dlib C++ Library - matrix_default_mul.h

    // Copyright (C) 2008  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_MATRIx_DEFAULT_MULTIPLY_
    #define DLIB_MATRIx_DEFAULT_MULTIPLY_

    #include "../geometry/rectangle.h"
    #include "matrix.h"
    #include "matrix_utilities.h"
    #include "../enable_if.h"

    namespace dlib
    {

    // ------------------------------------------------------------------------------------

        namespace ma
        {
            template < typename EXP, typename enable = void >
            struct matrix_is_vector { static const bool value = false; };
            template < typename EXP >
            struct matrix_is_vector<EXP, typename enable_if_c<EXP::NR==1 || EXP::NC==1>::type > { static const bool value = true; };
        }

    // ------------------------------------------------------------------------------------

        /*!  This file defines the default_matrix_multiply() function.  It is a function
             that conforms to the following definition:

            template <
                typename matrix_dest_type,
                typename EXP1,
                typename EXP2
                >
            void default_matrix_multiply (
                matrix_dest_type& dest,
                const EXP1& lhs,
                const EXP2& rhs
            );
                requires
                    - (lhs*rhs).destructively_aliases(dest) == false
                    - dest.nr() == (lhs*rhs).nr()
                    - dest.nc() == (lhs*rhs).nc()
                ensures
                    - #dest == dest + lhs*rhs
        !*/

    // ------------------------------------------------------------------------------------

        template <
            typename matrix_dest_type,
            typename EXP1,
            typename EXP2
            >
        typename enable_if_c<ma::matrix_is_vector<EXP1>::value == true || ma::matrix_is_vector<EXP2>::value == true>::type
        default_matrix_multiply (
            matrix_dest_type& dest,
            const EXP1& lhs,
            const EXP2& rhs
        )
        {
            matrix_assign_default(dest, lhs*rhs, 1, true);
        }

    // ------------------------------------------------------------------------------------

        template <
            typename matrix_dest_type,
            typename EXP1,
            typename EXP2
            >
        typename enable_if_c<ma::matrix_is_vector<EXP1>::value == false && ma::matrix_is_vector<EXP2>::value == false>::type
        default_matrix_multiply (
            matrix_dest_type& dest,
            const EXP1& lhs,
            const EXP2& rhs
        )
        {
            const long bs = 90;

            // if the matrices are small enough then just use the simple multiply algorithm
            if (lhs.nc() <= 2 || rhs.nc() <= 2 || lhs.nr() <= 2 || rhs.nr() <= 2 || (lhs.size() <= bs*10 && rhs.size() <= bs*10) )
            {
                matrix_assign_default(dest, lhs*rhs, 1, true);
            }
            else
            {
                // if the lhs and rhs matrices are big enough we should use a cache friendly
                // algorithm that computes the matrix multiply in blocks.

                // Loop over all the blocks in the lhs matrix
                for (long r = 0; r < lhs.nr(); r+=bs)
                {
                    for (long c = 0; c < lhs.nc(); c+=bs)
                    {
                        // make a rect for the block from lhs
                        rectangle lhs_block(c, r, std::min(c+bs-1,lhs.nc()-1), std::min(r+bs-1,lhs.nr()-1));

                        // now loop over all the rhs blocks we have to multiply with the current lhs block
                        for (long i = 0; i < rhs.nc(); i += bs)
                        {
                            // make a rect for the block from rhs
                            rectangle rhs_block(i, c, std::min(i+bs-1,rhs.nc()-1), std::min(c+bs-1,rhs.nr()-1));

                            // make a target rect in res
                            rectangle res_block(rhs_block.left(),lhs_block.top(), rhs_block.right(), lhs_block.bottom());

                            // This loop is optimized assuming that the data is laid out in
                            // row major order in memory.
                            for (long r = lhs_block.top(); r <= lhs_block.bottom(); ++r)
                            {
                                for (long c = lhs_block.left(); c<= lhs_block.right(); ++c)
                                {
                                    const typename EXP2::type temp = lhs(r,c);
                                    for (long i = rhs_block.left(); i <= rhs_block.right(); ++i)
                                    {
                                        dest(r,i) += rhs(c,i)*temp;
                                    }
                                }
                            }
                        }
                    }
                }
            }

        }

    // ------------------------------------------------------------------------------------

    }

    #endif // DLIB_MATRIx_DEFAULT_MULTIPLY_