feat(2024/12): 2024 day 12

2024/day12.scala

@@ -0,0 +1,54 @@
+package `2024`.day12
+
+import prelude.*
+import scala.annotation.tailrec
+
+extension (a: Pos)(using grid: Grid[Char])
+  inline infix def ===(b: Pos) = grid.get(a) == grid.get(b)
+  inline infix def !==(b: Pos) = !(a === b)
+
+object Garden:
+  def parse(input: String) =
+    given grid: Grid[Char] = Grid(input)
+    val visited = collection.mutable.Set.empty[Pos]
+
+    def getRegion(p: Pos): Set[Pos] =
+      val region = collection.mutable.Set.empty[Pos]
+
+      @tailrec def go(xs: Seq[Pos]): Unit = xs match
+        case Nil =>
+        case x :: xs if visited.add(x) =>
+          region += x; go(x.neighbours.filter(_ === p) ++ xs)
+        case x :: xs => go(xs)
+
+      go(Seq(p)); region.toSet
+
+    Garden(grid, grid.points.map(getRegion).filter(_.nonEmpty).toSeq)
+
+case class Garden(grid: Grid[Char], regions: Seq[Set[Pos]]):
+  given Grid[Char] = grid
+
+  def perimeter(region: Set[Pos]): Int =
+    region.sumBy { p => (p !== p.up).toInt + (p !== p.left).toInt } * 2
+
+  def corners(p: Pos): Int =
+    val Seq(nw, w, sw, n, s, ne, e, se) = p.neighbours8.map(_ === p)
+
+    (Seq((n && w && !nw), (n && e && !ne), (s && w && !sw), (s && e && !se))
+      ++ Seq(!(n || w), !(n || e), !(s || w), !(s || e))).sumBy(_.toInt)
+
+def part1(input: String): Int =
+  val garden = Garden.parse(input)
+
+  garden.regions.sumBy { xs => garden.perimeter(xs) * xs.size }
+
+def part2(input: String): Int =
+  val garden = Garden.parse(input)
+
+  garden.regions.sumBy { xs => xs.sumBy(garden.corners) * xs.size }
+
+@main def main() =
+  val input = readInput(this).mkString
+
+  println(part1(input))
+  println(part2(input))