Day 5

It took me a minute to figure out a good solution for part 2, but I'm pretty happy with the custom-sort-function implementation.

I also really like the LINQ one-liners for getting the actual totals. I feel like future-me is going to be mad and confused, but right now it feels fun!

src/05.cs

@@ -0,0 +1,88 @@
+namespace aoc2024;
+
+internal class Day05 : Day
+{
+    private static readonly List<(int, int)> orderingRules = [];
+    private static readonly List<List<int>> updates = [];
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}");
+        int mode = 0;
+        foreach (var line in lines)
+        {
+            if (string.IsNullOrWhiteSpace(line))
+            {
+                mode++;
+                continue;
+            }
+
+            if (mode == 0)
+            {
+                var split = line.Split('|');
+                orderingRules.Add((int.Parse(split[0]), int.Parse(split[1])));
+            }
+            else if (mode == 1)
+            {
+                var split = line.Split(',');
+                List<int> update = [];
+                split.ForEach(n => update.Add(int.Parse(n)));
+                updates.Add(update);
+            }
+        }
+    }
+
+    private static bool IsOrdered(List<int> update)
+    {
+        for (int i = 0; i < update.Count; i++)
+        {
+            var numsBefore = orderingRules.Where(r => r.Item2 == update[i]).Select(r => r.Item1).ToList();
+            var numsAfter = orderingRules.Where(r => r.Item1 == update[i]).Select(r => r.Item2).ToList();
+            if (update.Take(i).Any(a => numsAfter.Contains(a)))
+            {
+                return false;
+            }
+
+            if (update.Skip(i + 1).Any(a => numsBefore.Contains(a)))
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    internal override string Part1()
+    {
+        long total = updates.Where(IsOrdered).Sum(update => update[update.Count / 2]);
+        return $"Ordered update middle number sum: <+white>{total}";
+    }
+
+    private static List<int> FixOrder(List<int> update)
+    {
+        var fix = update.ToList();
+        fix.Sort((a, b) =>
+        {
+            var numsBefore = orderingRules.Where(r => r.Item2 == b).Select(r => r.Item1).ToList();
+            var numsAfter = orderingRules.Where(r => r.Item1 == b).Select(r => r.Item2).ToList();
+            if (numsBefore.Contains(a))
+            {
+                return -1;
+            }
+
+            if (numsAfter.Contains(a))
+            {
+                return 1;
+            }
+
+            return 0;
+        });
+
+        return fix;
+    }
+
+    internal override string Part2()
+    {
+        long total = updates.Where(update => !IsOrdered(update)).Select(FixOrder).Sum(update => update[update.Count / 2]);
+        return $"Fixed unordered middle number sum: <+white>{total}";
+    }
+}