Re: d3dx8 [6/7] Implement D3DXMatrixStack_Push

> > - This->current = This->current +1;
> > - HeapReAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, This->matrix, (This->current +1) * sizeof(D3DXMATRIX) );
> > - if ( This->matrix == NULL ) return E_OUTOFMEMORY;
> Aside from being a bit suboptimal (doing a realloc on every push),
> this probably doesn't do what you want. Consider what happens to the
> size of the array when you do something like push/pop/push/pop/...etc.
>
> It would be better to keep track of the current stack size, and then
> grow it by a factor (eg. 2 or 1.5) if a push would overflow the
> current stack. You could also consider shrinking the array again if a
> pop would make it use less than a certain percentage of the current
> size (eg. a third).
>
> You should also assign the result of HeapReAlloc() to This->matrix
> again. Although it's quite possible for HeapReAlloc to just grow the
> current block without changing its location, there's no guarantee it
> will. The NULL check is useless this way.

Just a little addition, I think we should resize the stack differently at different situations, i.e.:
- resize it to (This->current+3) if This->currect<5 (something around that is the minimum number of matrices for which anyone would even bother using a MatrixStack)
- resize it to (This->current*2) if This->current<12 (when it seems like the application's gonna push many matrices on the stack)
- resize it to (This->current+16) if This->current>=12 (when *2 could waste memory)
However, I don't know if these numbers are optimal, it's just a suggestion.

For the Pop function, I agree that you should free some memory when it's unused, but my suggestion from above fits here, too. However, one third is okay, too, as it isn't that relevant here.