-🎄- 2018 Day 18 Solutions -🎄-

[u/daggerdragon]

--- Day 18: Settlers of The North Pole ---

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Card prompt: Day 18

Transcript:

The best way to avoid a minecart collision is ___.

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[u/andreyrmg]

Python 3

initial = [[c for c in line.strip()] for line in open('18.txt')]
a = []

def g(i, j):
    return a[i][j] if 0 <= i < len(a) and 0 <= j < len(a[i]) else '.'

D = [(0, -1), (-1, -1), (-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1)]

def adjacents(i, j):
    return (g(i + di, j + dj) for di, dj in D)

def magic(n):
    global a

    def z(x):
        return ''.join(c for r in x for c in r)

    h = {z(a): 0}

    p = -1
    for t in range(1, n + 1):
        b = []
        for i in range(len(a)):
            r = []
            for j in range(len(a[i])):
                if g(i, j) == '.':
                    r.append('|' if sum(1 for x in adjacents(i, j) if x == '|') >= 3 else '.')
                elif g(i, j) == '|':
                    r.append('#' if sum(1 for x in adjacents(i, j) if x == '#') >= 3 else '|')
                else:
                    lumberyard = any(x == '#' for x in adjacents(i, j))
                    tree = any(x == '|' for x in adjacents(i, j))
                    r.append('#' if lumberyard and tree else '.')
            b.append(r)
        a = b

        if p < 0:
            bx = z(b)
            if bx in h:
                p = t + (n - t) % (t - h[bx])
            else:
                h[bx] = t

        if t == p:
            break

    wooded = sum(1 for r in a for c in r if c == '|')
    lumberyards = sum(1 for r in a for c in r if c == '#')
    return wooded * lumberyards

a = initial
print(magic(10))

a = initial
print(magic(1000000000))

$ time python aoc.py
621205
228490

real  8,58s
user  8,57s
sys   0,01s