Rename OldTypeHash, which is a confusing name

Also update comment based on research into exactly what this is.

Author: SkowronskiAndrew

Analyzer.Tests/FileDetectionTests.cs

@@ -232,8 +232,8 @@ public void TryParseMetadata_PlayerDataLevel0_ReturnsExpectedValues()
         {
             Assert.IsTrue(entry.InlineTypeTree,
                 $"InlineTypeTree should be true (persistentTypeID={entry.PersistentTypeID})");
-            Assert.IsFalse(entry.OldTypeHash.IsZero,
-                $"OldTypeHash should not be zero (persistentTypeID={entry.PersistentTypeID})");
+            Assert.IsFalse(entry.TypeTreeStructureHash.IsZero,
+                $"TypeTreeStructureHash should not be zero (persistentTypeID={entry.PersistentTypeID})");
             Assert.IsTrue(entry.TypeTreeContentHash.IsZero,
                 $"TypeTreeContentHash should be zero for version < 23 (persistentTypeID={entry.PersistentTypeID})");
             Assert.Greater(entry.TypeTreeSerializedSize, 0u,
@@ -288,8 +288,8 @@ public void TryParseMetadata_PlayerNoTypeTreeLevel1_ReturnsExpectedValues()
         {
             Assert.Greater(entry.PersistentTypeID, 0,
                 $"PersistentTypeID should be positive (got {entry.PersistentTypeID})");
-            Assert.IsFalse(entry.OldTypeHash.IsZero,
-                $"OldTypeHash should not be zero (persistentTypeID={entry.PersistentTypeID})");
+            Assert.IsFalse(entry.TypeTreeStructureHash.IsZero,
+                $"TypeTreeStructureHash should not be zero (persistentTypeID={entry.PersistentTypeID})");
             Assert.IsFalse(entry.InlineTypeTree,
                 $"InlineTypeTree should be false when EnableTypeTree=false (persistentTypeID={entry.PersistentTypeID})");
             Assert.IsTrue(entry.TypeTreeContentHash.IsZero,

Analyzer/Util/SerializedFileDetector.cs

@@ -70,19 +70,26 @@ public class TypeTreeInfo
     public UnityHash128 ScriptID { get; set; }
 
     /// <summary>
-    /// Hash of the TypeTree content as originally written into the file.
+    /// Hash of the TypeTree structure (field names, types, byte sizes, alignment flags),
+    /// computed via MD4 over the node graph. Used to detect type schema changes between
+    /// the version stored in the file and the current runtime type, and to deduplicate
+    /// type entries when writing serialized files.  Sometimes referred to as the "OldTypeHash"
+    /// because it refers to the type at the time it was serialized, which might be older than
+    /// the current type.
     /// Used for compatibility checking at load time.
     /// </summary>
-    public UnityHash128 OldTypeHash { get; set; }
+    public UnityHash128 TypeTreeStructureHash { get; set; }
 
     // -----------------------------------------------------------------------
     // TypeTree inline/extracted data (only when EnableTypeTree = true)
     // -----------------------------------------------------------------------
 
     /// <summary>
-    /// XXH3 content hash of the TypeTree blob. Stored explicitly in the metadata for
+    /// XXH3 content hash of the TypeTree blob, e.g. hash of the raw binary encoding
+    /// of the TypeTree definition. Stored explicitly in the metadata for
     /// version >= 23 (kExtractedTypeTreeSupport). IsZero == true indicates this field
     /// was not present in the metadata (version < 23 or no inline TypeTree).
+    /// This is used for TypeTree deduplication and caching.
     /// </summary>
     public UnityHash128 TypeTreeContentHash { get; set; }
 
@@ -751,7 +758,7 @@ private static TypeTreeInfo ReadTypeEntry(BinaryReader reader, uint version, boo
             info.ScriptID = BinaryFileHelper.ReadHash128(reader, swap);
 
         // oldTypeHash: always present. Hash of the TypeTree content as originally written.
-        info.OldTypeHash = BinaryFileHelper.ReadHash128(reader, swap);
+        info.TypeTreeStructureHash = BinaryFileHelper.ReadHash128(reader, swap);
 
         if (!enableTypeTree)
             return info;

Documentation/command-serialized-file.md

@@ -285,7 +285,7 @@ Each element of `typeTrees` and `serializedReferenceTypeTrees` in the JSON outpu
 | `isStrippedType` | `true` for types representing prefab-stripped objects (the `stripped` keyword in YAML). Orthogonal to TypeTree presence. |
 | `scriptTypeIndex` | Index into the file's MonoScript reference list. `-1` for native Unity types. |
 | `scriptID` | 128-bit hash (MD4 of assembly + namespace + class name) identifying the MonoScript. All-zeros when not applicable. |
-| `oldTypeHash` | Hash of the TypeTree content as originally written; used for compatibility checking at load time. |
+| `typeTreeStructureHash` | MD4 hash of the TypeTree structure as originally written; used for compatibility checking at load time. |
 | `typeTreeContentHash` | XXH3 hash of the TypeTree blob. All-zeros for files with version < 23. |
 | `typeTreeSerializedSize` | Byte size of the TypeTree blob for this entry. `0` when `inlineTypeTree` is false. |
 | `inlineTypeTree` | `true` when the TypeTree blob is present inline in the file's metadata. |