Day 1 part 2

src/main/scala/Day01.scala

@@ -8,8 +8,13 @@ object Day01:
     given Ordering[LocationId] = Ordering.by(_.value)
 
   case class Input(left: List[LocationId], right: List[LocationId]):
+
     def totalDistance: Long = left.sorted.zip(right.sorted).map((l, r) => Math.abs(l.value - r.value).toLong).sum
 
+    def similarityScore: Long =
+      val rightCount = right.groupBy(identity).map((k, vs) => (k, vs.length))
+      left.map(l => (l.value * rightCount.getOrElse(l, 0)).toLong).sum
+
   object Input:
     def parse(inputs: List[String]): Option[Input] =
       inputs.traverse {
@@ -18,9 +23,11 @@ object Day01:
           case _           => None
       }.map { lrs =>
         Input(
-          left = lrs.map((l, _) => LocationId(l)),
-          right = lrs.map((_, r) => LocationId(r))
+          left = lrs.map(_._1).map(LocationId.apply),
+          right = lrs.map(_._2).map(LocationId.apply)
         )
       }
 
   def totalDistance(inputs: List[String]): Option[Long] = Input.parse(inputs).map(_.totalDistance)
+
+  def similarityScore(inputs: List[String]): Option[Long] = Input.parse(inputs).map(_.similarityScore)

src/test/scala/Day01Suite.scala

@@ -9,8 +9,8 @@ class Day01Suite extends ScalaCheckSuite:
       Input.parse(smallInput),
       Some(
         Input(
-          left = List(3, 4, 2, 1, 3, 3).map(LocationId(_)),
-          right = List(4, 3, 5, 3, 9, 3).map(LocationId(_))
+          left = List(3, 4, 2, 1, 3, 3).map(LocationId.apply),
+          right = List(4, 3, 5, 3, 9, 3).map(LocationId.apply)
         )
       )
     )
@@ -21,6 +21,12 @@ class Day01Suite extends ScalaCheckSuite:
   test("total distance for big input is 1_223_326"):
     assertEquals(totalDistance(bigInput), Some(1_223_326L))
 
+  test("similarity score for small input is 31"):
+    assertEquals(similarityScore(smallInput), Some(31L))
+
+  test("similarity score for big input is 21_070_419"):
+    assertEquals(similarityScore(bigInput), Some(21_070_419L))
+
 object Day01Suite:
 
   val bigInput: List[String] = getLinesFromFile("src/test/scala/day01_input.txt")