Year 2019 Day 12

Author: maneatingape

README.md

@@ -79,3 +79,4 @@ Entries for the annual [Advent of Code](https://adventofcode.com/) challenge, wr
 | 9 | [Sensor Boost](https://adventofcode.com/2019/day/9) | [Source](src/main/scala/AdventOfCode2019/Day09.scala) |
 | 10 | [Monitoring Station](https://adventofcode.com/2019/day/10) | [Source](src/main/scala/AdventOfCode2019/Day10.scala) |
 | 11 | [Space Police](https://adventofcode.com/2019/day/11) | [Source](src/main/scala/AdventOfCode2019/Day11.scala) |
+| 12 | [The N-Body Problem](https://adventofcode.com/2019/day/12) | [Source](src/main/scala/AdventOfCode2019/Day12.scala) |

src/main/resources/AdventOfCode2019/Day12.txt

@@ -0,0 +1,4 @@
+<x=17, y=-12, z=13>
+<x=2, y=1, z=1>
+<x=-1, y=-17, z=7>
+<x=12, y=-14, z=18>

src/main/scala/AdventOfCode2019/Day12.scala

@@ -0,0 +1,47 @@
+package AdventOfCode2019
+
+import scala.annotation.tailrec
+
+object Day12:
+  case class Point(x: Int, y: Int, z: Int):
+    def +(other: Point): Point = Point(x + other.x, y + other.y, z + other.z)
+    def -(other: Point): Point = Point(x - other.x, y - other.y, z - other.z)
+    def sign: Point = Point(x.sign, y.sign, z.sign)
+    def manhattan: Int = x.abs + y.abs + z.abs
+
+  case class Moon(position: Point, velocity: Point):
+    def energy: Int = position.manhattan * velocity.manhattan
+
+  def parse(input: Seq[String]): Seq[Moon] =
+    val regex = """<x=(-?\d+), y=(-?\d+), z=(-?\d+)>""".r
+    input.map { case regex(x, y, z) => Moon(Point(x.toInt, y.toInt, z.toInt), Point(0, 0, 0)) }
+
+  def step(moons: Seq[Moon]): Seq[Moon] = moons.map { moon =>
+    val velocity = moons.foldLeft(moon.velocity)((total, next) => total + (next.position - moon.position).sign)
+    Moon(moon.position + velocity, velocity)
+  }
+
+  def gcd(a: Long, b: Long): Long = if a % b == 0 then b else gcd(b, a % b)
+
+  def lcm(a: Long, b: Long): Long = a * b / gcd(a, b)
+
+  def part1(input: Seq[String]): Int = Iterator.iterate(parse(input))(step).drop(1000).next().map(_.energy).sum
+
+  def part2(input: Seq[String]): Long =
+    @tailrec
+    def halfPeriod(moons: Seq[Moon], dimension: Point => Int, count: Int = 1): Long =
+      val next = step(moons)
+      if next.map(_.velocity).map(dimension).forall(_ == 0) then count else halfPeriod(next, dimension, count + 1)
+
+    val initial = parse(input)
+    val halfPeriodX = halfPeriod(initial, (p: Point) => p.x)
+    val halfPeriodY = halfPeriod(initial, (p: Point) => p.y)
+    val halfPeriodZ = halfPeriod(initial, (p: Point) => p.z)
+
+    2 * lcm(lcm(halfPeriodX, halfPeriodY), halfPeriodZ)
+  end part2
+
+  def main(args: Array[String]): Unit =
+    val data = io.Source.fromResource("AdventOfCode2019/Day12.txt").getLines().toSeq
+    println(part1(data))
+    println(part2(data))

src/test/scala/AdventOfCode2019/Day12Suite.scala

@@ -0,0 +1,18 @@
+package AdventOfCode2019
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day12Suite extends AnyFunSuite:
+  val sample = Seq(
+    "<x=-1, y=0, z=2>",
+    "<x=2, y=-10, z=-7>",
+    "<x=4, y=-8, z=8>",
+    "<x=3, y=5, z=-1>")
+
+  test("Part 1 should handle sample input correctly") {
+    assert(Day12.part1(sample) == 183)
+  }
+
+  test("Part 2 should handle sample input correctly") {
+    assert(Day12.part2(sample) == 2772)
+  }