Re: [boost] [rfc] A light weight statically typed matrix template class (again?)

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-12-07 12:05:02

On 12/06/07 18:40, Larry Evans wrote:
[snip]
> Then, the rank of array_fix_size would be Shape::size.
> You could calculate of strides using some mpl metafunction on
> Shape:
> 
>          typedef
>        some-mpl-function
>        < Shape
>        >::type
>      strides
>      ;
> 
>   and then allocate storage as:
> 
>       Value
>     values
>       [ mpl::at_c<strides,Strides::size>::value
>       ]
>     ;
> 
> Then operator[] could be defined as:
> 
>      Value&
>    operator[](boost::array<unsigned,Shape::size>const& indices)
>    {
>         return values[some_offset_function_<strides>(indices)];
>    }
> 
> where some_offset_function simply performs a dot product of the
> compile time vector, strides, with the runtime vector, indices.
Andreas,

There actually some code to illustrate this in the fold_seq_test.zip
file at:

http://www.boost-consulting.com/vault/index.php?&directory=Template%20Metaprogramming

It's in the test_array_index_offset function.  The fold_seq.hpp file 
includes several other files on my local disk and I could upload those 
if you're interested, but I think the test_array_index_offset code
illustrates the general idea.  Hm... No actually it differs from
what's suggested in my previous post.  Instead of that, there would
be something like:

     typedef
   unsigned
index_type
;
   struct
inner_product_static_dynamic_ftor
{
     explicit inner_product_static_dynamic_ftor
       ( index_type const* indices
       )
     : cur_index_ptr(indices)
     , offset(0)
     {}
       index_type const*
     cur_index_ptr
     ;
       index_type
     offset
     ;
     template<class PartialProductElement>
     void operator()(PartialProductElement)const
     {
         offset+=PartialProductElement::value*(*cur_index_ptr);
         ++cur_index_ptr;
     }
};
   template
   < class Strides
   >
   index_type
offset_at_indices
   ( array<index_type,mpl::size<Strides>::value>const& a
   )
{
     index_type const*indices=a.data();
     inner_product_static_dynamic_ftor ip(indices);
     typedef mpl::pop_back<Strides>::type strides;//rm total array size.
     mpl::for_each<strides>(ip);
     return ip.offset;
}

*WARNING*: not compiled.