AoC Day 23: Experimental Emergency Teleportation

#adventofcode #discuss

Day 23! Two days left until Christmas! We have now officially graduated from 2D coordinates, and we've got nanobots flying around in three dimensions! These nanobots are going to help us save a reindeer friend of Santa's, somehow. But first, we'll need to figure out which nanobots can talk to which.

Good luck!

Top comments (2)

Neil Gall • Dec 23 '18

Yesterday's was tough and if I'm honest I'm getting tired. So it was nice to find a straightforward part 1 at least.

Model

Simple stuff, we've done this many times already.

data class Pos(val x: Int, val y: Int, val z: Int)

data class Nanobot(val pos: Pos, val radius: Int)

typealias Manhattan = Int

operator fun Pos.minus(other: Pos): Manhattan = 
    Math.abs(x - other.x) + Math.abs(y - other.y) + Math.abs(z - other.z)

Parser

I've used parser combinators on nearly 2 out of 3 days this year, and I'm not going to stop now! At this point I'm writing them without requiring much reference to the documentation.

fun parse(input: String): List<Nanobot> {
    val integer = or(
        string("-").next(INTEGER).map { s -> -s.toInt() },
        INTEGER.map(String::toInt)
    )

    val pos = sequence(string("pos=<").next(integer),
                       string(",").next(integer),
                       string(",").next(integer).followedBy(string(">")),
                       ::Pos)

    val nanobot = sequence(pos, string(", r=").next(integer), ::Nanobot)

    return nanobot.sepBy(WHITESPACES).parse(input.trim())
}

Part 1

Easy stuff.

fun part1(nanobots: List<Nanobot>): Int {
    val strongest = nanobots.maxBy { n -> n.radius }!!
    return nanobots.count { n -> (strongest.pos - n.pos) <= strongest.radius }
}