Add intersection solution to 2025 day 9 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2025/Day9.scala

@@ -31,6 +31,8 @@ object Day9 {
       _ => true
   }
 
+  trait Part2Solution extends Part
+
   // Copied & modified from 2018 day 11
   class ArrayPartialSumGrid(f: Pos => Int, box: Box) extends SumGrid {
     val Box(min, max) = box
@@ -56,7 +58,11 @@ object Day9 {
     }
   }
 
-  object Part2 extends Part {
+  /**
+   * Solution, which compresses the grid to 2-unit distances between occurring coordinates
+   * and checks validity by counting outside cells (in the compressed grid) using a [[SumGrid]].
+   */
+  object CompressGridPart2Solution extends Part2Solution {
     def makeCompressPos(redTiles: Seq[Pos]): Pos => Pos = {
       val xs = redTiles.map(_.x).distinct.sorted
       val ys = redTiles.map(_.y).distinct.sorted
@@ -105,9 +111,31 @@ object Day9 {
     }
   }
 
+  /**
+   * Solution, which checks validity geometrically using axis-aligned line-box intersection.
+   */
+  object IntersectionPart2Solution extends Part2Solution {
+    override def makeIsValid(redTiles: Seq[Pos]): Box => Boolean = {
+      val lines =
+        (redTiles lazyZip redTiles.rotateLeft(1))
+          .toSeq
+          .map((p1, p2) => Box.bounding(Seq(p1, p2))) // convert line to box of width/height 1
+
+      def isValid(box: Box): Boolean = {
+        // construct box interior, otherwise box intersects with its own edges
+        val innerMin = box.min + Pos(1, 1)
+        val innerMax = box.max - Pos(1, 1)
+        //innerMin <= innerMax &&
+        !(innerMin <= innerMax) || // box has no/empty interior // TODO: this is wrong for line-line intersections
+          !lines.exists(_.intersect(Box(innerMin, innerMax)).nonEmpty) // interior doesn't intersect any line
+      }
+
+      isValid
+    }
+  }
+
   // TODO: line of sight in all directions solution (single iteration over corners, not all pairs!)
   // TODO: glguy's solution
-  // TODO: polygon-based solution?
 
   def parseRedTile(s: String): Pos = s match {
     case s"$x,$y" => Pos(x.toInt, y.toInt)
@@ -119,6 +147,6 @@ object Day9 {
 
   def main(args: Array[String]): Unit = {
     println(Part1.largestArea(parseRedTiles(input)))
-    println(Part2.largestArea(parseRedTiles(input)))
+    println(CompressGridPart2Solution.largestArea(parseRedTiles(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2025/Day9Test.scala

@@ -1,9 +1,17 @@
 package eu.sim642.adventofcode2025
 
-import Day9._
+import Day9.*
+import Day9Test.*
+import org.scalatest.Suites
 import org.scalatest.funsuite.AnyFunSuite
 
-class Day9Test extends AnyFunSuite {
+class Day9Test extends Suites(
+  new Part1Test,
+  new CompressGridPart2SolutionTest,
+  new IntersectionPart2SolutionTest,
+)
+
+object Day9Test {
 
   val exampleInput =
     """7,1
@@ -15,19 +23,29 @@ class Day9Test extends AnyFunSuite {
       |2,3
       |7,3""".stripMargin
 
-  test("Part 1 examples") {
-    assert(Part1.largestArea(parseRedTiles(exampleInput)) == 50)
-  }
+  class Part1Test extends AnyFunSuite {
 
-  test("Part 1 input answer") {
-    assert(Part1.largestArea(parseRedTiles(input)) == 4729332959L)
-  }
+    test("Part 1 examples") {
+      assert(Part1.largestArea(parseRedTiles(exampleInput)) == 50)
+    }
 
-  test("Part 2 examples") {
-    assert(Part2.largestArea(parseRedTiles(exampleInput)) == 24)
+    test("Part 1 input answer") {
+      assert(Part1.largestArea(parseRedTiles(input)) == 4729332959L)
+    }
   }
 
-  test("Part 2 input answer") {
-    assert(Part2.largestArea(parseRedTiles(input)) == 1474477524L)
+  abstract class Part2SolutionTest(part2Solution: Part2Solution) extends AnyFunSuite {
+
+    test("Part 2 examples") {
+      assert(part2Solution.largestArea(parseRedTiles(exampleInput)) == 24)
+    }
+
+    test("Part 2 input answer") {
+      assert(part2Solution.largestArea(parseRedTiles(input)) == 1474477524L)
+    }
   }
+
+  class CompressGridPart2SolutionTest extends Part2SolutionTest(CompressGridPart2Solution)
+
+  class IntersectionPart2SolutionTest extends Part2SolutionTest(IntersectionPart2Solution)
 }