Implement D3DXMatrixStack_Push

[H. Verbeet]

On 09/04/2008, David Adam <david.adam.cnrs at gmail.com> wrote:
> This would increase the size of the stack exponentially. Would it be better
> to do this:
>
> Take a stack_size_reference (for instance 32 items)
> When This->current =32, then increase the size of the stack of
> stack_size_reference (that is 64 items now)
> Then when This->current is =64, again increase the size of the stack of
> stack_size_reference (that is 98 items now), and so on....
>
>
> This would increase the size of the stack linearly instead of exponentially.
>
Growing exponentially is intended. If HeapReAlloc can't simply grow
the current block, it has to copy the contents over to a new memory
location. If the stack becomes larger, the relative time it costs to
do the copy becomes more and more expensive compared to the actual
push. Growing by a factor instead of by a constant size keeps the
amortized running time for a series of pushes constant independent of
the the array size. (Not that I expect a matrix stack to become large
enough for this to matter a whole lot, but that's the reasoning.)

> > You should also assign the result of HeapReAlloc() to This->matrix
> > again.
>
> I thought that
>
>  HeapReAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, This->matrix, ..........);
>
>
> do exactly what you say
It can't do that. HeapReAlloc takes a pointer to a memory location, so
it can change the memory at that location, but it can't change the
pointer itself.