module createhelixmod {                      # Create helix filter lags and mis
use helix
use cartesian
contains
  function createhelix( nd, center, gap, na)  result( aa) {
    type( filter)                     :: aa       # needed by helicon.
    integer, dimension(:), intent(in) :: nd, na   # data and filter axes
    integer, dimension(:), intent(in) :: center   # normally (na1/2,na2/2,...,1)
    integer, dimension(:), intent(in) :: gap      # normally ( 0,   0,  0,...,0)
    integer, dimension( size( nd))    :: ii       # cartesian indexes
    integer                           :: na123, ia, ndim, nh, lag0a,lag0d
    integer, dimension(:), allocatable:: lag
           nh= 0;   na123 = product( na);   ndim = size( nd)
    allocate(  lag( na123 ) )		  # filter cube size
    call cart2line ( na, center, lag0a)   # lag0a = index pointing to the "1.0"
    do ia = 1+lag0a, na123 {              #    ia  is fortran linear index.
       call line2cart( na, ia, ii)        # ii(ia) is fortran array  indices.
       if( any( ii <= gap))    next       # ignore some locations
       nh = nh + 1                        # got another live one.
       call cart2line( nd, ii, lag(nh))	  # get its fortran linear index
       }
    call cart2line(  nd, center, lag0d)   # lag0d is center shift for nd_cube
    call allocatehelix( aa, nh)		  # nh becomes size of filter on helix.
    aa%lag = lag(1:nh) - lag0d;		  # lag = fortran_linear_index - center
    aa%flt = 0.0;             deallocate( lag)
    }      
}