perf: Cache Stable Hashes (#3377)

* perf: Cache Stable Hashes
- Static dictionary of message hashes
- ResetStatics

* Added debug log for future debugging.

* Update Extensions.cs

---------

Co-authored-by: mischa <16416509+vis2k@users.noreply.github.com>

Assets/Mirror/Core/Tools/Extensions.cs

@@ -9,15 +9,33 @@ public static class Extensions
         public static string ToHexString(this ArraySegment<byte> segment) =>
             BitConverter.ToString(segment.Array, segment.Offset, segment.Count);
 
+        // GetStableHashCode is O(N).
+        // the longer the string, the more it needs to compute:
+        // https://github.com/MirrorNetworking/Mirror/pull/3377
+        // cache results for O(1) lookups.
+        static readonly Dictionary<string, int> StableHashes = new Dictionary<string, int>();
+
+        [UnityEngine.RuntimeInitializeOnLoadMethod]
+        public static void ResetStatics()
+        {
+            StableHashes.Clear();
+        }
+
         // string.GetHashCode is not guaranteed to be the same on all machines, but
         // we need one that is the same on all machines. simple and stupid:
         public static int GetStableHashCode(this string text)
         {
+            if (StableHashes.TryGetValue(text, out int cachedHash))
+                return cachedHash;
+
             unchecked
             {
                 int hash = 23;
                 foreach (char c in text)
                     hash = hash * 31 + c;
+
+                //UnityEngine.Debug.Log($"Caching stable hash {(ushort)hash} for {text}");
+                StableHashes[text] = hash;
                 return hash;
             }
         }