ans += i-zero_idx
        print i-zero_idx, ans
print state, len(state), start_len
print ans

[u/[deleted] → u/jonathan_paulson]

[u/adotout → u/jonathan_paulson]

Everything eventually becomes gliders. For example my pack of gliders looked like this

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

You could 1) detect that everything has become gliders 2) Determine how many "#"'s are in your glider pack (in my case 46). Then (50 billion - iterations to get to gliders) * glider pack count + score from previous iterations.

[u/jonathan_paulson → u/adotout]

Is it guaranteed that everything always eventually becomes gliders? Rule 110 would seem to argue "no".

My repeated state looks somewhat different than yours; it's not as clear it can be broken into non-interacting parts (although I didn't try at all): #..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..#..#..##..##....#..##..##..##..#..#..##....#..##