RebuildLeaves_QuadTreeMesh2D Subroutine

  subroutine RebuildLeaves_QuadTreeMesh2D(this)
    !! Recompute the active leaf set by depth-first traversal from the roots. Traversal (rather
    !! than a scan of all nodes) is what makes orphaned nodes left behind by coarsening invisible.
    implicit none
    class(QuadTreeMesh2D),intent(inout) :: this
    ! Local
    integer :: r,n

    ! First pass: count leaves.
    n = 0
    do r = 1,this%nRoots
      call count_leaves(r,n)
    enddo
    this%nLeaves = n
    if(allocated(this%leaf)) deallocate(this%leaf)
    allocate(this%leaf(1:max(n,1)))
    this%leaf = 0

    ! Second pass: collect leaves in root-major DFS order.
    n = 0
    do r = 1,this%nRoots
      call collect_leaves(r,n)
    enddo

  contains

    recursive subroutine count_leaves(node,cnt)
      integer,intent(in) :: node
      integer,intent(inout) :: cnt
      integer :: k
      if(this%child(1,node) == 0) then
        cnt = cnt+1
      else
        do k = 1,4
          call count_leaves(this%child(k,node),cnt)
        enddo
      endif
    endsubroutine count_leaves

    recursive subroutine collect_leaves(node,idx)
      integer,intent(in) :: node
      integer,intent(inout) :: idx
      integer :: k
      if(this%child(1,node) == 0) then
        idx = idx+1
        this%leaf(idx) = node
      else
        do k = 1,4
          call collect_leaves(this%child(k,node),idx)
        enddo
      endif
    endsubroutine collect_leaves

  endsubroutine RebuildLeaves_QuadTreeMesh2D