Solve 2023 day 3 part 2

src/main/scala/eu/sim642/adventofcode2023/Day3.scala

@@ -10,6 +10,8 @@ object Day3 {
 
   def isSymbol(c: Char): Boolean = c != '.' && !c.isDigit
 
+  def isGear(c: Char): Boolean = c == '*'
+
   private val numberRegex = """\d+""".r
 
   def iteratePartNumbers(schematic: Schematic): Iterator[Int] = {
@@ -25,12 +27,31 @@ object Day3 {
 
   def sumPartNumbers(schematic: Schematic): Int = iteratePartNumbers(schematic).sum
 
+  def sumGearRatios(schematic: Schematic): Int = {
+    val schematicBox = Box(Pos.zero, Pos(schematic(0).length - 1, schematic.length - 1))
+
+    val gears = for {
+      (row, y) <- schematic.iterator.zipWithIndex
+      m <- numberRegex.findAllMatchIn(row)
+      box = Box(Pos(m.start, y) - Pos(1, 1), Pos(m.end - 1, y) + Pos(1, 1))
+      gearPos <- box.iterator
+      if schematicBox.contains(gearPos) && isGear(schematic(gearPos.y)(gearPos.x))
+    } yield gearPos -> m.toString().toInt
+
+    gears.toSeq
+      .groupMap(_._1)(_._2) // TODO: groupMap to IteratorImplicits
+      .filter(_._2.size == 2)
+      .map(_._2.product)
+      .sum
+  }
+
 
   def parseSchematic(input: String): Schematic = input.linesIterator.toVector
 
   lazy val input: String = io.Source.fromInputStream(getClass.getResourceAsStream("day3.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
     println(sumPartNumbers(parseSchematic(input)))
+    println(sumGearRatios(parseSchematic(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2023/Day3Test.scala

@@ -24,4 +24,12 @@ class Day3Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(sumPartNumbers(parseSchematic(input)) == 521515)
   }
+
+  test("Part 2 examples") {
+    assert(sumGearRatios(parseSchematic(exampleInput)) == 467835)
+  }
+
+  test("Part 2 input answer") {
+    assert(sumGearRatios(parseSchematic(input)) == 69527306)
+  }
 }