Document 2025 day 9 part 2 solution assumptions

Author: sim642

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

@@ -61,6 +61,8 @@ object Day9 {
   /**
    * 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]].
+   *
+   * Grid compression assumes no adjacent coordinates occur.
    */
   object CompressGridPart2Solution extends Part2Solution {
     def makeCompressPos(redTiles: Seq[Pos]): Pos => Pos = {
@@ -113,6 +115,9 @@ object Day9 {
 
   /**
    * Solution, which checks validity geometrically using axis-aligned line-box intersection.
+   *
+   * Assumes largest area won't be outside.
+   * Assumes no adjacent coordinates occur.
    */
   object IntersectionPart2Solution extends Part2Solution {
     override def makeIsValid(redTiles: Seq[Pos]): Box => Boolean = {

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

@@ -23,29 +23,74 @@ object Day9Test {
       |2,3
       |7,3""".stripMargin
 
+  val largeUInput =
+    """1,1
+      |3,1
+      |3,1000
+      |1000,1000
+      |1000,1
+      |1002,1
+      |1002,1002
+      |1,1002""".stripMargin
+
+  val adjacentCoordinateInput = // from Timvde on Libera IRC
+    """1,1
+      |3,1
+      |3,6
+      |4,6
+      |4,1
+      |6,1
+      |6,10
+      |1,10""".stripMargin
+
   class Part1Test extends AnyFunSuite {
 
     test("Part 1 examples") {
       assert(Part1.largestArea(parseRedTiles(exampleInput)) == 50)
     }
 
+    test("Part 1 large U") {
+      assert(Part1.largestArea(parseRedTiles(largeUInput)) == 1004004)
+    }
+
+    test("Part 1 adjacent coordinate") {
+      assert(Part1.largestArea(parseRedTiles(adjacentCoordinateInput)) == 60)
+    }
+
     test("Part 1 input answer") {
       assert(Part1.largestArea(parseRedTiles(input)) == 4729332959L)
     }
   }
 
-  abstract class Part2SolutionTest(part2Solution: Part2Solution) extends AnyFunSuite {
+  abstract class Part2SolutionTest(val part2Solution: Part2Solution) extends AnyFunSuite {
 
     test("Part 2 examples") {
       assert(part2Solution.largestArea(parseRedTiles(exampleInput)) == 24)
     }
 
+    ignore("Part 2 adjacent coordinate") {
+      // should not be 30
+      assert(part2Solution.largestArea(parseRedTiles(adjacentCoordinateInput)) == 60)
+    }
+
     test("Part 2 input answer") {
       assert(part2Solution.largestArea(parseRedTiles(input)) == 1474477524L)
     }
   }
 
-  class CompressGridPart2SolutionTest extends Part2SolutionTest(CompressGridPart2Solution)
+  class CompressGridPart2SolutionTest extends Part2SolutionTest(CompressGridPart2Solution) {
+
+    test("Part 2 large U") {
+      // should not be 998000, which would be the completely outside box in the large U
+      assert(part2Solution.largestArea(parseRedTiles(largeUInput)) == 3006)
+    }
+  }
 
-  class IntersectionPart2SolutionTest extends Part2SolutionTest(IntersectionPart2Solution)
+  class IntersectionPart2SolutionTest extends Part2SolutionTest(IntersectionPart2Solution) {
+
+    ignore("Part 2 large U") { // fails due to assumption
+      // should not be 998000, which would be the completely outside box in the large U
+      assert(part2Solution.largestArea(parseRedTiles(largeUInput)) == 3006)
+    }
+  }
 }