Variable Size Multi Dimensional Array Casting

[Tankz]

Hi all.

This is my first post so before I start begging for help let me first say that I am very grateful to the people who contribute and created this site. It has been invaluable to me for quick and easy reference. So thank you, hugely.

Now to my "problem". I am working with a game engine library, and for the most part everything is great. I just have a small issue that I would ideally like to fix. The library does many things, one of which is supply Game Map data.

int s_x, int s_y;
char* mapData=(char*)MapData(s_x,s_y);

s_x and s_y are output vars which receive the dimensions of the 2D map (W x H), each map element is comprised of 8 bytes, s_x of these form 1 row and s_y rows form the entire data block.

At the moment I can access byte (n) of the element at a particular row (y) and column (x) like this:
mapData[ (s_x*y*8) + (x*8) + n ]

What would be very useful to me is if I could access an element like this:
mapData[y][x][n]
So far I have not been able to find any information on casting the pointer to a multidimsional array type. I can do it if the map size is always the same using a typedef but I cannot work out or find out how to cast to variable sizes or if it's even possible.

Many thanks for any help.
Marc.

[hamsterman]

It is possible to build such structure which would work as you want it. If you wanted a 2d array, you would first need an array of pointers to the beginning of each row. If you took a pointer to that array, you could apply [] twice. With a 3d array, of course, you would need another layer of arrays (should I explain more?). However this requires some work, gives no benefit to the resulting application and might be hard to debug. It would be better to simply write

char get_tile(int x, int y, int n) {
   return mapData[ (s_x*y*8) + (x*8) + n ];
}

[Tankz]

Thanks for the reply. The function you posted is pretty much how am am doing things already. I was concerned that having 3 multiplications in the element retrieval would be less efficient than a 3d array. Map elements are used frequently in the application. Since the elements are 8 bytes long i can use a left 3 bit shift instead of *8 which might reduce overhead, but I was hoping a 3d array situation would be a lot faster.

I could reduce complexity by using a 2d array of UINT64 types i guess.

[hamsterman]

The compiler should optimize *8 to <<3 for you. Also, each [] result in a pointer being dereferenced and memory being read. I can't say how that compares to a multiplication. I suppose that depends on many things.
But that doesn't matter. This is a trivial procedure, it takes negligible amount of time. I don't think this is a place where you should look for ways to optimize.