Major Version update to reflect Symbol packing and unpacking

Author: Asmod4n

README.md

@@ -4,7 +4,7 @@ mruby-simplemsgpack
 Breaking changes
 ================
 
-Starting with Release 2.0 only mruby-3 is supported, if you are on an older version check out a commit from before 2021.
+Starting with Version 4, Symbol packing changed, see Symbol Handling (Updated) down below.
 
 Installation
 ============
@@ -120,24 +120,39 @@ lazy.at_pointer("/0/name/foo")
 # => TypeError (cannot navigate into a string)
 ```
 
-Extension Types
----------------
+Symbol Handling (Updated)
+-------------------------
+MessagePack for mruby now provides a **built‑in, user‑configurable symbol packing strategy**.
+This replaces the old pattern where users had to register extension types manually for symbols.
 
-To customize how objects are packed, define an [extension type](https://github.com/msgpack/msgpack/blob/master/spec.md#types-extension-type).
+You can choose between three strategies:
+
+- **`:raw`** (default)
+  Symbols are packed as normal MessagePack strings.
+
+- **`:string`**
+  Symbols are packed as an extension type containing their string name.
+
+- **`:int`**
+  Symbols are packed as an extension type containing their internal symbol ID.
 
-By default, MessagePack packs symbols as strings and does not convert them
-back when unpacking them. Symbols can be preserved by registering an extension
-type for them:
+Configure the strategy globally:
 
 ```ruby
-sym_ext_type = 0
-MessagePack.register_pack_type(sym_ext_type, Symbol) { |symbol| symbol.to_s }
-MessagePack.register_unpack_type(sym_ext_type) { |data| data.to_sym }
+# Pack symbols as raw strings (default)
+MessagePack.sym_strategy(:raw)
 
-MessagePack.unpack(:symbol.to_msgpack) # => :symbol
+# Pack symbols as ext with their string name
+MessagePack.sym_strategy(:string, 1)
+
+# Pack symbols as ext with their integer ID
+MessagePack.sym_strategy(:int, 1)
 ```
 
-Other objects like classes can also be preserved:
+Extension Types
+---------------
+
+To customize how objects are packed, define an [extension type](https://github.com/msgpack/msgpack/blob/master/spec.md#types-extension-type).
 
 ```ruby
 cls_ext_type = 1

mrblib/version.rb

@@ -1,3 +1,3 @@
 module MessagePack
-    VERSION="3.1"
+    VERSION="4.0"
 end

src/mrb_msgpack.cpp

@@ -502,9 +502,10 @@ mrb_msgpack_register_pack_type(mrb_state* mrb, mrb_value self)
     mrb_raise(mrb, E_TYPE_ERROR, "not a block");
   }
 
+  mrb_msgpack_ctx* ctx = MRB_MSGPACK_CONTEXT(mrb);
   if (mrb_class_ptr(mrb_class) == mrb->symbol_class) {
     mrb_raise(mrb, E_ARGUMENT_ERROR,
-              "cannot register ext packer for Symbols");
+              "cannot register ext packer for Symbols, use the new MessagePack.sym_strategy function.");
   }
 
   ext_packers = mrb_const_get(mrb, self, MRB_SYM(_ExtPackers));
@@ -517,6 +518,7 @@ mrb_msgpack_register_pack_type(mrb_state* mrb, mrb_value self)
   return mrb_nil_value();
 }
 
+
 static mrb_value
 mrb_msgpack_ext_packer_registered(mrb_state *mrb, mrb_value self)
 {
@@ -930,6 +932,16 @@ mrb_msgpack_register_unpack_type(mrb_state* mrb, mrb_value self)
     mrb_raise(mrb, E_TYPE_ERROR, "not a block");
   }
 
+  mrb_msgpack_ctx* ctx = MRB_MSGPACK_CONTEXT(mrb);
+
+  // If the user is using an ext-based symbol strategy,
+  // forbid overriding the symbol ext type.
+  if (ctx->sym_unpacker != nullptr && type == ctx->ext_type) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR,
+      "cannot register ext unpacker for Symbols, use MessagePack.sym_strategy instead.");
+  }
+
+  // Otherwise: safe to register
   mrb_hash_set(mrb,
                mrb_const_get(mrb, self, MRB_SYM(_ExtUnpackers)),
                mrb_int_value(mrb, type),
@@ -938,6 +950,7 @@ mrb_msgpack_register_unpack_type(mrb_state* mrb, mrb_value self)
   return mrb_nil_value();
 }
 
+
 static mrb_value
 mrb_msgpack_ext_unpacker_registered(mrb_state *mrb, mrb_value self)
 {