[day10] solution. a recursive one, and still fast, take that reddit :P

src/main/scala/Day10.scala

+package aoc2020
+import aoc2020.lib._
+
+def day10(ratings: List[Int]): String =
+  val adapters = (0 :: (ratings.max + 3) :: ratings).sorted
+  val joltages = adapters.sliding(2,1).toSeq
+    .groupMapReduce{case List(low, high) => high - low}(_ => 1)(_ + _)
+
+  val variants = adapterVariants(0, adapters.tail)
+
+  s"Joltage differences:\n 1 Jolt: ${joltages(1)}\n 3 Jolts: ${joltages(3)}\n"+
+  s" Product: ${joltages(1) * joltages(3)}\n" +
+  s"\nPossible variants: ${variants}"
+
+
+
+
+val adapterVariants: ((Int, List[Int])) => Long = memoize{ case (prev, remaining) => {
+  remaining match {
+    case h :: t if h < prev + 4 => adapterVariants(h, t) + adapterVariants(prev, t)
+    case last :: Nil => 1
+    case _ => 0
+  }
+}}

src/main/scala/Main.scala

@@ -11,6 +11,7 @@ package aoc2020
     case 7 => input()(day7)
     case 8 => input()(day8)
     case 9 => input(_.toLong)(day9)
+    case 10 => input(_.toInt)(day10)
     case _ => "No such day implemented"
   }
   println(out)