Improve read performance with different tag

src/main/java/net/minestom/server/tag/Tag.java

@@ -178,6 +178,12 @@ public class Tag<T> {
         write(nbtCompound, (T) value);
     }
 
+    final boolean shareValue(@NotNull Tag<?> other) {
+        // Verify if these 2 tags can share the same cached value
+        // Key/Default value/Path are ignored
+        return this == other || this.readFunction == other.readFunction;
+    }
+
     public static @NotNull Tag<Byte> Byte(@NotNull String key) {
         return tag(key, NBTByte::getValue, NBT::Byte);
     }

src/main/java/net/minestom/server/tag/TagHandlerImpl.java

@@ -172,6 +172,12 @@ final class TagHandlerImpl implements TagHandler {
             this.tag = tag;
             this.value = value;
         }
+
+        NBT updatedNbt() {
+            NBT nbt = this.nbt;
+            if (nbt == null) this.nbt = nbt = tag.writeFunction.apply(value);
+            return nbt;
+        }
     }
 
     private record Cache(Entry<?>[] entries, NBTCompound compound) implements TagReadable {
@@ -196,6 +202,10 @@ final class TagHandlerImpl implements TagHandler {
                 }
                 if (entry.value instanceof TagHandlerImpl handler) {
                     entries = handler.entries;
+                } else if (entry.updatedNbt() instanceof NBTCompound compound) {
+                    var tmp = new TagHandlerImpl();
+                    tmp.updateContent(compound);
+                    entries = tmp.entries;
                 }
             }
         }
@@ -206,14 +216,13 @@ final class TagHandlerImpl implements TagHandler {
         if (value instanceof TagHandlerImpl)
             throw new IllegalStateException("Cannot read path-able tag " + tag.getKey());
         final Tag entryTag = entry.tag;
-        if (entryTag == tag) {
+        if (entryTag.shareValue(tag)) {
             // Tag is the same, return the value
             //noinspection unchecked
             return (T) value;
         }
         // Value must be parsed from nbt if the tag is different
-        NBT nbt = entry.nbt;
-        if (nbt == null) entry.nbt = nbt = (NBT) entryTag.writeFunction.apply(value);
+        final NBT nbt = entry.updatedNbt();
         try {
             return tag.readFunction.apply(nbt);
         } catch (ClassCastException e) {

src/test/java/net/minestom/server/tag/TagEqualityTest.java

@@ -0,0 +1,53 @@
+package net.minestom.server.tag;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Test tags that can share cached values.
+ */
+public class TagEqualityTest {
+
+    @Test
+    public void same() {
+        var tag = Tag.String("test");
+        assertTrue(tag.shareValue(tag));
+    }
+
+    @Test
+    public void similar() {
+        var tag = Tag.String("test");
+        var tag2 = Tag.String("test");
+        assertTrue(tag.shareValue(tag2));
+    }
+
+    @Test
+    public void differentDefault() {
+        var tag = Tag.String("test").defaultValue("test2");
+        var tag2 = Tag.String("test").defaultValue("test3");
+        assertTrue(tag.shareValue(tag2));
+    }
+
+    @Test
+    public void differentType() {
+        var tag = Tag.String("test");
+        var tag2 = Tag.Integer("test");
+        assertFalse(tag.shareValue(tag2));
+    }
+
+    @Test
+    public void list() {
+        var tag = Tag.String("test").list();
+        assertTrue(tag.shareValue(tag));
+    }
+
+    //@Test
+    public void similarList() {
+        // TODO make work
+        var tag = Tag.String("test").list();
+        var tag2 = Tag.String("test").list();
+        assertTrue(tag.shareValue(tag2));
+    }
+}

src/test/java/net/minestom/server/tag/TagPathTest.java

@@ -236,6 +236,7 @@ public class TagPathTest {
                   }
                 }
                 """, handler.asCompound());
+        assertEquals(5, handler.getTag(tag.path("struct")));
 
         handler.setTag(tag, 5);
         assertEqualsSNBT("""