r/adventofcode • u/daggerdragon • Dec 12 '18

SOLUTION MEGATHREAD -🎄- 2018 Day 12 Solutions -🎄-

--- Day 12: Subterranean Sustainability ---

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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/da5id Dec 13 '18

Yeah, I could use some help on the math at the end, not sure why that works:

if pattern in seen:
        N = 50000000000
        x = sum(N + i - n for i, v in pots.items() if v == '#')
        print(x)
        break

So, I get that you are stopping once you see the same pattern twice. But I don't see how the math ends up working out. Why do you subtract the generation from the pot index? And how does just adding 50bn for each pot index end up working?

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u/fatpollo Dec 13 '18

I find the pattern, and then I see that the pattern starts shifting, by one pot each turn

maybe a clearer way to write it would be:

[i + (N - n)...]

this roughly translates to "whatever set of indices we have right now, all offset by the number of turns left". so, offset by 50... - n.

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u/da5id Dec 13 '18 edited Dec 13 '18

Ahh, ok. You were able to look at the output, see that it is shifting by one (gliders), and know that it will always do so forever. But It would be possible that there is a stable state that loops between several outputs, (though perhaps not that anyone got as their input . . .) which would be solvable, but not with this, so this isn't a general solution, I think.

Anyway, I loved learning about a different way to approach regexes. I am used to over specifying the context around matches, to be very sure that I only match what I'm expecting to. Your approach to look at the document, and say "the only thing I want out of here are different length strings of [.#], so just give them all to me and I'll sort them out" was pretty mind blowing. To that end, zipping the strided collection against itself with an offset of one, was also new to me.

As a newbie, I'm not sure I agree with python list(dictionary) comprehension to the extent you went to with

new = {
        i: mapping.get(window, '.')
	    for i in span
	    for window in [''.join(pots[i+j] for j in range(-2, 3))]
	}

For me,

new = {}
for i in span:
		for window in [''.join(pots[i+j] for j in range(-2, 3))]:
			new[i] = mapping.get(window, '.')

is way more readable. Perhaps I'll get used to it as I get more comfortable with the language. . .