Day 06

src/day06.scala

@@ -0,0 +1,72 @@
+package aoc
+package day06
+
+import scala.annotation.tailrec
+
+val dayNumber = "06"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  val (gridMap, guardPos) = parseInput(input)
+  walk(guardPos, Dir.Up, Set())(using gridMap).size.toString
+
+def part2(input: String): String =
+  val (gridMap, guardPos) = parseInput(input)
+  (walk(guardPos, Dir.Up, Set())(using gridMap) - guardPos)
+    .count((x, y) =>
+      isLoop(guardPos, Dir.Up, Set())(using gridMap + ((x -> y, '#')))
+    )
+    .toString
+
+def parseInput(input: String): (Map[(Int, Int), Char], (Int, Int)) =
+  val grid = input.split("\n")
+  val coords =
+    for
+      y <- 0 until grid.length
+      x <- 0 until grid.head.length
+    yield (x, y) -> grid(y)(x)
+  val gridMap = coords.toMap
+  val guardPos = coords.collectFirst { case (x, y) -> '^' => (x, y) }.get
+  (gridMap, guardPos)
+
+enum Dir:
+  case Up, Right, Down, Left
+  def next = this match
+    case Up    => Right
+    case Right => Down
+    case Down  => Left
+    case Left  => Up
+  def move(x: Int, y: Int) = this match
+    case Up    => (x, y - 1)
+    case Right => (x + 1, y)
+    case Down  => (x, y + 1)
+    case Left  => (x - 1, y)
+
+@tailrec def walk(pos: (Int, Int), dir: Dir, visited: Set[(Int, Int)])(using
+    gridMap: Map[(Int, Int), Char]
+): Set[(Int, Int)] =
+  val nextPos = dir.move.tupled(pos)
+  gridMap.get(nextPos) match
+    case Some('.' | '^') =>
+      walk(nextPos, dir, visited + pos)
+    case Some('#') => walk(pos, dir.next, visited)
+    case Some(_)   => ???
+    case None      => visited + pos
+
+@tailrec def isLoop(pos: (Int, Int), dir: Dir, visited: Set[(Int, Int, Dir)])(
+    using gridMap: Map[(Int, Int), Char]
+): Boolean =
+  val nextPos = dir.move.tupled(pos)
+  if visited.contains(nextPos :* dir) then true
+  else
+    gridMap.get(nextPos) match
+      case Some('.' | '^') =>
+        isLoop(nextPos, dir, visited + (pos :* dir))
+      case Some('#') => isLoop(pos, dir.next, visited)
+      case Some(_)   => ???
+      case None      => false