Solve 2024 day 23 part 2

sim642 · Dec 23, 2024 · fe45dcf · fe45dcf
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diff --git a/src/main/scala/eu/sim642/adventofcode2024/Day23.scala b/src/main/scala/eu/sim642/adventofcode2024/Day23.scala
@@ -16,6 +16,44 @@ object Day23 {
 
   def count3CliquesT(edges: Set[Edge]): Int = find3Cliques(edges).count(_.exists(_.startsWith("t")))
 
+  // copied from 2018 day 23
+  // TODO: move to library
+  def maximumClique(neighbors: Map[Computer, Set[Computer]]): Set[Computer] = {
+    var best: Set[Computer] = Set.empty
+
+    def bronKerbosh(r: Set[Computer], p: Set[Computer], x: Set[Computer]): Unit = {
+      if (p.isEmpty && x.isEmpty) {
+        //println(r)
+        if (r.size > best.size)
+          best = r
+      }
+      else {
+        //val u = p.headOption.getOrElse(x.head)
+        val u = (p ++ x).maxBy(neighbors(_).size) // pivot on highest degree
+        var p2 = p
+        var x2 = x
+        for (v <- p -- neighbors(u)) {
+          bronKerbosh(r + v, p2 intersect neighbors(v), x2 intersect neighbors(v))
+          p2 -= v
+          x2 += v
+        }
+      }
+    }
+
+    bronKerbosh(Set.empty, neighbors.keySet, Set.empty)
+    best
+  }
+
+  def maximumClique(edges: Set[Edge]): Set[Computer] = {
+    val neighbors = (edges ++ edges.map(_.swap)).groupMap(_._1)(_._2)
+    maximumClique(neighbors)
+  }
+
+  def lanPartyPassword(edges: Set[Edge]): String = {
+    val clique = maximumClique(edges)
+    clique.toSeq.sorted.mkString(",")
+  }
+
   def parseEdge(s: String): Edge = s match {
     case s"$from-$to" => (from, to)
   }
@@ -26,6 +64,7 @@ object Day23 {
 
   def main(args: Array[String]): Unit = {
     println(count3CliquesT(parseEdges(input)))
+    println(lanPartyPassword(parseEdges(input)))
 
     // part 1: 2366 - too high (used contains 't' instead of startsWith 't')
   }

diff --git a/src/test/scala/eu/sim642/adventofcode2024/Day23Test.scala b/src/test/scala/eu/sim642/adventofcode2024/Day23Test.scala
@@ -46,4 +46,12 @@ class Day23Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(count3CliquesT(parseEdges(input)) == 1437)
   }
+
+  test("Part 2 examples") {
+    assert(lanPartyPassword(parseEdges(exampleInput)) == "co,de,ka,ta")
+  }
+
+  test("Part 2 input answer") {
+    assert(lanPartyPassword(parseEdges(input)) == "da,do,gx,ly,mb,ns,nt,pz,sc,si,tp,ul,vl")
+  }
 }