Day09Test.scala

package org.lemon.advent.year2022

import scala.collection.mutable
import org.lemon.advent._

class Day09Test extends UnitTest {

  type Coord = (Int, Int)
  extension (coord: Coord)
    def x = coord._1
    def y = coord._2

  type Rope = Seq[Coord]

  def drag(head: Coord, tail: Coord): Coord =
    (head, tail) match
      case ((hx, hy), (tx, ty)) if (hx - tx).abs == 2 && hy == ty => ((hx + tx) / 2, ty)
      case ((hx, hy), (tx, ty)) if hx == tx && (hy - ty).abs == 2 => (tx, (hy + ty) / 2)
      case ((hx, hy), (tx, ty)) if (hx - tx).abs == 1 && (hy - ty).abs == 2 => (hx, (hy + ty) / 2)
      case ((hx, hy), (tx, ty)) if (hx - tx).abs == 2 && (hy - ty).abs == 1 => ((hx + tx) / 2, hy)
      case ((hx, hy), (tx, ty)) if (hx - tx).abs == 2 && (hy - ty).abs == 2 => ((hx + tx) / 2, (hy + ty) / 2)
      case _ => tail

  def update(rope: Rope, step: (Coord => Coord), n: Int, tailPos: mutable.Map[Coord, Int]): Rope =
    val newRope = mutable.Seq(rope: _*)
    for _ <- 1 to n do
      newRope(0) = step(newRope(0))
      for i <- 1 to rope.size - 1 do newRope(i) = drag(newRope(i - 1), newRope(i))
      tailPos(newRope.last) += 1
    newRope.toSeq

  def move(move: String, rope: Rope, tailPos: mutable.Map[Coord, Int]) = move match
    case s"L $n" => update(rope, h => (h.x - 1, h.y), n.toInt, tailPos)
    case s"R $n" => update(rope, h => (h.x + 1, h.y), n.toInt, tailPos)
    case s"U $n" => update(rope, h => (h.x, h.y + 1), n.toInt, tailPos)
    case s"D $n" => update(rope, h => (h.x, h.y - 1), n.toInt, tailPos)

  def trackTail(in: String, numberOfKnots: Int) =
    val tailPos = mutable.Map.empty[Coord, Int].withDefaultValue(0)
    var rope = Seq.fill(numberOfKnots)((0, 0))
    in.linesIterator.foldLeft(rope)((r, m) => move(m, r, tailPos))
    tailPos

  def part1(in: String) = trackTail(in, 2).size

  def part2(in: String) = trackTail(in, 10).size

  test("part 1 example") {
    val in = """|R 4
                |U 4
                |L 3
                |D 1
                |R 4
                |D 1
                |L 5
                |R 2""".stripMargin

    part1(in) shouldBe 13
  }

  test("part 1") {
    part1(read(file(2022)(9))) shouldBe 6503
  }

  test("part 2 example 1") {
    val in = """|R 4
                |U 4
                |L 3
                |D 1
                |R 4
                |D 1
                |L 5
                |R 2""".stripMargin

    part2(in) shouldBe 1
  }

  test("part 2 example 2") {
    val in = """|R 5
                |U 8
                |L 8
                |D 3
                |R 17
                |D 10
                |L 25
                |U 20""".stripMargin

    part2(in) shouldBe 36
  }

  test("part 2") {
    part2(read(file(2022)(9))) shouldBe 2724
  }
}