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            天行健 君子當自強而不息

            Working with Maps and Levels(6)

            cTrigger::add_trigger

            add_trigger is the heart of all the other functions that add a trigger. This function
            allocates a sTrigger structure, sets its type, identification number, and enable flag,
            and then links the structure into the linked list of triggers. Once you allocate your
            program using the add_trigger function, the program can fill in the returned sTrigger
            structure with the coordinates, radius, height, or whatever other information the
            trigger needs to have defined.

            Keeping in mind that the add_trigger function allocates only a sTrigger structure and
            fills it with the minimal data as just mentioned.

             

            cTrigger::add_sphere, cTrigger::add_box, cTrigger::add_cylinder, cTrigger::add_triangle

            This group of functions adds a trigger of a specific type to the linked list of triggers.
            Each function has its own list of arguments to use for creation (you can
            check the comments preceding each function to see what each argument does).
            Regardless of the type of trigger, each function first calls the add_trigger function
            to get a sTrigger structure with which to work.

            Let’s start with the add_sphere function, which takes, in addition to the trigger’s identification
            number and default enabled state (as each of the four functions here
            do), the sphere’s radius and the X-, Y-, and Z-coordinates for the sphere.

            Short and to the point, the add_sphere function calls on the add_trigger function to
            allocate and link in a sTrigger structure to the linked list. Once created, the sTrigger
            structure instance is filled with the sphere trigger’s coordinates and radius.

            add_box, add_cylinder, and add_triangle operate in much the same way the add_sphere function
            does. The add_box function takes the identification number and default enabled
            state, as well as the coordinates for the opposing corners of the box.

            The add_cylinder function uses the lower-middle coordinates of the cylinder, the
            radius, and height for the trigger.

            Wrapping up the bunch is add_triangle, which takes the three pairs of X- and Zcoordinates
            that define each of the triangle’s three corners. The Y-coordinate to
            use for those three corners, as well as the height of the triangular trigger shape,
            follows.

            NOTE
            All functions that use a radius as an argument square the value when it is stored in the structure.This speeds
            up distance checks later on. How does the trigger class speed up distance checks? A standard distance check
            uses a sqrt call to calculate the correct distance. Tossing out the sqrt speeds up the engine, but then
            you must square the values to match the distance.

             

            cTrigger::remove and cTrigger::free

            These two functions remove triggers from the linked list by referring to the identification
            number of the trigger to remove in the remove function or by allowing the
            class to remove all triggers in the list using the free function.

            The remove function operates by scanning the entire linked list—for each trigger
            that shares the same identification number as the number passed in the ID argument,
            the remove function removes the structure from the linked list and frees the
            structure’s memory:

            void cTrigger::remove(long id)
            {
                sTrigger* trigger = m_root_trigger;

                
            // scan through list of triggers
                while(trigger != NULL)
                {
                    sTrigger* next_trigger = trigger->next;

                    
            if(trigger->id == id)
                    {
                        
            // remove from list

                        
            if(trigger->prev)
                            trigger->prev->next = trigger->next;
                        
            else
                            m_root_trigger = trigger->next;

                        
            if(trigger->next)
                            trigger->next->prev = trigger->prev;

                        trigger->next = NULL;
                        delete trigger;

                        m_num_triggers--;
                    }

                    trigger = next_trigger;
                }
            }

            At this point, the linked list of sTrigger structures is being scanned. Now you store
            a pointer to the next structure in the linked list and check the currently iterated
            sTrigger structure for a match in the identification number being removed.

            Once it is determined that a structure needs to be removed, the code
            alters the linked list’s pointers and releases the structure’s memory resources.

            From this point on, the number of triggers stored in the linked list is reduced and the
            loop that scans for structures to remove continues until all structures are scanned.

            Whereas the remove function removes triggers according to their identification numbers,
            the free function can skip all the hoopla and delete the entire linked list in
            one fell swoop.


            posted on 2007-12-10 10:58 lovedday 閱讀(173) 評論(0)  編輯 收藏 引用

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