Day 23: LAN Party

Author: ahorner

lib/2024/23.rb

@@ -0,0 +1,39 @@
+NETWORK = INPUT.split("\n").each_with_object(Hash.new { |h, k| h[k] = Set.new }) do |line, network|
+  computer, connected = line.split("-")
+
+  network[computer] << connected
+  network[connected] << computer
+end
+
+def cliques(computer, network)
+  network[computer].to_a.combination(2).select do |a, b|
+    network[a].include?(b)
+  end
+end
+
+parties = NETWORK.keys.each_with_object(Set.new) do |computer, lans|
+  cliques(computer, NETWORK).each { |peers| lans << Set.new([computer, *peers]) }
+end
+matches = parties.count { |party| party.any? { |cpu| cpu.start_with?("t") } }
+
+solve!("The number of groups containing a 't' computer is:", matches)
+
+def largest_clique(network, clique = Set.new, ignore = Set.new)
+  return clique if network.empty? && ignore.empty?
+
+  network.keys.map do |computer|
+    largest = largest_clique(
+      network.slice(*network[computer]),
+      clique + [computer],
+      ignore.intersection(network[computer])
+    )
+
+    network.delete(computer)
+    ignore << computer
+
+    largest
+  end.reject(&:nil?).max_by(&:size)
+end
+
+max_party = largest_clique(NETWORK.dup)
+solve!("The LAN party password is:", max_party.sort.join(","))

spec/2024/23_spec.rb

@@ -0,0 +1,57 @@
+require "spec_helper"
+
+RSpec.describe "Day 23: LAN Party" do
+  let(:runner) { Runner.new("2024/23") }
+  let(:input) do
+    <<~TXT
+      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
+    TXT
+  end
+
+  describe "Part One" do
+    let(:solution) { runner.execute!(input, part: 1) }
+
+    it "counts the sets of potential computers" do
+      expect(solution).to eq(7)
+    end
+  end
+
+  describe "Part Two" do
+    let(:solution) { runner.execute!(input, part: 2) }
+
+    it "finds the largest group of mutual peers" do
+      expect(solution).to eq("co,de,ka,ta")
+    end
+  end
+end