2023 Day 15

src/2023/15/Program.cs

@@ -1,3 +1,56 @@
 using AdventOfCode.Common;
 
-var lines = Resources.GetInputFileLines();
+var instructions = Resources.GetInputFileLines()
+    .Single()
+    .SplitBy(",");
+
+var boxes = Enumerable.Range(0, 256)
+    .Select(_ => new LinkedList<Lens>())
+    .ToArray();
+
+foreach (var instruction in instructions)
+{
+    var index1 = instruction.IndexOf('=');
+    var index2 = instruction.IndexOf('-');
+    var label = instruction[0..Math.Max(index1, index2)];
+    var box = boxes[Hash(label)];
+    var lens = box.FirstOrDefault(lens => lens.Label == label);
+
+    if (index1 != -1) // =
+    {
+        var length = int.Parse(instruction.Substring(index1 + 1));
+        if (lens != null)
+        {
+            lens.FocalLength = length;
+        }
+        else
+        {
+            box.AddLast(new Lens(label, length));
+        }
+    }
+    else if (lens != null) // -
+    {
+        box.Remove(lens);
+    }
+}
+
+var part1 = instructions
+    .Select(Hash)
+    .Sum();
+
+var part2 = boxes
+    .SelectMany((box, boxId) =>
+        box.Select((lens, lensId) => (1 + boxId) * (lensId + 1) * lens.FocalLength))
+    .Sum();
+
+Console.WriteLine($"Part 1: {part1}");
+Console.WriteLine($"Part 2: {part2}");
+
+static int Hash(string input)
+    => input.Aggregate(0, (acc, c) => acc = (acc + c) * 17 % 256);
+
+file class Lens(string label, int focalLength)
+{
+    public string Label { get; } = label;
+    public int FocalLength { get; set; } = focalLength;
+}