Day 08

src/day08.scala

@@ -0,0 +1,54 @@
+package aoc
+package day08
+
+import scala.annotation.tailrec
+
+val dayNumber = "08"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def findPathLength(
+    nodeMap: Map[String, (String, String)],
+    instructions: String,
+    startNode: String,
+    endNode: String => Boolean
+): Long =
+  LazyList
+    .continually(instructions)
+    .flatMap(_.toSeq)
+    .scanLeft(startNode) {
+      case (node, 'L') => nodeMap(node)._1
+      case (node, 'R') => nodeMap(node)._2
+    }
+    .takeWhile(!endNode(_))
+    .length
+
+def part1(input: String) =
+  val Array(instructions, nodesStr) = input.split("\n\n")
+  val nodeMap = nodesStr
+    .split('\n')
+    .map { case s"$name = ($left, $right)" => (name, (left, right)) }
+    .toMap
+  findPathLength(nodeMap, instructions, "AAA", _ == "ZZZ").toString
+
+def part2(input: String) =
+  val Array(instructions, nodesStr) = input.split("\n\n")
+  val nodeMap = nodesStr
+    .split('\n')
+    .map { case s"$name = ($left, $right)" => (name, (left, right)) }
+    .toMap
+
+  @tailrec def gcd(a: Long, b: Long): Long =
+    if (b == 0) a.abs else gcd(b, a % b)
+  def lcm(a: Long, b: Long) =
+    (a * b).abs / gcd(a, b)
+
+  nodeMap.keySet
+    .filter(_.endsWith("A"))
+    .map(node => findPathLength(nodeMap, instructions, node, _.endsWith("Z")))
+    .reduce(lcm)
+    .toString