calculate_jend Subroutine

  subroutine calculate_jend(jend)
    use theta_grid, only: ntgrid, bmag
    use mp, only: mp_abort
    implicit none
    integer, dimension(-ntgrid:), intent(out) :: jend
    integer :: ig, il

    jend = 0

    ! jend(ig) is total # of valid al grid points at each theta value

    !CMR, 1/11/2013:
    ! Above, with no trapped particles, we set: jend(ig)=   0
    ! Here, with trapped particles, we set:     jend(ig)=  il
    !  where il is the lambda index of the trapped particle bouncing at theta(ig)

    ! Note it might be better to set jend(ig) > nlambda if we have no
    ! trapped particles so il <= jend(ig) is always true for a particle
    ! which isn't forbidden. This would require a change in logic elsewhere

    ! Exit now if there are no trapped particles
    if (.not. grid_has_trapped_particles()) return

    do ig = -ntgrid, ntgrid
       ! We could initialise jend to ng2 and start this loop at ng2 + 1
       ! as we assume that all lambda up to and including with ng2 satisfy
       ! 1-lambda*bmag > -bouncefuzz for all theta. This is enforced/checked
       ! in [[calculate_forbidden_region]].
       do il = 1, nlambda
          if (1.0 - al(il)*bmag(ig) > -bouncefuzz) jend(ig) = jend(ig) + 1
       end do
    end do
  end subroutine calculate_jend