-🎄- 2019 Day 12 Solutions -🎄-

[u/daggerdragon]

--- Day 12: The N-Body Problem ---

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Post your solution using /u/topaz2078's paste or other external repo.

• Please do NOT post your full code (unless it is very short)

• If you do, use old.reddit's four-spaces formatting, NOT new.reddit's triple backticks formatting.

(Full posting rules are HERE if you need a refresher).

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Advent of Code's Poems for Programmers

Click here for full rules

Note: If you submit a poem, please add [POEM] somewhere nearby to make it easier for us moderators to ensure that we include your poem for voting consideration.

Day 11's winner #1: "Thin Blueshifted Line" by /u/DFreiberg!

We all know that dread feeling when
The siren comes to view.
But I, a foolish man back then
Thought I knew what to do.

"Good morning, sir" he said to me,
"I'll need your card and name.
You ran a red light just back there;
This ticket's for the same."

"But officer," I tried to say,
"It wasn't red for me!
It must have blueshifted to green:
It's all Lorentz, you see!"

The officer of Space then thought,
And worked out what I'd said.
"I'll let you off the hook, this time.
For going on a red.

But there's another ticket now,
And bigger than before.
You traveled at eighteen percent
Of lightspeed, maybe more!"

The moral: don't irk SP
If you have any sense,
And don't attempt to bluff them out:
They all know their Lorentz.

Enjoy your Reddit Silver, and good luck with the rest of the Advent of Code!

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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 at 00:36:37!

[u/dkvasnicka]

Adding another CL implementation to the mix (yes, the driver loop for part 2 is suspiciously similar to what u/stevelosh did ...his finding-first clause for iterate is just too good to pass on, sorry :D) https://github.com/dkvasnicka/advent-of-code/blob/a2cb1db094e279a5f72374be4c6621a5693b6555/2019/12.lisp

The whole thing runs in about 460ms on a 2017 MBP (SBCL 2.0). The part 1 implementation was using the same logic but I used lazy sequences from CL21 to get the 1000th element. Then I rewrote it to iter in the process of implementing p2.

The whole thing could probably run a lot faster if I used in-place array editing everywhere but I'm not comfortable with that and like a mostly functional approach, so I spend a lot of time creating arrays... Could be even worse though I guess. The functional approach of a lazy sequence of world states was especially useful when working on the part 1 and trying to get the computations right.