[day15] solution

build.sbt

@@ -11,7 +11,8 @@ lazy val root = project
       "-Yexplicit-nulls",
       "-language:strict-equality",
     ),
-    libraryDependencies += "org.typelevel" %% "cats-core" % "2.3.0"
+    libraryDependencies += "org.typelevel" %% "cats-core" % "2.3.0",
   )
+  .enablePlugins(JmhPlugin)
 
-Runtime / unmanagedSources += baseDirectory.value / "input",
+Runtime / unmanagedSources += baseDirectory.value / "input"

project/plugins.sbt

 addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.6")
+addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.0")

src/main/scala/Day13.scala

 package aoc2020
 import aoc2020.lib._
-import cats.implicits.given
 import scala.math.{min,max}
 import cats._, cats.implicits.given
 

src/main/scala/Day15.scala

+package aoc2020
+
+import aoc2020.lib._
+
+def day15(input: List[String]): String =
+  val starting: Map[Long, Long] = parse(input.head)
+  val (num2020, _) = playMut(starting, 2020)
+  val (num30mil, _) = playMut(starting, 30000000)
+  s"""2020th spoken number: $num2020
+     |30 millionth spoken number: $num30mil
+     """.stripMargin
+
+def parse(input: String): Map[Long, Long] =
+    input.split(",").nn
+      .zipWithIndex
+      .map(t => (t._1.toLong, t._2 + 1L))
+      .toMap
+
+def play(starting: Map[Long, Long], maxTurns: Long) =
+  val (lastNum, lastTurn) = starting.maxBy(_._2)
+
+  (lastTurn to maxTurns-1).foldLeft((lastNum, starting - lastNum)){
+    case ((last, spoken), turn) => 
+      val next = spoken.get(last).map(turn - _).getOrElse(0L)
+      val nowSpoken = spoken.updated(last, turn)
+      (next, nowSpoken)
+  }
+
+/* about twice as fast for a single run */
+import scala.collection.mutable
+def playMut(starting: Map[Long, Long], maxTurns: Long) =
+  val (lastNum, lastTurn) = starting.maxBy(_._2)
+  val spoken = mutable.Map.from(starting - lastNum)
+  var last: Long = lastNum
+  for (turn <- (lastTurn to maxTurns-1)) {
+      val next = spoken.get(last).map(turn - _).getOrElse(0L)
+      spoken += last -> turn
+      last = next
+  }
+  (last, spoken)

src/main/scala/Main.scala

@@ -16,6 +16,7 @@ package aoc2020
     case 12 => input()(day12)
     case 13 => input()(day13)
     case 14 => input()(day14)
+    case 15 => input()(day15)
     case _ => "No such day implemented"
   }
   if (sample.nonEmpty) println("SAMPLE VALUES!")

src/main/scala/bench/Benchs.scala

+package aoc2020
+
+import org.openjdk.jmh.annotations._
+import java.util.concurrent.TimeUnit
+
+object Input { val in = parse("2,1,10,11,0,6") }
+
+class Benchs {
+  @Benchmark
+  @BenchmarkMode(Array(Mode.AverageTime))
+  @OutputTimeUnit(TimeUnit.MICROSECONDS)
+  def measureAvgTime = playMut(Input.in, 2020)
+}