day(23): Part 2

src/AdventOfCode/Day23.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
 using AdventOfCode.Utilities;
@@ -36,9 +37,7 @@ public int Part1(string[] input)
 
             int total = 0;
 
-            string[] nodes = graph.Keys.Order().ToArray();
-
-            foreach (string node in nodes)
+            foreach (string node in graph.Keys)
             {
                 foreach (string left in graph[node].Where(l => string.CompareOrdinal(node, l) < 0))
                 {
@@ -49,6 +48,7 @@ public int Part1(string[] input)
                             continue;
                         }
 
+                        // found a triangle
                         if (node.StartsWith('t') || left.StartsWith('t') || right.StartsWith('t'))
                         {
                             total++;
@@ -62,14 +62,58 @@ public int Part1(string[] input)
             return total;
         }
 
-        public int Part2(string[] input)
+        public string Part2(string[] input)
         {
+            SortedDictionary<string, ICollection<string>> graph = new();
+
             foreach (string line in input)
             {
-                throw new NotImplementedException("Part 2 not implemented");
+                string[] elements = line.Split('-');
+                Debug.Assert(elements.Length == 2);
+
+                graph.GetOrCreate(elements[0], () => new SortedSet<string>()).Add(elements[1]);
+                graph.GetOrCreate(elements[1], () => new SortedSet<string>()).Add(elements[0]);
             }
 
-            return 0;
+            var cliques = new List<IImmutableSet<string>>();
+            BronKerbosch(graph, [], graph.Keys.ToImmutableSortedSet(), [], cliques);
+
+            IImmutableSet<string> biggest = cliques.MaxBy(c => c.Count);
+
+            return string.Join(',', biggest.Order());
+        }
+
+        /// <summary>
+        /// Had to do some Googling for this one... Never heard of it before
+        /// </summary>
+        /// <param name="graph">Graph of node ID to the IDs of all connected nodes</param>
+        /// <param name="clique">Current clique being considered</param>
+        /// <param name="candidates">Candidate nodes to be added to the clique</param>
+        /// <param name="visited">Nodes already visited</param>
+        /// <param name="cliques">All cliques found</param>
+        private static void BronKerbosch(IDictionary<string, ICollection<string>> graph,
+                                         IImmutableSet<string> clique,
+                                         IImmutableSet<string> candidates,
+                                         IImmutableSet<string> visited,
+                                         ICollection<IImmutableSet<string>> cliques)
+        {
+            if (candidates.Count == 0)
+            {
+                cliques.Add(clique);
+                return;
+            }
+
+            foreach (var candidate in candidates.ToArray())
+            {
+                BronKerbosch(graph,
+                             clique.Add(candidate),
+                             candidates.Intersect(graph[candidate]),
+                             visited.Intersect(graph[candidate]),
+                             cliques);
+
+                candidates = candidates.Remove(candidate);
+                visited = visited.Add(candidate);
+            }
         }
     }
 }

tests/AdventOfCode.Tests/Day23Tests.cs

@@ -84,7 +84,7 @@ public void Part1_RealInput_ProducesCorrectResponse()
         [Fact]
         public void Part2_SampleInput_ProducesCorrectResponse()
         {
-            var expected = -1;
+            var expected = "co,de,ka,ta";
 
             var result = solver.Part2(GetSampleInput());
 
@@ -94,7 +94,7 @@ public void Part2_SampleInput_ProducesCorrectResponse()
         [Fact]
         public void Part2_RealInput_ProducesCorrectResponse()
         {
-            var expected = -1;
+            var expected = "av,ax,dg,di,dw,fa,ge,kh,ki,ot,qw,vz,yw";
 
             var result = solver.Part2(GetRealInput());
             output.WriteLine($"Day 23 - Part 2 - {result}");