update_voxels Subroutine

    subroutine update_voxels(grid, pos, celli, cellj, cellk)
    !! updates the current voxel based upon position

        use vector_class
        use gridmod
        
        !> grid
        type(cart_grid), intent(IN)    :: grid
        !> current photon packet position
        type(vector),    intent(IN)    :: pos
        !> position of photon packet in grid
        integer,         intent(INOUT) :: celli, cellj, cellk

        !accurate but slow
        ! celli = find(pos%x, grid%xface) 
        ! cellj = find(pos%y, grid%yface)
        ! cellk = find(pos%z, grid%zface) 

        !fast but can be inaccurate in some cases...
        celli = floor(grid%nxg * (pos%x) / (2. * grid%xmax)) + 1
        cellj = floor(grid%nyg * (pos%y) / (2. * grid%ymax)) + 1
        cellk = floor(grid%nzg * (pos%z) / (2. * grid%zmax)) + 1

        if(celli > grid%nxg .or. celli < 1)celli = -1
        if(cellj > grid%nyg .or. cellj < 1)cellj = -1
        if(cellk > grid%nzg .or. cellk < 1)cellk = -1

    end subroutine update_voxels