#$
#$=head1 NAME
#$
#$hconest - convolution using helix filters, adjoint is filter
#$
#$=head1 SYNOPSIS
#$
#$Initializer - C<call hconest_init(x,aa)>
#$
#$Operator    - C<ierr=hconest_lop(adj,add,xx,yy)>
#$
#$=head1 PARAMETERS
#$
#$=over 4
#$
#$=item x  - C<real(:)>   
#$
#$      data
#$
#$=item aa - type(filter)  
#$
#$      helix filter to perform convolution with
#$
#$=item adj,add,xx,yy - 
#$
#$      standard operators parameters
#$
#$=back
#$
#$=head1 DESCRIPTION
#$
#$  Masked helix convolution, adjoint is the filter$
#$
#$=head1 SEE ALSO
#$
#$L<helix>,L<nhconest>,L<helicon>,L<polydiv>
#$
#$=head1 LIBRARY
#$
#$B<geef90>
#$
#$=cut
module hconest {             # masked helix convolution, adjoint is the filter.
use helix
  real, dimension (:), pointer :: x
  type( filter)                :: aa
#% _init( x, aa)
#% _lop( a, y)
    integer  ia, ix, iy
    do ia = 1, size( a) {
         do iy = 1  + aa%lag( ia), size( y) {    if( aa%mis( iy)) cycle  
            ix = iy - aa%lag( ia)

             if( adj)    a( ia) +=  y( iy) * x( ix)
             else        y( iy) +=  a( ia) * x( ix)
         }
    }
}