feat: registry hash method (#270)

ecsact/runtime/core.h

@@ -53,13 +53,26 @@ ECSACT_CORE_API_FN(void, ecsact_destroy_registry)
 /**
  * Creates a new registry from an existing one with all its entities and
  * components intact.
+ *
+ * If `ecsact_hash_registry` is defined then the cloned registry hash must
+ * match the original registry.
  */
 ECSACT_CORE_API_FN(ecsact_registry_id, ecsact_clone_registry)
 ( //
 	ecsact_registry_id registry,
 	const char*        registry_name
 );
 
+/**
+ * Creates a hash of current state of the registry. The algorithm is
+ * implementation defined, but must represent both user state and internal
+ * state.
+ */
+ECSACT_CORE_API_FN(uint64_t, ecsact_hash_registry)
+( //
+	ecsact_registry_id registry
+);
+
 /**
  * Destroy all entities
  */
@@ -307,6 +320,7 @@ ECSACT_CORE_API_FN(ecsact_stream_error, ecsact_stream)
 		fn(ecsact_create_registry, __VA_ARGS__);             \
 		fn(ecsact_destroy_registry, __VA_ARGS__);            \
 		fn(ecsact_clone_registry, __VA_ARGS__);              \
+		fn(ecsact_hash_registry, __VA_ARGS__);               \
 		fn(ecsact_clear_registry, __VA_ARGS__);              \
 		fn(ecsact_create_entity, __VA_ARGS__);               \
 		fn(ecsact_ensure_entity, __VA_ARGS__);               \

ecsact/runtime/core.hh

@@ -642,6 +642,10 @@ public:
 		return cloned_registry;
 	}
 
+	ECSACT_ALWAYS_INLINE auto hash() const -> uint64_t {
+		return ecsact_hash_registry(_id);
+	}
+
 	template<typename Component, typename... AssocFields>
 		requires(!std::is_empty_v<Component>)
 	ECSACT_ALWAYS_INLINE auto get_component( //