[ldapext] VLV: absolute position of the list (negative offsets revisited)

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

VLV currently allows the list to be positioned (in addition
to other means) by an absolute offset from the head of the
list.  That, the client can send byoffset.contentCount=0
and byoffset.offset of some offset into the list.  This
allows the client to select the Nth entry from the beginning
of the list.

An application which offers a paged interface to the use
can use this feature to offer direct navigation to the
Nth page (from the head of the list) even where the
current content count is unknown (or unstable)

I presume there are other uses of absolute positioning.

Just as useful would be support direct navigation to the
Nth to Last Page even where the current content count is
unknown (or unstable).

Currently to select the Nth to Last Page, the client has
to reverse the list order, obtain the Nth Page, and reverse
the entries in the page.  This is expensive.

The VLV operation should allow the client to select the Nth
entry from either end of the list in a natural way.

The positive (head) and negative (tail) offsets are a common
approach used in GUI list view widgets.  Since VLV's design
is based upon the list view metaphor, I think it natural to
modify VLV to support both positive and negative offsets.

It has been argued by some that absolute positioning is not
useful.  If consensus is that absolute positioning is not
useful, then I would propose that VLV not standardize an
absolute positioning mechanism.  However, if consensus is
that absolute positioning is useful, I believe VLV should
support both absolute position from either end of the list.

That is, neither or both.  I prefer both.

Kurt

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