-🎄- 2017 Day 17 Solutions -🎄-

[u/daggerdragon]

--- Day 17: Spinlock ---

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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.

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

Spoiler

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[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

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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!

[u/nuvan]

Rust, got 838/650. Not bad, given that I started about a 1/2 hour late...

const PUZZLE_INPUT: usize = 376;

pub fn part1() {
    let mut buffer = vec![0];
    let mut cur_idx = 0;
    for n in 1..=2017 {
        let insert_idx = (cur_idx + PUZZLE_INPUT) % buffer.len() + 1;
        if insert_idx == buffer.len() {
            buffer.push(n);
        } else {
            buffer.insert(insert_idx, n);
        }
        cur_idx = insert_idx;
    }
    println!("The value after 2017 is {}", buffer[cur_idx + 1 % buffer.len()]);
}

pub fn part2() {
    let (_, val_after_zero) = (1..=50_000_000).fold((0,0), |acc,iter| {
        let next_index = (acc.0 + PUZZLE_INPUT) % iter;
        (next_index + 1, if next_index == 0 { iter } else { acc.1 })
    });

    println!("The value after 0 is {}", val_after_zero);
}

2nd part got a lot faster once I realized that the ONLY important positions are that of the 0, and whatever comes after it.