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From: Junio C Hamano <junkio@cox.net>
Subject: Re: Gitk feature - show nearby tags
Date: Sun, 04 Jun 2006 02:21:39 -0700
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To: "Marco Costalba" <mcostalba@gmail.com>
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	(Marco Costalba's message of "Sat, 3 Jun 2006 13:16:45 +0200")
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"Marco Costalba" <mcostalba@gmail.com> writes:

> As example given a selected revision with id <sha> is it possible to
> do something like this to fond the ancestor?
>
> 1) get the tag list with git-peek-remote or something similar if tags
> are not already loaded
>
> 2) given the tagList vector with n elements run
>
>    git-rev-list  --topo-order <sha> ^tagList[0]  ^tagList[1]   ....
>  ^tagList[n-1]
>
> 3) take the last sha spit out by git-rev-list, be it <lastSha>.
>
> 4) Previous nearest tag is the parent of lastSha

Sorry, I do not understand what you are doing here at all.
Suppose you have this simple history.

  (root)

         a---b---d---e---f---g
         t1   \ /        t3        
            ---c
               t2

and <sha1> in (2) is "e".  When tagList = (t1, t2, t3), the
above rev-list would return empty.  For tagList = (t1, t2), the
rev-list would return "e", and then "d".  Parent of "d" are "b"
and "c", and c is tagged but b is not.  So if you are willing to
try 2^N permutations of what tagList to use, then the above
sequence, with a bit of tweak to step (4) to see which parents
are tagged, would yield the closest tag in the past of "e", but
is that what you are suggesting?