# 1 "<stdin>"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "<stdin>"
module createhelixmod
! Create helix filter lags and mis
  use helix
  use cartesian
  implicit none
  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)) then
        cycle ! ignore some locations
      end if
      nh = nh + 1 ! got another live one.
      call cart2line( nd, ii, lag(nh))
      ! get its fortran linear index
    end do
    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)
  end function
end module