esubi Subroutine

   pure subroutine esubi(n, m, wd, ldwd, ld2wd, alpha, tt, ldtt, i, e)
   !! Compute `e = wd + alpha*tt**2`.

      use odrpack_kinds, only: zero

      integer, intent(in) :: n
         !! Number of observations.
      integer, intent(in) :: m
         !! Number of columns of data in the independent variable.
      real(wp), intent(in) :: wd(ldwd, ld2wd, m)
         !! Squared `delta` weights.
      integer, intent(in) :: ldwd
         !! Leading dimension of array `wd`.
      integer, intent(in) :: ld2wd
         !! Second dimension of array `wd`.
      real(wp), intent(in) :: alpha
         !! Levenberg-Marquardt parameter.
      real(wp), intent(in) :: tt(ldtt, m)
         !! Scaling values used for `delta`.
      integer, intent(in) :: ldtt
         !! Leading dimension of array `tt`.
      integer, intent(in) :: i
         !! Indexing variable.
      real(wp), intent(out) :: e(m, m)
         !! Value of the array `e = wd + alpha*tt**2`.

      ! Local scalars
      integer :: j

      ! Variable Definitions (alphabetically)
      !  J:      An indexing variable.

      ! N.B. the locations of WD and TT accessed depend on the value of the first element of
      ! each array and the leading dimensions of the multiply subscripted arrays.

      if (n == 0 .or. m == 0) return

      if (wd(1, 1, 1) >= zero) then
         if (ldwd >= n) then
            ! The elements of WD have been individually specified
            if (ld2wd == 1) then
               ! The arrays stored in WD are diagonal
               e = zero
               do j = 1, m
                  e(j, j) = wd(i, 1, j)
               end do
            else
               ! The arrays stored in WD are full positive semidefinite matrices
               e = wd(i, :, :)
            end if

            if (tt(1, 1) > zero) then
               if (ldtt >= n) then
                  do j = 1, m
                     e(j, j) = e(j, j) + alpha*tt(i, j)**2
                  end do
               else
                  do j = 1, m
                     e(j, j) = e(j, j) + alpha*tt(1, j)**2
                  end do
               end if
            else
               do j = 1, m
                  e(j, j) = e(j, j) + alpha*tt(1, 1)**2
               end do
            end if
         else
            ! WD is an M by M matrix
            if (ld2wd == 1) then
               ! The array stored in WD is diagonal
               e = zero
               do j = 1, m
                  e(j, j) = wd(1, 1, j)
               end do
            else
               ! The array stored in WD is a full positive semidefinite matrix
               e = wd(1, :, :)
            end if

            if (tt(1, 1) > zero) then
               if (ldtt >= n) then
                  do j = 1, m
                     e(j, j) = e(j, j) + alpha*tt(i, j)**2
                  end do
               else
                  do j = 1, m
                     e(j, j) = e(j, j) + alpha*tt(1, j)**2
                  end do
               end if
            else
               do j = 1, m
                  e(j, j) = e(j, j) + alpha*tt(1, 1)**2
               end do
            end if
         end if
      else
         ! WD is a diagonal matrix with elements ABS(WD(1,1,1))
         e = zero
         if (tt(1, 1) > zero) then
            if (ldtt >= n) then
               do j = 1, m
                  e(j, j) = abs(wd(1, 1, 1)) + alpha*tt(i, j)**2
               end do
            else
               do j = 1, m
                  e(j, j) = abs(wd(1, 1, 1)) + alpha*tt(1, j)**2
               end do
            end if
         else
            do j = 1, m
               e(j, j) = abs(wd(1, 1, 1)) + alpha*tt(1, 1)**2
            end do
         end if
      end if

   end subroutine esubi