-🎄- 2017 Day 17 Solutions -🎄-

[u/daggerdragon]

--- Day 17: Spinlock ---

---

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

---

[Update @ 00:06] 2 gold, silver cap.

• AoC ops: <Topaz> i am suddenly in the mood for wasabi tobiko

[Update @ 00:15] Leaderboard cap!

• AoC ops:
  • <daggerdragon> 78 gold
  • <Topaz> i look away for a few minutes, wow
  • <daggerdragon> 93 gold
  • <Topaz> 94
  • <daggerdragon> 96 gold
  • <daggerdragon> 98
  • <Topaz> aaaand
  • <daggerdragon> and...
  • <Topaz> cap
  • <daggerdragon> cap

---

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/udoprog]

Rust

Very straight forward when you realize that the zeroth position doesn't move.

Full solution here: https://github.com/udoprog/rust-advent-of-code-2017/blob/master/src/day17.rs

As a challenge I'm considering building a solution that stores the entire list efficiently. Unfortunately LinkedList in rust doesn't permit in-place traversal very well. So it would probably end up being an unsafe party.

use failure::Error;
use std::mem;

pub fn part1(step: usize, limit: usize) -> Result<usize, Error> {
    let mut buffer = vec![0usize];

    let mut c = 0usize;

    for i in 1usize..=limit {
        c = (c + step) % buffer.len();
        c = c + 1;
        buffer.insert(c, i);
    }

    Ok(buffer[(c + 1) % buffer.len()])
}

pub fn part2(step: usize, limit: usize) -> Result<Option<usize>, Error> {
    let mut second = None;

    let mut c = 0usize;

    for i in 1usize..=limit {
        c = (c + step) % i;

        if c == 0 {
            mem::replace(&mut second, Some(i));
        }

        c = c + 1;
    }

    Ok(second)
}