!+
! Subroutine em_field_custom (ele, param, s_rel, orbit, local_ref_frame, field, calc_dfield, err_flag)
!
! Routine for handling custom (user supplied) EM fields.
! This routine is called when ele%field_calc = custom$ or when ele is a custom element (ele%key = custom$)
! In order to be used, this stub file must be modified appropriately. See the Bmad manual for more details.
!
! Note: Unlike all other elements, "s_rel" and "here" areguments for a patch element are with respect to 
! the exit reference frame of the element. See the Bmad manual for more details.
!
! Note: Fields should not have any unphysical discontinuities. 
! Discontinuities may cause Runge-Kutta integration to fail resulting in particles getting marked as "lost".
! The mode of failure here is that RK will try smaller and smaller steps to integrate through the 
! discontinuity until the step size gets lower than bmad_com%min_ds_adaptive_tracking. At this
! point the particle gets marked as lost.
!
! General rule: Your code may NOT modify any argument that is not listed as
! an output agument below.
!
! Input:
!   ele         -- Ele_struct: Custom element.
!   param       -- lat_param_struct: Lattice parameters.
!   s_rel       -- Real(rp): Longitudinal position relative to the start of the element.
!   orbit       -- Coord_struct: Coords with respect to the reference particle.
!   local_ref_frame 
!               -- Logical, If True then take the 
!                     input coordinates and output fields as being with 
!                     respect to the frame of referene of the element. 
!   calc_dfield -- Logical, optional: If present and True then the field 
!                     derivative matrix is wanted by the calling program.
!
! Output:
!   field    -- Em_field_struct: Structure hoding the field values.
!   err_flag -- Logical, optional: Set true if there is an error. False otherwise.
!-

recursive subroutine em_field_custom (ele, param, s_rel, orbit, local_ref_frame, field, calc_dfield, err_flag, &
                                       calc_potential, use_overlap, grid_allow_s_out_of_bounds, rf_time, used_eles)

use em_field_mod, except_dummy2 => em_field_custom

implicit none

type (ele_struct), target :: ele
type (lat_param_struct) param
type (coord_struct), intent(in) :: orbit
type (em_field_struct) :: field
type (ele_pointer_struct), allocatable, optional :: used_eles(:)

real(rp), intent(in) :: s_rel
real(rp) f
logical local_ref_frame
real(rp), optional :: rf_time
logical, optional :: err_flag, grid_allow_s_out_of_bounds
logical, optional :: calc_dfield, calc_potential, use_overlap
character(*), parameter :: r_name = 'em_field_custom'

!

if (s_rel < -1 .or. s_rel > 10) then
  if (present(err_flag)) err_flag = .true.
  call out_io (s_fatal$, r_name, 'OUT OF RANGE!')
  call err_exit
endif

f = -0.001 * (1 + s_rel)
field%e = 0
field%b = [f*orbit%vec(3), -f*orbit%vec(1), 100*f*orbit%vec(1)]

if (present(err_flag)) err_flag = .false.

end subroutine