Re: Geometry and spatial indexes, my opinion

>> In VLSI layout we sometimes represent the layer as the z axis of a 3D
>> coordinate system.  Because there are only tens of layers we can use
>> something smaller than int to store it.  However, the compiler will
>> often pad it back to word aligned addressing and insert padding bytes
>> into a data structure, reducing the memory savings.  Also,
internally,
>> all arithmetic is 32 bit or greater, so there is no advantage in
using
>> smaller data types for local operations.  I think it is perfectly
>> reasonable to allow point classes to have heterogeneous coordinate
>> types, but require them to cast to and from a homogeneous coordinate
>> type at the interface between that object and the geometry library.
In
>> all three examples, we would make the coordinate type of the
interface
>> between the point and the library int and allow the 16 bit value to
cast
>> up to 32 bits when it is read into an algorithm and back down to 16
bits
>> when it is written out.

--Michael Fawcett wrote:
>I don't understand why the interface or algorithm cares whether it's
>homogeneous or heterogeneous.

The interface or the algorithm must necessarily be much more complex to
accommodate heterogeneous coordinates.  Moreover, you have a real
problem with casting when you do arithmetic between heterogeneous
coordinates in an algorithm.

x_coordinate_type x;
y_coordinate_type y;
?_coordinate_type result = x+y;

You would probably have to register a type for casting to:

x_coordinate_type x;
y_coordinate_type y;
general_coordinate_type result = (general_coordinate_type)x +
(general_coordinate_type)y;

Which is exactly what you do if you cast to the general coordinate type
in the interface between the algorithm and the data type.

Luke
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