AoC Day 19

Advent19.scala

package advent

class Advent19 extends Advent {
  type Grid = Array[Array[Char]]
  case class Position(x: Int, y: Int)
  sealed trait Move {
    def apply(position: Position): Position
  }
  case object Up extends Move {
    def apply(p: Position): Position = p.copy(y = p.y - 1)
  }
  case object Down extends Move {
    def apply(p: Position): Position = p.copy(y = p.y + 1)
  }
  case object Left extends Move {
    def apply(p: Position): Position = p.copy(x = p.x - 1)
  }
  case object Right extends Move {
    def apply(p: Position): Position = p.copy(x = p.x + 1)
  }
  def charAt(position: Position, grid: Grid) = grid.lift(position.y).flatMap(_.lift(position.x)).getOrElse(' ')
  case class State(move: Move, position: Position) {
    def apply(grid: Grid): State = {
      val c = charAt(position, grid)
      if (c == '+') {
        move match {
          case Down | Up =>
            if (charAt(Left(position), grid) != ' ') {
              copy(Left, Left(position))
            } else {
              copy(Right, Right(position))
            }
          case Right | Left =>
            if (charAt(Up(position), grid) != ' ') {
              copy(Up, Up(position))
            } else {
              copy(Down, Down(position))
            }
          case _ => this
        }
      } else if (c == ' ') this
      else { copy(position = move(position)) }
    }
  }
  object State {
    def apply(g: Grid) = new State(Down, Position(g(0).indexOf('|'), 0))
  }
  def stream(g: Grid): Stream[State] = {
    def inner(s: State): Stream[State] = {
      val next = s(g)
      if (next == s) Stream.empty else s #:: inner(next)
    }
    inner(State(g))
  }
  def parse(s: String): Grid = {
    s.splitOnNewline.map(_.toCharArray)
  }
  def part1(s: String) = {
    val grid = parse(s)
    stream(grid).map(s => charAt(s.position, grid)).filter(_.isLetter).mkString
  }
  def part2(s: String) = {
    val grid = parse(s)
    stream(grid).length
  }
  val sample =
    "     |          \n     |  +--+    \n     A  |  C    \n F---|----E|--+ \n     |  |  |  D \n     +B-+  +--+ \n"
  "part1" should "work" in {
    part1(sample) should be("ABCDEF")
    part1(Inputs.day19) should be("BPDKCZWHGT")
  }
  "part2" should "work" in {
    part2(sample) should be(38)
    part2(Inputs.day19) should be(17728)
  }

}