ddot3.f

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c Copyright (C) 2009-2018  VZLU Prague, a.s., Czech Republic
c
c Author: Jaroslav Hajek <highegg@gmail.com>
c
c This file is part of Octave.
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c
      subroutine ddot3(m,n,k,a,b,c)
c purpose:      a 3-dimensional dot product.
c               c = sum (a .* b, 2), where a and b are 3d arrays.
c arguments:
c m,n,k (in)    the dimensions of a and b
c a,b (in)      double prec. input arrays of size (m,k,n)
c c (out)       double prec. output array, size (m,n)
      integer m,n,k,i,j,l
      double precision a(m,k,n),b(m,k,n)
      double precision c(m,n)

      double precision ddot
      external ddot


c quick return if possible.
      if (m <= 0 .or. n <= 0) return

      if (m == 1) then
c the column-major case.
        do j = 1,n
          c(1,j) = ddot(k,a(1,1,j),1,b(1,1,j),1)
        end do
      else
c We prefer performance here, because that's what we generally
c do by default in reduction functions. Besides, the accuracy
c of xDOT is questionable. Hence, do a cache-aligned nested loop.
        do j = 1,n
          do i = 1,m
            c(i,j) = 0d0
          end do
          do l = 1,k
            do i = 1,m
              c(i,j) = c(i,j) + a(i,l,j)*b(i,l,j)
            end do
          end do
        end do
      end if

      end subroutine