PlanWindows Subroutine

  subroutine PlanWindows(plan,nRanks,newOffset,winFirst,winLast)
    !! For every rank r, the contiguous window [winFirst(r),winLast(r)] of GLOBAL old element
    !! indices that rank r's new element range references through the transfer plan. The plan is
    !! rank-replicated, so this is a purely local computation: no communication is needed to
    !! learn what a peer wants, which is what makes the point-to-point migration cheap here.
    !!
    !! A rank owning no new elements gets the empty sentinel winFirst > winLast.
    !!
    !! Correctness does not depend on the leaf ordering being monotone in the old ordering: the
    !! window is the min/max hull of the referenced indices, so a non-monotone ordering only
    !! widens it, the worst case being the whole old element list - i.e. exactly what the v1
    !! gather-then-slice migration moves. Efficiency, not correctness, rests on the
    !! space-filling-curve locality.
    !!
    !! Cost is ONE pass over the plan, not nRanks x nNew: the rank loop partitions 1..nNew.
    implicit none
    type(TransferPlan3D),intent(in) :: plan
    integer,intent(in) :: nRanks
    integer,intent(in) :: newOffset(1:nRanks+1)
    integer,intent(out) :: winFirst(1:nRanks)
    integer,intent(out) :: winLast(1:nRanks)
    ! Local
    integer :: r,li,c,src

    do r = 1,nRanks
      winFirst(r) = plan%nOld+1
      winLast(r) = 0
      do li = newOffset(r)+1,newOffset(r+1)
        if(plan%sourceKind(li) == SELF_TRANSFER_RESTRICT) then
          do c = 1,8
            src = plan%family(c,li)
            winFirst(r) = min(winFirst(r),src)
            winLast(r) = max(winLast(r),src)
          enddo
        else
          src = plan%sourceElem(li)
          winFirst(r) = min(winFirst(r),src)
          winLast(r) = max(winLast(r),src)
        endif
      enddo
    enddo

  endsubroutine PlanWindows