Solve 2025 day 1 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2025/Day1.scala

@@ -4,10 +4,28 @@ import eu.sim642.adventofcodelib.IntegralImplicits._
 
 object Day1 {
 
-  def actualPassword(rotations: Seq[Int]): Int = {
-    rotations
-      .scanLeft[Int](50)((a, b) => (a + b) %+ 100) // TODO: why can't use implicit arguments?
-      .count(_ == 0)
+  trait Part {
+    def password(rotations: Seq[Int]): Int
+  }
+
+  object Part1 extends Part {
+    override def password(rotations: Seq[Int]): Int = {
+      rotations
+        .scanLeft[Int](50)((a, b) => (a + b) %+ 100) // TODO: why can't use implicit arguments?
+        .count(_ == 0)
+    }
+  }
+
+  object Part2 extends Part {
+    override def password(rotations: Seq[Int]): Int = {
+      rotations
+        .flatMap({ // expand all rotations to single to make each tick observable, this is silly but works
+          case i if i >= 0 => Seq.fill(i)(1)
+          case i if i < 0 => Seq.fill(-i)(-1)
+        })
+        .scanLeft[Int](50)((a, b) => (a + b) %+ 100) // TODO: why can't use implicit arguments?
+        .count(_ == 0)
+    }
   }
 
   def parseRotation(s: String): Int = s match {
@@ -20,6 +38,7 @@ object Day1 {
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day1.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
-    println(actualPassword(parseRotations(input)))
+    println(Part1.password(parseRotations(input)))
+    println(Part2.password(parseRotations(input)))
   }
 }

src/test/scala/eu/sim642/adventofcode2025/Day1Test.scala

@@ -17,11 +17,20 @@ class Day1Test extends AnyFunSuite {
       |R14
       |L82""".stripMargin
 
-  test("Part 1 example") {
-    assert(actualPassword(parseRotations(exampleInput)) == 3)
+  test("Part 1 examples") {
+    assert(Part1.password(parseRotations(exampleInput)) == 3)
   }
 
   test("Part 1 input answer") {
-    assert(actualPassword(parseRotations(input)) == 995)
+    assert(Part1.password(parseRotations(input)) == 995)
+  }
+
+  test("Part 2 examples") {
+    assert(Part2.password(parseRotations(exampleInput)) == 6)
+    assert(Part2.password(parseRotations("R1000")) == 10)
+  }
+
+  test("Part 2 input answer") {
+    assert(Part2.password(parseRotations(input)) == 5847)
   }
 }