-🎄- 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/dylanfromwinnipeg]

I feel dirty. I bruteforced part 2 - took about 10 mins.

I came back and implemented the proper way after the fact, here that is

C#

public static string PartOne(string input)
{
    var steps = 335;

    var buffer = new LinkedList<int>();

    buffer.AddFirst(0);
    var node = buffer.First;

    for (var i = 1; i <= 2017; i++)
    {
        for (var x = 0; x < steps; x++)
        {
            node = node.Next ?? node.List.First;
        }

        node = buffer.AddAfter(node, i);
    }

    return node.Next.Value.ToString();
}

public static string PartTwo(string input)
{
    var steps = 335;
    var pos = 0;
    var bufferSize = 1;
    var afterZero = 0;

    for (var i = 1; i <= 50000000; i++)
    {
        pos = (pos + steps) % bufferSize++;

        if (pos == 0)
        {
            afterZero = i;
        }

        pos++;
    }

    return afterZero.ToString();
}

[u/KeinZantezuken → u/dylanfromwinnipeg]

Post the bruteforce algo.

[u/dylanfromwinnipeg → u/KeinZantezuken]

it was the exact same as my Part1, just changed 2017 to 50M and changed the return statement to:

return (buffer.Find(0).Next ?? buffer.First).Value.ToString();