incubator-cassandra-user mailing list archives

Site index · List index
Message view
Top
Subject Re: Order of the cyclic group of hashed partitioners
Date Thu, 06 Sep 2012 14:32:37 GMT
```> I meant that what the OP spotted was it's an inclusive maximum <=

That's it Tim, you understood what I meant. Thank you for taking the time
to consider my question.
0 <= hash < (2**127) corresponds to the cyclic group Z/(2**127) so the
maximum is (2**127)-1
So there is indeed a mix up in sources between the order -- 2**127 -- and
the maximum  -- (2**127)-1

But I don't agree with you when you say:
> The code does exclusive comparisons and excludes the value 0 though.

CMIIW but 0 is allowed:
if (i.compareTo(ZERO) < 0) // if i is negative. 0 is OK.
throw ...

The same goes for 2**127:
if (i.compareTo(MAXIMUM) > 0) // if i is greater than MAXIMUM. MAXIMUM is
OK.
throw ...

Exclusive bounds would be: "compareTo(...) == 0".

IMHO the maximum const should be:
public static final BigInteger MAXIMUM = new
BigInteger("2").pow(127).subtract(BigInteger.ONE);

And the correct comment should be:
throw ...("Token must be < 2**127");

Are you agree?

Romain

Tim Wintle <timwintle@gmail.com> a écrit sur 06/09/2012 08:41:27 :

> On Wed, 2012-09-05 at 13:23 +1200, aaron morton wrote:
> > > I believe the question is why is the maximum 2**127 and not
> > > 0xffffffffffffffff
>
> oops - I got the wrong number of digits there.
>
> > The maximum is the size of the digest created by MD5.
>
> (I may be mistaken) - isn't the range of MD5 values
> 0 <= hash < (2**128)
> ?
>
> If you're dropping one bit to store as a signed integer to give 127 bits
> of entropy then it would be in the range:
>
> 0 <= hash < (2**127)
>
> but the range being checked is:
>
> 0 <= hash <= (2**127)
>
> > Does that answer the question?
>
> I meant that what the OP spotted was it's an inclusive maximum <=
>
> 0 <= hash <= 2**127 gives (2**127) + 1 different values, and is
> mathematically the clock-arithmetic (cyclic) group:
>     Z/(2**127 + 1)   [0]
>
>
> I _believe_ the issue is actually the other way around in
> AbstractHashedPartitioner (upper and lower bounds are exclusive) - but
>
> i.e. both the code and the comments have off-by-one errors.
>
>
> {{{
>
> if (i.compareTo(ZERO) < 0)
>     throw new ConfigurationException("Token must be >= 0");
> if (i.compareTo(MAXIMUM) > 0)
>     throw new ConfigurationException("Token must be <= 2**127");
> }}}
>
> The comments imply that 0 and 2**127 are both valid tokens (which they
> shouldn't be).
>
> The code does exclusive comparisons and excludes the value 0 though.
>
> Tim
> >
> [0] I believe the OP mistyped that as Z/(127+1)
>
> >
> >
> > -----------------
> > Aaron Morton
> > Freelance Developer
> > @aaronmorton
> > http://www.thelastpickle.com
> >
> > On 3/09/2012, at 8:20 PM, Tim Wintle <timwintle@gmail.com> wrote:
> >
> > > On Tue, 2012-08-28 at 16:57 +1200, aaron morton wrote:
> > > > Sorry I don't understand your question.
> > > >
> > > > Can you explain it a bit more or maybe someone else knows.
> > >
> > > I believe the question is why is the maximum 2**127 and not
> > > 0xffffffffffffffff
> > >
> > > Tim
> > >
> > > >
> > > > Cheers
> > > >
> > > > -----------------
> > > > Aaron Morton
> > > > Freelance Developer
> > > > @aaronmorton
> > > > http://www.thelastpickle.com
> > > >
> > > > On 27/08/2012, at 7:16 PM, Romain HARDOUIN
> > > > <romain.hardouin@urssaf.fr> wrote:
> > > >
> > > > >
> > > > > Thank you Aaron.
> > > > > This limit was pushed down in RandomPartitioner but the question
> > > > > still exists...
> > > > >
> > > > >
> > > > > aaron morton <aaron@thelastpickle.com> a écrit sur 26/08/2012
> > > > > 23:35:50 :
> > > > >
> > > > > > > AbstractHashedPartitioner
> > > > > > does not exist in the trunk.
> > > > > > https://git-wip-us.apache.org/repos/asf?p=cassandra.git;
> > > > > > a=commitdiff;h=a89ef1ffd4cd2ee39a2751f37044dba3015d72f1
> > > > > >
> > > > > >
> > > > > > Cheers
> > > > > >
> > > > > > -----------------
> > > > > > Aaron Morton
> > > > > > Freelance Developer
> > > > > > @aaronmorton
> > > > > > http://www.thelastpickle.com
> > > > > >
> > > > > > On 24/08/2012, at 10:51 PM, Romain HARDOUIN
> > > > > > <romain.hardouin@urssaf.fr> wrote:
> > > > > >
> > > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > AbstractHashedPartitioner defines a maximum of 2**127 hence
> > > > > > > an
> > > > > > order of (2**127)+1.
> > > > > > > I'd say that tokens of such partitioners are intented to
be
> > > > > > distributed in Z/(127), hence a maximum of (2**127)-1.
> > > > > > > Could there be a mix up between maximum and order?
> > > > > > > This is a detail but could someone confirm/invalidate?

> > > > > > >
> > > > > > > Regards,
> > > > > > >
> > > > > > > Romain
> > > > > >
> > > >
> > >
> > >
> >
> >
>
>

```
Mime
View raw message