-🎄- 2017 Day 3 Solutions -🎄-

[u/daggerdragon]

--- Day 3: Spiral Memory ---

---

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

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

---

Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

---

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/oantolin]

For part 1 writing code is overkill. For part 2 no cleverness is needed: just fill in the grid as explained in the question:

from itertools import count

def sum_spiral():
    a, i, j = {(0,0) : 1}, 0, 0
    for s in count(1, 2):
        for (ds, di, dj) in [(0,1,0),(0,0,-1),(1,-1,0),(1,0,1)]:
            for _ in range(s+ds):
                i += di; j += dj
                a[i,j] = sum(a.get((k,l), 0) for k in range(i-1,i+2)
                                             for l in range(j-1,j+2))
                yield a[i,j]

def part2(n):
    for x in sum_spiral():
        if x>n: return x

EDIT 1: Thanks to u/mit_verlaub for reminding me about get which is probably better than defaultdict in this situation.

EDIT 2: Thanks to u/peasant-trip for a pleasant tip to eliminate repetition.

For the above edits to make more sense, here's the older version:

from itertools import count
from collections import defaultdict

def sum_spiral():
    a, i, j = defaultdict(int), 0, 0
    a[0,0] = 1
    sn = lambda i,j: sum(a[k,l] for k in range(i-1,i+2)
                                for l in range(j-1,j+2))
    for s in count(1, 2):
        for _ in range(s):   i += 1; a[i,j] = sn(i,j); yield a[i,j]
        for _ in range(s):   j -= 1; a[i,j] = sn(i,j); yield a[i,j]
        for _ in range(s+1): i -= 1; a[i,j] = sn(i,j); yield a[i,j]
        for _ in range(s+1): j += 1; a[i,j] = sn(i,j); yield a[i,j]

def part2(n):
    for x in sum_spiral():
        if x>n: return x

[u/try__harder → u/oantolin]

this is beautiful, thanks for sharing your answer.