feat(2024): day 23

2024/day23.scala

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+package `2024`.day23
+
+import prelude.*
+import scala.collection.parallel.CollectionConverters.*
+import scala.annotation.tailrec
+
+type Connection = Map[String, Set[String]]
+
+def parse(input: String): Connection = input
+  .split('\n')
+  .toSet
+  .flatMap { case s"$a-$b" => Set(a -> b, b -> a) }
+  .groupMap(_._1)(_._2)
+
+def part1(input: String) =
+  val connection = parse(input)
+
+  extension (a: String)
+    inline infix def <->(b: String) =
+      connection(a).contains(b) && connection(b).contains(a)
+
+  def isValidTriangle(vertices: Set[String]): Boolean = vertices.toList match
+    case List(v1, v2, v3) => v1 <-> v2 && v2 <-> v3 && v3 <-> v1
+    case _                => false
+
+  connection.par
+    .flatMap { (vertex, neighbors) =>
+      neighbors
+        .subsets(2)
+        .map(_ + vertex)
+        .withFilter(_.exists(_.startsWith("t")))
+        .filter(isValidTriangle)
+    }
+    .toSet
+    .size
+
+def part2(input: String) =
+  val connection = parse(input)
+  findMaximumCliqueBronKerbosch(connection).toList.sorted.mkString(",")
+
+@main def main() =
+  val input = fromFile(".cache/2024/23.txt").mkString
+  println(part1(input))
+  println(part2(input))
+
+def findMaximumCliqueBronKerbosch(connections: Connection): Set[String] =
+  def bronKerbosch(
+    potential: Set[String],
+    excluded: Set[String] = Set.empty,
+    result: Set[String] = Set.empty,
+  ): Set[String] =
+    if (potential.isEmpty && excluded.isEmpty) then result
+    else
+      // Choose pivot to minimize branching
+      val pivot = (potential ++ excluded)
+        .maxBy(vertex => potential.count(connections(vertex).contains))
+
+      val remaining = potential -- connections(pivot)
+
+      remaining.foldLeft(Set.empty[String]) { (currentMax, vertex) =>
+        val neighbors = connections(vertex)
+        val newClique = bronKerbosch(
+          result = result + vertex,
+          potential = potential & neighbors,
+          excluded = excluded & neighbors,
+        )
+        if (newClique.size > currentMax.size) newClique else currentMax
+      }
+
+  bronKerbosch(potential = connections.keySet)

2024/day23.test.scala

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+package `2024`.day23
+
+import munit.FunSuite
+import prelude.dedent
+
+val example = """
+kh-tc
+qp-kh
+de-cg
+ka-co
+yn-aq
+qp-ub
+cg-tb
+vc-aq
+tb-ka
+wh-tc
+yn-cg
+kh-ub
+ta-co
+de-co
+tc-td
+tb-wq
+wh-td
+ta-ka
+td-qp
+aq-cg
+wq-ub
+ub-vc
+de-ta
+wq-aq
+wq-vc
+wh-yn
+ka-de
+kh-ta
+co-tc
+wh-qp
+tb-vc
+td-yn
+""".dedent
+
+class Test extends FunSuite:
+  test("example"):
+    assertEquals(part1(example), 7)
+    assertEquals(part2(example), "co,de,ka,ta")