Advent of Code 2023 day 8 bug

d8bug.scala

// Guide to run:
// + Install scala-cli: https://scala-cli.virtuslab.org/install
// + Run: scala-cli test d8bug.scala
// Guide to dev: https://scala-cli.virtuslab.org/docs/cookbooks/ide/vscode
//> using scala 3.3.1
//> using dep org.scalameta::munit::1.0.0-M10

// With input like bellow, we can NOT use LCM to solve part2
val input = """LLR

AAA = (BBA, AAA)
BBA = (BBB, DDD)
BBB = (BBA, CCC)
CCC = (AAZ, CCC)
AAZ = (BBB, AAZ)
DDD = (DDD, ZZZ)
ZZZ = (AAZ, ZZZ)"""

type Node = String
type Direction = Char // 'L' or 'R'
case class Choices(left: Node, right: Node)

val (instructions: LazyList[Direction], net: Map[Node, Choices]) =
    val lines = input.linesIterator
    val firstLinne = lines.next()
    val net = lines.drop(1).map { line =>
        val s"$from = ($left, $right)" = line: @unchecked
        from -> Choices(left, right)
    }.toMap
    val instructions = LazyList.from(0).map(i => firstLinne.charAt(i % firstLinne.length))
    instructions -> net

def moves(start: Node => Boolean) =
    instructions.scanLeft(net.keys.filter(start).toList) {
        case (nodes, 'L') => nodes.map(net(_).left)
        case (nodes, _) => nodes.map(net(_).right)
    }

// brute force
def part2() = moves(_.endsWith("A")).indexWhere(_.forall(_.endsWith("Z")))

class MyTests extends munit.FunSuite:
    val stop = (n: Node) => n.endsWith("Z")
    val moves1 = moves(_ == "AAA").flatten
    val moves2 = moves(_ == "BBA").flatten

    test("AAA escape after 4 + 3x steps"):
        moves1.zipWithIndex.take(20).foreach: (n, i) =>
            assertEquals(stop(n), i >= 3 && i % 3 == 1)

    test("BBA escape after 6 or 7 + 3x steps"):
        moves2.zipWithIndex.take(20).foreach: (n, i) =>
            assertEquals(stop(n), i == 6 || i >= 7 && i % 3 == 1)
        
    test("Using LCM to solve part2 is NOT correct"): // fail!!!
        val l1 = moves1.indexWhere(stop)
        val l2 = moves2.indexWhere(stop)
        assertEquals(l1, 4)
        assertEquals(l2, 6)
        val lcm = 12
        val at12 = moves1(lcm)
        assert(stop(at12), s"\nat12 == $at12")

    test("Correct answer for part2 is 7"):
        val n = part2()
        assertEquals(n, 7)
        assert(stop(moves1(7)))
        assert(stop(moves2(7)))

network visualize for the input