Day 13

13.cs

@@ -0,0 +1,108 @@
+List<Node> packets = new();
+while (Console.ReadLine() is { } line)
+{
+    if (line.Length > 1)
+    {
+        packets.Add(ParseLine(line));
+    }
+}
+
+var sum = 0;
+for (var i = 0; i < packets.Count / 2; ++i)
+{
+    var c = NodeComparer.Instance.Compare(packets[i * 2], packets[i * 2 + 1]);
+    sum += c <= 0 ? i + 1 : 0;
+}
+
+Console.WriteLine($"Sum of indices of correctly ordered pairs: {sum}");
+
+var dividers = new[]
+{
+    ParseLine("[[2]]"),
+    ParseLine("[[6]]")
+};
+
+packets.AddRange(dividers);
+packets.Sort(NodeComparer.Instance);
+
+var key = (packets.IndexOf(dividers[0]) + 1) * (packets.IndexOf(dividers[1]) + 1);
+
+Console.WriteLine($"Decoder key: {key}");
+
+Node ParseLine(string line)
+{
+    var i = 0;
+    return Parse(line, ref i);
+}
+
+Node Parse(string line, ref int i)
+{
+    switch (line[i])
+    {
+        case '[':
+            i++;
+            List<Node> elements = new();
+            while (line[i] != ']')
+            {
+                elements.Add(Parse(line, ref i));
+                if (line[i] == ',')
+                    i++;
+            }
+
+            i++;
+            return new List(elements);
+        case var c:
+            int d;
+            for (d = 0; char.IsDigit(line[i]); i++)
+            {
+                d = d * 10 + (c - '0');
+            }
+
+            return new Number(d);
+    }
+}
+
+internal class NodeComparer : IComparer<Node>
+{
+    public static NodeComparer Instance { get; } = new();
+
+    private int CompareLists(List a, List b)
+    {
+        var n = Math.Min(a.Values.Count, b.Values.Count);
+        for (var i = 0; i < n; ++i)
+        {
+            var c = Compare(a.Values[i], b.Values[i]);
+            if (c != 0)
+                return c;
+        }
+
+        return a.Values.Count.CompareTo(b.Values.Count);
+    }
+
+    public int Compare(Node? x, Node? y) => (x, y) switch
+    {
+        (Number a, Number b) => a.Value.CompareTo(b.Value),
+        (List a, List b) => CompareLists(a, b),
+        (Number a, List b) => Compare(new List(new List<Node> {a}), b),
+        (List a, Number b) => Compare(a, new List(new List<Node> {b})),
+        _ => throw new NotImplementedException()
+    };
+}
+
+internal abstract record Node;
+
+internal sealed record Number(int Value) : Node
+{
+    public override string ToString() => Value.ToString();
+}
+
+internal sealed record List(List<Node> Values) : Node
+{
+    public bool Equals(List? other)
+        => other != null && other.Values.Count == Values.Count &&
+           Values.Zip(other.Values).All(p => p.First.Equals(p.Second));
+
+    public override int GetHashCode() => Values.Count;
+
+    public override string ToString() => $"[{string.Join(",", Values)}]";
+}