feat(day21): part 2

Author: fdlk

day21-2.sc

@@ -0,0 +1,58 @@
+import common.loadPackets
+
+import scala.annotation.tailrec
+
+val input = loadPackets(List("day21.txt")).map(_.split(": ")(1).toInt)
+
+val threeThrows =
+  (for(x <- Range.inclusive(1, 3);
+       y <- Range.inclusive(1, 3);
+       z <- Range.inclusive(1, 3)) yield (x + y + z, 1)).groupMapReduce(_._1)(_._2)(_ + _)
+
+case class State(player1Position: Int = input.head,
+                 player2Position: Int = input(1),
+                 player1Score: Int = 0,
+                 player2Score: Int = 0,
+                 player1Turn: Boolean = true) {
+  def wrap(v: Int): Int = (v - 1) % 10 + 1
+
+  def won: Boolean = player1Score >= 21 | player2Score >= 21
+
+  def winner: Option[Int] =
+    if (player1Score >= 21) Some(1)
+    else if (player2Score >= 21) Some(2)
+    else None
+
+  def next(): Map[State, Int] = {
+    if (won) Map(this -> 1)
+    else threeThrows.map { case (roll, count) =>
+      if (player1Turn) {
+        val newPos = wrap(player1Position + roll)
+        copy(player1Position = newPos,
+          player1Score = player1Score + newPos,
+          player1Turn = !player1Turn) -> count
+      } else {
+        val newPos = wrap(player2Position + roll)
+        copy(player2Position = newPos,
+          player2Score = player2Score + newPos,
+          player1Turn = !player1Turn) -> count
+      }
+    }
+  }
+}
+
+val initial: Map[State, Long] = Map(State(input.head, input(1)) -> 1L)
+def nextState(universes: Map[State, Long]): Map[State, Long] =
+  universes.toList.flatMap {
+    case (universe: State, count: Long) => universe.next().view.mapValues(_ * count)
+  }.groupMapReduce(_._1)(_._2)(_ + _)
+
+@tailrec
+def part2(universes: Map[State, Long]): Long = {
+  if (universes.keySet.forall(_.won)) {
+    val counts: Map[Int, Long] = universes.groupMapReduce(_._1.winner.get)(_._2)(_ + _)
+    counts.values.max
+  } else part2(nextState(universes))
+}
+
+part2(initial)