fix: correct relative offsets in getfield and setfield polyfills

Author: YoRyan

src/binding-factory.ts

@@ -29,7 +29,11 @@ const luaBindings: Record<string, luaBindingFactoryFunc> = {
             },
             lua_getfield: function(L: LuaState, index: number, k: string) {
                 (this as Lua).lua_pushstring(L, k);
-                return (this as Lua).lua_gettable(L, index);
+
+                // If the index is a relative offset, it needs to be corrected for the grown stack
+                const isOffset = index < 0 && index !== LUA_GLOBALSINDEX_50;
+
+                return (this as Lua).lua_gettable(L, isOffset ? index - 1 : index);
             },
             lua_setfield: function(L: LuaState, index: number, k: string) {
                 // The value to set is expected to be on the top of the stack
@@ -38,14 +42,12 @@ const luaBindings: Record<string, luaBindingFactoryFunc> = {
                 (this as Lua).lua_pushstring(L, k);
 
                 // Swap key and value because settable expects stack in that order
-                
-                // Copy value to top of stack
-                (this as Lua).lua_pushvalue(L, -2);
+                (this as Lua).lua_insert(L, -2);
 
-                // Remove original value from stack
-                (this as Lua).lua_remove(L, -3);
-    
-                const result = (this as Lua).lua_settable(L, index);
+                // If the index is a relative offset, it needs to be corrected for the grown stack
+                const isOffset = index < 0 && index !== LUA_GLOBALSINDEX_50;
+
+                const result = (this as Lua).lua_settable(L, isOffset ? index - 1 : index);
 
                 return result;
             },