#$=head1 NAME
#$
#$conv - convolve helix filters
#$
#$=head1 SYNOPSIS
#$
#$C<ss=conv(aa,bb)>
#$
#$=head1 INPUT PARAMETERS
#$
#$=over 4
#$
#$=item aa,bb - type(filter) 
#$
#$      Helix filters
#$
#$=back
#$
#$=head1 RETURN VALUE
#$
#$=over 4
#$
#$=item  ss - type(filter)   
#$
#$       Result of helix filter convolution
#$
#$=back
#$
#$=head1 DESCRIPTION
#$
#$Convolves two helix filters and returns result
#$
#$=head1 SEE ALSO
#$
#$L<helix>,L<compress>,L<helicon>
#$
#$=head1 LIBRARY
#$
#$B<geef90>
#$
#$=cut
module conv {
  use helix
  use compress
contains
  function conv1( aa, bb) result ( ss) {
    type( filter) :: aa, bb, ss
    integer       :: na, nb, ns, ia, ib, i	
    na = size (aa%lag)
    nb = size (bb%lag)
    ns = maxval (aa%lag) + maxval (bb%lag)
    call allocatehelix (ss, ns)
    ss%lag = (/ (i, i = 1, ns) /)     
    do ia = 1, na { 
       i = aa%lag (ia)      
       ss%flt (i) = aa%flt (ia)
    }
    do ib = 1, nb { 
       i = bb%lag (ib)      
       ss%flt (i) = ss%flt (i) + bb%flt (ib)
    }    
    do ia = 1, na {	
    do ib = 1, nb {	
        i = aa%lag (ia) + bb%lag (ib) 
        ss%flt (i) = ss%flt (i) + aa%flt (ia) * bb%flt (ib)
    }}
    call resize (ss)
  }
}