2016.19

ebowman · ebowman · commit 9fa206c0db7a · 2022-01-14T23:50:14.000+01:00
diff --git a/src/main/scala/y2016/Day19.scala b/src/main/scala/y2016/Day19.scala
@@ -0,0 +1,64 @@
+package y2016
+
+import scala.annotation.tailrec
+
+trait Day19 {
+
+  def solve1(n: Int): Int = {
+    val elves = Array.fill(n)(1)
+
+    @tailrec def recurse(): Int = {
+      val winner = elves.indexOf(n)
+      if (winner >= 0) winner
+      else {
+        var i = 0
+        while (i < n) {
+          if (elves(i) != 0) {
+            val next = elves.indexWhere(_ != 0, i + 1) match {
+              case -1 => elves.indexWhere(_ != 0, 0)
+              case n => n
+            }
+            elves(i) += elves(next % n)
+            elves(next % n) = 0
+          }
+          i += 1
+        }
+        recurse()
+      }
+    }
+
+    recurse() + 1
+  }
+
+  def solve2(n: Int): Int = {
+    val fast = true
+    if (fast) {
+      @tailrec def recurse(i: Int): Int =
+        if (i * 3 < n) recurse(i * 3)
+        else n - i
+      recurse(1)
+    } else {
+      val values = Array.fill(n)(1)
+      val next = { val nx = Array.range(1, n + 1); nx(nx.length - 1) = 0; nx }
+      val prev = { val pr = Array.range(-1, n - 1); pr(0) = pr.length - 1; pr }
+      var count = n
+
+      @inline @tailrec def advance(c: Int, n: Int): Int = if (n == 0) c else advance(next(c), n - 1)
+      @inline def opposite(c: Int): Int = advance(c, count / 2)
+      var cursor = 0
+      while (values.indexOf(n) == -1) {
+        if (count > 0 && (count % 1000) == 0) println(count)
+        val opp = opposite(cursor)
+        values(cursor) += values(opp)
+        values(opp) = 0
+        next(prev(opp)) = next(opp)
+        prev(next(opp)) = prev(opp)
+        next(opp) = -1
+        prev(opp) = -1
+        cursor = next(cursor)
+        count -= 1
+      }
+      values.indexOf(n) + 1
+    }
+  }
+}
diff --git a/src/test/scala/y2016/Day19Spec.scala b/src/test/scala/y2016/Day19Spec.scala
@@ -0,0 +1,13 @@
+package y2016
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should._
+
+class Day19Spec extends AnyFlatSpec with Matchers with Day19 {
+  it should "solve part 1" in {
+    solve1(3014387) shouldBe 1834471
+  }
+  it should "solve part 2" in {
+    solve2(3014387) shouldBe 1420064
+  }
+}
