[tor-dev] tor's definition of 'median'

[Virgil Griffith]

Is there some implementation-specific reason not to use the standard
mathematical definition of "median"?  If not, I propose changing the
implementation to become it.

-V

On Tue, Aug 11, 2015 at 2:44 AM Nick Mathewson <nickm at alum.mit.edu> wrote:

> On Mon, Aug 10, 2015 at 1:11 PM, nusenu <nusenu at openmailbox.org> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA512
> >
> > Hi,
> >
> > https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2028
> >
> >> If 3 or more authorities provide a Measured= keyword for a router,
> >> the authorities produce a consensus containing a "w" Bandwidth=
> >> keyword equal to the median of the Measured= votes.
> >
> > a random sample from recent votes:
> >
> > grep 37.59.38.117 -A 3 *|grep Measured
> > w Bandwidth=6869 Measured=7570
> > w Bandwidth=6869 Measured=15500
> > w Bandwidth=6869 Measured=18100
> > w Bandwidth=6869 Measured=30500
> >
> > Tor says the median value is
> > 15500
> >
> > 2015-08-10-16-00-00-consensus:
> > w Bandwidth=15500
> >
> > but the median of these 4 values is actually:
> > (18100+15500)/2 = 16800
> > no?
> >
> > Has tor a different definition of 'median' and simply takes always the
> > second ordered measurement vote out of 4 votes or is there a bug in
> > the spec or implementation?
>
> There's one misplaced throwaway sentence in dir-spec.txt:
>
> "  All ties in computing medians are broken in favor of the smaller or
>    earlier item.
> "
>
> We should bring this, and probably other things, into a "definitions"
> section earlier in dir-spec.txt.  Patches welcome. ;)
>
> --
> Nick
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