Fix multiple damage instances in certain areas during explosions

Client/multiplayer_sa/CMultiplayerSA.cpp

@@ -1595,6 +1595,7 @@ void CMultiplayerSA::InitHooks()
     InitHooks_ObjectStreamerOptimization();
 
     InitHooks_Postprocess();
+    InitHooks_Explosions();
 }
 
 // Used to store copied pointers for explosions in the FxSystem

Client/multiplayer_sa/CMultiplayerSA.h

@@ -81,6 +81,7 @@ class CMultiplayerSA : public CMultiplayer
     void                InitHooks_ObjectStreamerOptimization();
     void                InitHooks_Postprocess();
     void                InitHooks_DeviceSelection();
+    void                InitHooks_Explosions();
     CRemoteDataStorage* CreateRemoteDataStorage();
     void                DestroyRemoteDataStorage(CRemoteDataStorage* pData);
     void                AddRemoteDataStorage(CPlayerPed* pPed, CRemoteDataStorage* pData);

Client/multiplayer_sa/CMultiplayerSA_Explosions.cpp

@@ -0,0 +1,78 @@
+/*****************************************************************************
+ *
+ *  PROJECT:     Multi Theft Auto
+ *  LICENSE:     See LICENSE in the top level directory
+ *  FILE:        multiplayer_sa/CMultiplayerSA_Explosions.cpp
+ *
+ *  Multi Theft Auto is available from https://www.multitheftauto.com/
+ *
+ *****************************************************************************/
+#include "StdInc.h"
+
+//////////////////////////////////////////////////////////////////////////////////////////
+//
+// CWorld::TriggerExplosion
+//
+// Fix for multiple damage instances in certain areas during explosions (GH #4125, #997)
+//
+//////////////////////////////////////////////////////////////////////////////////////////
+#define HOOKPOS_CWorld_TriggerExplosion  0x56B82E
+#define HOOKSIZE_CWorld_TriggerExplosion 8
+static constexpr std::uintptr_t RETURN_CWorld_TriggerExplosion = 0x56B836;
+static void _declspec(naked) HOOK_CWorld_TriggerExplosion()
+{
+    _asm
+    {
+        mov [esp+1Ch-8h], eax
+        mov [esp+1Ch-10h], ecx
+
+        // Call SetNextScanCode
+        mov ecx, 0x4072E0
+        call ecx
+        mov ecx, esi
+
+        // SetNextScanCode overwrote the result of the cmp instruction at 0x56B82A
+        // so we call it again
+        cmp esi, eax
+        jmp RETURN_CWorld_TriggerExplosion
+    }
+}
+
+#define HOOKPOS_CWorld_TriggerExplosionSectorList  0x5677F4
+#define HOOKSIZE_CWorld_TriggerExplosionSectorList 7
+static constexpr std::uintptr_t RETURN_CWorld_TriggerExplosionSectorList = 0x5677FB;
+static constexpr std::uintptr_t SKIP_CWorld_TriggerExplosionSectorList = 0x568473;
+static void _declspec(naked) HOOK_CWorld_TriggerExplosionSectorList()
+{
+    _asm
+    {
+        // check entity->m_nScanCode == CWorld::ms_nCurrentScanCode
+        mov ecx, dword ptr ds:[0xB7CD78]
+        cmp [esi+2Ch], ecx
+        jz skip
+
+        // set entity current scan code
+        mov [esi+2Ch], ecx
+
+        mov al, [esi+36h]
+        and al, 7
+        cmp al, 4
+        jmp RETURN_CWorld_TriggerExplosionSectorList
+
+        skip:
+        jmp SKIP_CWorld_TriggerExplosionSectorList
+    }
+}
+
+//////////////////////////////////////////////////////////////////////////////////////////
+//
+// CMultiplayerSA::InitHooks_Explosions
+//
+// Setup hooks
+//
+//////////////////////////////////////////////////////////////////////////////////////////
+void CMultiplayerSA::InitHooks_Explosions()
+{
+    EZHookInstall(CWorld_TriggerExplosion);
+    EZHookInstall(CWorld_TriggerExplosionSectorList);
+}