[NNTP] Large article numbers extension

[Clive D.W. Feather]

Thomas Gschwind said:
> It is necessary to provide an explicit renumbering table or to keep the
> old newsgroup around.  As you say if the numbers differ by a large
> constant, the implementation should be fairly simple.  As soon as the
> low and high watermark exceed 2^30, simply subtract 2^30 from all the
> articles (or 2^31 in case of unsigned article numbers).  Note the
> constant SHOULD be highest possible watermark divided by 2.

Actually, a better approach is to reduce the numbers by 2^30 when both the
watermarks exceed 2^31 (assuming a 2^32-1 maximum). This means a
wrap-around is much less likely to be confused with a reset.

However, I don't think either approach is something we should be
supporting.

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