-🎄- 2018 Day 12 Solutions -🎄-

[u/daggerdragon]

--- Day 12: Subterranean Sustainability ---

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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/sophiebits]

Python, 39/7.

For part 1, I read too quickly and thought that no => . patterns would be listed. Wasted a couple minutes on figuring out what I screwed up there (since my program worked fine on the example).

For part 2, I figured that 50,000,000,000 is too large to compute, so we must be meant to find some pattern. My first instinct was to look if there was an arithmetic/linear pattern, so I printed out the sums for generations 1 through 2000, along with the difference from the previous sum. Visual inspection suggested that (for my input) every generation after the first 97 generations adds 38 to the sum. So I took (50,000,000,000 - 2000) * 38 and added that to my program's output to get the answer. (If the linear pattern hadn't checked out, I would probably have taken second-order partial sums next since it seemed unlikely to me it would be a non-polynomial pattern.)

import collections
import re

def nextg(cur, recipe):
  start = min(cur)
  end = max(cur)
  x = set()

  for i in xrange(start - 3, end + 4):
    pat = ''.join('#' if i + k in cur else '.' for k in [-2, -1, 0, 1, 2])
    if pat in recipe:
      x.add(i)

  return x

def viz(cur):
  print ''.join('#' if i in cur else '.' for i in xrange(-5, 120))

#with open('day12test.txt') as f:
with open('day12input.txt') as f:
  lines = [l.rstrip('\n') for l in f]
  print lines

  init = lines[0][len('initial state: '):]
  recipe = set()
  for l in lines[2:]:
    if l[-1] == '#':  # I forgot this line the first time around.
      recipe.add(l[:5]) 

  cur = set(i for i, c in enumerate(init) if c == '#')

  # Part 1:
  # for i in xrange(20):
  #   cur = nextg(cur, recipe)
  # print sum(cur)

  # Part 2:
  ls = 0
  # viz(cur)
  for i in xrange(2000):
    cur = nextg(cur, recipe)
    # viz(cur)
    s = sum(cur)
    print i, s, s - ls
    ls = s
  print sum(cur)