Day 7

This solution is bad and slow and there are probably ways to make it not as bad and not as slow, but I'll take < 1 second runtime for now.

src/07.cs

@@ -0,0 +1,55 @@
+namespace aoc2024;
+
+internal class Day07 : Day
+{
+    private readonly List<(long result, List<long> inputs)> eqs = [];
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}");
+        foreach (var line in lines)
+        {
+            var split = line.Split(':');
+            var nums = split[1].Split(' ', StringSplitOptions.RemoveEmptyEntries).Select(long.Parse).ToList();
+            eqs.Add((long.Parse(split[0]), nums));
+        }
+    }
+
+    private static bool CanSolveAddMult((long result, List<long> inputs) eq)
+    {
+        if (eq.inputs.Count > 2)
+        {
+            var added = eq.inputs[0] + eq.inputs[1];
+            var multd = eq.inputs[0] * eq.inputs[1];
+            return CanSolveAddMult(eq with { inputs = [added, .. eq.inputs.Skip(2)] }) || CanSolveAddMult(eq with { inputs = [multd, .. eq.inputs.Skip(2)] });
+        }
+
+        return eq.inputs[0] + eq.inputs[1] == eq.result || eq.inputs[0] * eq.inputs[1] == eq.result;
+    }
+
+    internal override string Part1()
+    {
+        long total = eqs.Where(CanSolveAddMult).Sum(eq => eq.result);
+        return $"Add | Mult calibration result: <+white>{total}";
+    }
+
+    private static bool CanSolveAddMultConcat((long result, List<long> inputs) eq)
+    {
+        if (eq.inputs.Count > 2)
+        {
+            var added = eq.inputs[0] + eq.inputs[1];
+            var multd = eq.inputs[0] * eq.inputs[1];
+            var cated = long.Parse($"{eq.inputs[0]}{eq.inputs[1]}");
+            return CanSolveAddMultConcat(eq with { inputs = [added, .. eq.inputs.Skip(2)] })
+                   || CanSolveAddMultConcat(eq with { inputs = [multd, .. eq.inputs.Skip(2)] })
+                   || CanSolveAddMultConcat(eq with { inputs = [cated, .. eq.inputs.Skip(2)] });
+        }
+
+        return eq.inputs[0] + eq.inputs[1] == eq.result || eq.inputs[0] * eq.inputs[1] == eq.result || long.Parse($"{eq.inputs[0]}{eq.inputs[1]}") == eq.result;
+    }
+
+    internal override string Part2()
+    {
+        long total = eqs.Where(CanSolveAddMultConcat).Sum(eq => eq.result);
+        return $"Add | Mult | Concat calibration result: <+white>{total}";
+    }
+}