Rework HSM transitions bookkeeping

Fixes a potential rare hash collision situation, and allows additional
attributes to be stored with the transition data in future.

hsm/limbo_hsm.cpp

@@ -104,19 +104,31 @@ void LimboHSM::add_transition(LimboState *p_from_state, LimboState *p_to_state,
 	ERR_FAIL_COND_MSG(p_to_state->get_parent() != this, "LimboHSM: Unable to add a transition to a state that is not an immediate child of mine.");
 	ERR_FAIL_COND_MSG(p_event == StringName(), "LimboHSM: Failed to add transition due to empty event string.");
 
-	uint64_t key = _get_transition_key(p_from_state, p_event);
+	TransitionKey key = Transition::make_key(p_from_state, p_event);
-	transitions[key] = Object::cast_to<LimboState>(p_to_state);
+	ERR_FAIL_COND_MSG(transitions.has(key), "LimboHSM: Unable to add another transition with the same event and origin.");
+	// Note: Explicit casting needed for GDExtension.
+	transitions[key] = { p_from_state != nullptr ? ObjectID(p_from_state->get_instance_id()) : ObjectID(), ObjectID(p_to_state->get_instance_id()), p_event };
 }
 
 void LimboHSM::remove_transition(LimboState *p_from_state, const StringName &p_event) {
 	ERR_FAIL_COND_MSG(p_from_state != nullptr && p_from_state->get_parent() != this, "LimboHSM: Unable to remove a transition from a state that is not an immediate child of mine.");
 	ERR_FAIL_COND_MSG(p_event == StringName(), "LimboHSM: Unable to remove a transition due to empty event string.");
 
-	uint64_t key = _get_transition_key(p_from_state, p_event);
+	TransitionKey key = Transition::make_key(p_from_state, p_event);
 	ERR_FAIL_COND_MSG(!transitions.has(key), "LimboHSM: Unable to remove a transition that does not exist.");
 	transitions.erase(key);
 }
 
+void LimboHSM::get_transition(LimboState *p_from_state, const StringName &p_event, Transition &r_transition) const {
+	ERR_FAIL_COND_MSG(p_from_state != nullptr && p_from_state->get_parent() != this, "LimboHSM: Unable to get a transition from a state that is not an immediate child of this HSM.");
+	ERR_FAIL_COND_MSG(p_event == StringName(), "LimboHSM: Unable to get a transition with an empty event string.");
+
+	TransitionKey key = Transition::make_key(p_from_state, p_event);
+	if (transitions.has(key)) {
+		r_transition = transitions[key];
+	}
+}
+
 LimboState *LimboHSM::get_leaf_state() const {
 	LimboHSM *hsm = const_cast<LimboHSM *>(this);
 	while (hsm->active_state != nullptr && hsm->active_state->is_class("LimboHSM")) {
@@ -148,16 +160,18 @@ bool LimboHSM::_dispatch(const StringName &p_event, const Variant &p_cargo) {
 	}
 
 	if (!event_consumed && active_state) {
-		uint64_t key = _get_transition_key(active_state, p_event);
 		LimboState *to_state = nullptr;
-		if (transitions.has(key)) {
+
-			to_state = transitions[key];
+		Transition transition;
+		get_transition(active_state, p_event, transition);
+		if (transition.is_valid()) {
+			to_state = Object::cast_to<LimboState>(ObjectDB::get_instance(transition.to_state));
 		}
 		if (to_state == nullptr) {
 			// Get ANYSTATE transition.
-			key = _get_transition_key(nullptr, p_event);
+			get_transition(nullptr, p_event, transition);
-			if (transitions.has(key)) {
+			if (transition.is_valid()) {
-				to_state = transitions[key];
+				to_state = Object::cast_to<LimboState>(ObjectDB::get_instance(transition.to_state));
 				if (to_state == active_state) {
 					// Transitions to self are not allowed with ANYSTATE.
 					to_state = nullptr;

hsm/limbo_hsm.h

@@ -14,6 +14,16 @@
 
 #include "limbo_state.h"
 
+#define TransitionKey Pair<uint64_t, StringName>
+
+struct TransitionKeyHasher {
+	static uint32_t hash(const TransitionKey &P) {
+		uint64_t h1 = HashMapHasherDefault::hash(P.first);
+		uint64_t h2 = HashMapHasherDefault::hash(P.second);
+		return hash_one_uint64((h1 << 32) | h2);
+	}
+};
+
 class LimboHSM : public LimboState {
 	GDCLASS(LimboHSM, LimboState);
 
@@ -25,22 +35,28 @@ public:
 	};
 
 private:
+	struct Transition {
+		ObjectID from_state;
+		ObjectID to_state;
+		StringName event;
+
+		inline bool is_valid() const { return to_state != ObjectID(); }
+
+		static _FORCE_INLINE_ TransitionKey make_key(LimboState *p_from_state, const StringName &p_event) {
+			return TransitionKey(
+					p_from_state != nullptr ? uint64_t(p_from_state->get_instance_id()) : 0,
+					p_event);
+		}
+	};
+
 	UpdateMode update_mode;
 	LimboState *initial_state;
 	LimboState *active_state;
 	LimboState *previous_active;
 	LimboState *next_active;
-	HashMap<uint64_t, LimboState *> transitions;
 	bool updating = false;
 
-	_FORCE_INLINE_ uint64_t _get_transition_key(LimboState *p_from_state, const StringName &p_event) {
+	HashMap<TransitionKey, Transition, TransitionKeyHasher> transitions;
-		uint64_t key = hash_djb2_one_64(Variant::OBJECT);
-		if (p_from_state != nullptr) {
-			key = hash_djb2_one_64(hash_one_uint64(hash_make_uint64_t(p_from_state)), key);
-		}
-		key = hash_djb2_one_64(p_event.hash(), key);
-		return key;
-	}
 
 protected:
 	static void _bind_methods();
@@ -75,6 +91,7 @@ public:
 
 	void add_transition(LimboState *p_from_state, LimboState *p_to_state, const StringName &p_event);
 	void remove_transition(LimboState *p_from_state, const StringName &p_event);
+	void get_transition(LimboState *p_from_state, const StringName &p_event, Transition &r_transition) const;
 
 	LimboState *anystate() const { return nullptr; }