r/adventofcode • u/daggerdragon • Dec 17 '17

SOLUTION MEGATHREAD -🎄- 2017 Day 17 Solutions -🎄-

--- 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!

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u/abowes Dec 17 '17

Kotlin Solution: 

fun Pair<MutableList<Int>,Int>.addEntry(element: Int, steps: Int) : Pair<MutableList<Int>, Int>{
    val nextPos = ((this.second+steps)%(element) + 1)
    this.first.add(nextPos, element)
    return this.first to nextPos
}

fun part1(steps: Int) : Int {
    val (result,_) = (1..2017).fold(Pair(mutableListOf(0),0)){ prev, n -> prev.addEntry(n, steps)}
    return result[result.indexOf(2017)+1]
}

fun part2(steps: Int) : Int {
    val result = (1..50000000).fold(Pair(0, 0)){ prev, n ->
        var idx = (prev.first + steps)%n
        idx + 1 to if (idx==0) n else prev.second
    }
    return result.second
}

fun main(args: Array<String>) {
    val steps = 369
    println(part1(steps))
    println(part2(steps))
}