NextGeomBuffer_AMRController3D Subroutine

  subroutine NextGeomBuffer_AMRController3D(this,nElem,geom,slot)
    !! Select the geometry buffer to fill this epoch: whichever of the two is not currently
    !! active, so the previous epoch's geometry stays intact and readable.
    !!
    !! Each buffer is Init-ed the first time it is used and Resized on every subsequent epoch,
    !! so after the first two adaptations geometry performs no allocation at all - which is the
    !! point of the change. On the very first adaptation activeGeom is still the caller's
    !! geometry (geomSlot == 0); that object belongs to the caller and is never freed or reused
    !! here.
    implicit none
    class(AMRController3D),intent(inout) :: this
    integer,intent(in) :: nElem
    type(SEMHex),pointer,intent(out) :: geom
    integer,intent(out) :: slot

    if(this%geomSlot == 1) then
      slot = 2
    else
      slot = 1
    endif

    if(slot == 1) then
      if(.not. associated(this%geomA)) allocate(this%geomA)
      geom => this%geomA
    else
      if(.not. associated(this%geomB)) allocate(this%geomB)
      geom => this%geomB
    endif

    if(geom%nElem == 0) then
      call geom%Init(this%interp,nElem)
    else
      call geom%Resize(this%interp,nElem)
    endif

  endsubroutine NextGeomBuffer_AMRController3D