🍋 restore original day 8 solution

twentylemon · Dec 8, 2023 · cda49da · cda49da
1 parent 0fde24f
commit cda49da
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diff --git a/src/main/scala/org/lemon/advent/year2023/Day08.scala b/src/main/scala/org/lemon/advent/year2023/Day08.scala
@@ -35,3 +35,22 @@ private object Day08:
       .map(_.toLong)
 
     cycle.fold(1L)(lcm)
+
+  def cycleGraph(directions: String, graph: Map[String, (String, String)]) = graph.keySet
+    .map(loc => (loc, directions.foldLeft(loc)((p, d) => move(p, d, graph))))
+    .toMap
+
+  def cyclesUntilEnd(cycle: Map[String, String], start: String) =
+    Iterator.iterate(start)(loc => cycle(loc))
+      .zipWithIndex
+      .dropWhile(!_._1.endsWith("Z"))
+      .next._2.toLong
+
+  def part2_alternate(input: String) =
+    val (directions, graph) = parse(input)
+    val cycle = cycleGraph(directions, graph)
+    val loop = graph.keySet.toSeq
+      .filter(_.endsWith("A"))
+      .map(cyclesUntilEnd(cycle, _))
+
+    loop.fold(1L)(lcm) * directions.length
diff --git a/src/test/scala/org/lemon/advent/year2023/Day08Test.scala b/src/test/scala/org/lemon/advent/year2023/Day08Test.scala
@@ -48,3 +48,21 @@ class Day08Test extends UnitTest:
   test("part 2") {
     part2(read(file(2023)(8))) shouldBe 13663968099527L
   }
+
+  test("part 2 alternate example") {
+    val in = """|LR
+                |
+                |11A = (11B, XXX)
+                |11B = (XXX, 11Z)
+                |11Z = (11B, XXX)
+                |22A = (22B, XXX)
+                |22B = (22C, 22C)
+                |22C = (22Z, 22Z)
+                |22Z = (22B, 22B)
+                |XXX = (XXX, XXX)""".stripMargin
+    part2_alternate(in) shouldBe 6
+  }
+
+  test("part 2 alternate") {
+    part2_alternate(read(file(2023)(8))) shouldBe 13663968099527L
+  }