Day 15...slower than I'd like...

15input.txt

@@ -0,0 +1 @@
+7,14,0,17,11,1,2

2020.csproj

@@ -53,6 +53,9 @@
     <None Update="14input.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="15input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 
 </Project>

Program.cs

@@ -19,6 +19,7 @@
          Q12.Go();
          Q13.Go();
          Q14.Go();
+         Q15.Go();
          Util.Log($"Total time={(System.DateTime.Now - start).TotalMilliseconds}ms");
       }
    }

Q15.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+
+namespace _2020
+{
+   class Q15
+   {
+      static List<int> list = new List<int>();
+
+      public static void Go()
+      {
+         var start = DateTime.Now;
+         MakeList();
+         Util.Log($"Q15 MakeList took {(DateTime.Now - start).TotalMilliseconds}ms");
+         var p1start = DateTime.Now;
+         Part1();
+         Util.Log($"Q15 part1 took {(DateTime.Now - p1start).TotalMilliseconds}ms");
+         var p2start = DateTime.Now;
+         Part2();
+         Util.Log($"Q15 part2 took {(DateTime.Now - p2start).TotalMilliseconds}ms");
+
+         Util.Log($"Q15 took {(DateTime.Now - start).TotalMilliseconds}ms");
+      }
+
+      static void MakeList()
+      {
+         var input = File.ReadAllText("15input.txt");
+         foreach (var num in input.Split(','))
+         {
+            list.Add(Convert.ToInt32(num));
+         }
+      }
+
+      static int GetNumAt(int turn)
+      {
+         var numsSaidDict = new Dictionary<int, List<int>>();
+
+         int lastNumSaid = 0;
+         List<int> entry = null;
+         for (int i = 0; i < turn; i++)
+         {
+            if (i < list.Count)
+            {
+               numsSaidDict.Add(list[i], new List<int>() { i });
+               lastNumSaid = list[i];
+            }
+            else
+            {
+               if (i == list.Count)
+               {
+                  lastNumSaid = 0;
+               }
+               else
+               {
+                  if (entry != null)
+                  {
+                     lastNumSaid = entry.Count == 1 ? 0 : entry[^1] - entry[^2];
+                  }
+                  else
+                  {
+                     lastNumSaid = 0;
+                  }
+               }
+
+               numsSaidDict.TryGetValue(lastNumSaid, out entry);
+               if (entry == null)
+               {
+                  entry = new List<int>();
+                  numsSaidDict.Add(lastNumSaid, entry);
+               }
+
+               entry.Add(i);
+            }
+         }
+
+         return lastNumSaid;
+      }
+
+      static void Part1()
+      {
+         Util.Log($"Q15Part1: last num said={GetNumAt(2020)}");
+      }
+
+      static void Part2()
+      {
+         Util.Log($"Q15Part2: last num said={GetNumAt(30000000)}");
+      }
+   }
+}