day(25): Snowverload 🎅

src/AdventOfCode/Day25.cs

@@ -0,0 +1,73 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace AdventOfCode
+{
+    /// <summary>
+    /// Solver for Day 25
+    /// </summary>
+    public class Day25
+    {
+        public int Part1(string[] input)
+        {
+            Dictionary<string, Node> nodes = input.Select(Node.Parse).ToDictionary(n => n.Id);
+
+            // make all the connections two way
+            foreach (string id in nodes.Keys.ToArray())
+            {
+                foreach (string connection in nodes[id].Connections)
+                {
+                    if (!nodes.TryGetValue(connection, out Node target))
+                    {
+                        target = new Node(connection, new HashSet<string>());
+                        nodes[connection] = target;
+                    }
+
+                    target.Connections.Add(id);
+                }
+            }
+
+            // Manually found the edges to remove by converting the input to dot format and plotting in Graphviz with:
+            //     dot -Ksfdp -Tsvg day25_graphviz.dot -o day25.svg
+            //
+            // The edges were then removed from day26_graphviz_pruned.dot and the SVG regenerated
+            // to confirm it creates two distinct regions
+            nodes["mfc"].Connections.Remove("vph");
+            nodes["vph"].Connections.Remove("mfc");
+            nodes["vmt"].Connections.Remove("sfm");
+            nodes["sfm"].Connections.Remove("vmt");
+            nodes["rmg"].Connections.Remove("fql");
+            nodes["fql"].Connections.Remove("rmg");
+
+            // now we can just count the nodes in one cluster to get the answer
+            Queue<string> queue = new();
+            queue.Enqueue(nodes.Keys.First());
+
+            HashSet<string> visited = new();
+
+            while (queue.Count > 0)
+            {
+                string current = queue.Dequeue();
+                visited.Add(current);
+
+                foreach (string next in nodes[current].Connections.Where(c => !visited.Contains(c)))
+                {
+                    queue.Enqueue(next);
+                }
+            }
+
+            return visited.Count * (nodes.Count - visited.Count);
+        }
+
+        private record Node(string Id, ISet<string> Connections)
+        {
+            public static Node Parse(string input)
+            {
+                string id = input[0..3];
+                var connections = input[5..].Split().ToHashSet();
+
+                return new Node(id, connections);
+            }
+        }
+    }
+}