Day 19

Nothing a little caching can't solve!

src/19.cs

@@ -0,0 +1,102 @@
+namespace aoc2024;
+
+internal class Day19 : Day
+{
+    private readonly List<string> towels = [];
+    private readonly List<string> desiredPatterns = [];
+
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}").ToList();
+        towels.AddRange(lines[0].Split(", ", StringSplitOptions.RemoveEmptyEntries));
+        for (int i = 2; i < lines.Count; i++)
+        {
+            desiredPatterns.Add(lines[i]);
+        }
+    }
+
+    private readonly HashSet<string> knownGood = [];
+    private bool IsPossible(ReadOnlySpan<char> desired)
+    {
+        if (knownGood.Contains(desired.ToString()))
+        {
+            return true;
+        }
+
+        foreach (var towel in towels)
+        {
+            if (!desired.StartsWith(towel))
+            {
+                continue;
+            }
+
+            if (desired.Length == towel.Length || IsPossible(desired[towel.Length..]))
+            {
+                knownGood.Add(towel);
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private readonly Dictionary<string, long> knownGoodCount = [];
+    private long NumPossible(ReadOnlySpan<char> desired)
+    {
+        var desiredStr = desired.ToString();
+        if (knownGoodCount.TryGetValue(desiredStr, out var count))
+        {
+            return count;
+        }
+
+        foreach (var towel in towels)
+        {
+            if (!desired.StartsWith(towel))
+            {
+                continue;
+            }
+
+            var numPossible = desired.Length != towel.Length ? NumPossible(desired[towel.Length..]) : 1;
+            if (numPossible == 0)
+            {
+                continue;
+            }
+
+            if (!knownGoodCount.TryAdd(desiredStr, numPossible))
+            {
+                knownGoodCount[desiredStr] += numPossible;
+            }
+        }
+
+        return knownGoodCount.GetValueOrDefault(desiredStr, 0);
+    }
+
+    internal override string Part1()
+    {
+        int numPossible = 0;
+        foreach (var desired in desiredPatterns)
+        {
+            if (IsPossible(desired))
+            {
+                numPossible++;
+            }
+        }
+
+        return $"Possible designs: <+white>{numPossible}";
+    }
+
+    internal override string Part2()
+    {
+        long total = 0;
+        foreach (var desired in desiredPatterns)
+        {
+            var combinations = NumPossible(desired);
+            if (combinations > 0)
+            {
+                total += combinations;
+            }
+        }
+
+        return $"Ways to make each design: <+white>{total}";
+    }
+}