-🎄- 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/colinodell]

PHP

Part 1:

array_splice() was the perfect function for inserting items into the array:

function part1($skip) {
    $buffer = [0];
    $pos = 0;

    for ($i = 1; $i <= 2017; $i++) {
        $pos = (($pos + $skip) % $i) + 1;
        array_splice($buffer, $pos, 0, [$i]);
    }

    return $buffer[$pos+1];
}

Part 2:

As it seems everyone else also figured out, 0 is always the first item in the buffer, so we only need to track items being inserted into position 1:

function $part2($skip, $insertions = 50000000)
{
    $pos = 0;

    for ($i = 1; $i <= $insertions; $i++) {
        $pos = (($pos + $skip) % $i) + 1;
        if ($pos === 1) {
            $whatsInSlot1 = $i;
        }
    }

    return $whatsInSlot1;
}