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From Avinash Lakshman <avinash.laksh...@gmail.com>
Subject Re: OPHF
Date Wed, 01 Apr 2009 21:40:17 GMT
Doing what you are suggesting scares the hell out of me for a couple of
reasons -  All work in P2P be it random/OPHF does the token handling the way
it is setup. I cannot try something that has not been well explored in
academia. I insist this must be doable. I am going to think about this more.

Avinash

On Wed, Apr 1, 2009 at 2:31 PM, Jonathan Ellis <jbellis@gmail.com> wrote:

> Avinash already commited his new order-preserving hash function and I
> missed it.  It's in OrderPreservingHashPartitioner.  It takes the
> approach that Todd and I discussed back in January: turn the string
> into a base-Char.MAX_VALUE number.
> (
> http://groups.google.com/group/cassandra-dev/browse_thread/thread/6bda8518466210e7/f53b79c19010a9ed
> ).
>  I chatted with Avinash a little on IM but we didn't finish, so I'm
> picking it up here.
>
> There are two problems with this approach.  One is that the hashes
> will only be order-preserving for a subset of unicode (all of UCS-2,
> but not all of UTF-16; see http://en.wikipedia.org/wiki/UTF-16/UCS-2).
>  The other is that this only gives you a naive ordering by code point
> value, which for unicode is not what you want and even for ascii
> sometimes you want another collation.  (see
> http://www.unicode.org/reports/tr10/ and
> http://java.sun.com/javase/6/docs/api/java/text/Collator.html).
>
> Say for instance you have inserted keys ['a', 'b', '--a', '--b'] and
> you are going to do range queries on them.  The correct
> collation-aware sort is ['a', '--a', 'b', '--b'].  But ordering by
> char value gives ['--a', '--b', 'a', 'b'].
>
> Switching to a more flexibile system like the one I wrote for
> CASSANDRA-3 will let use use Token<BigInteger> for random distribution
> or Token<String> for order-preserving, with user-defined collation.  I
> don't see a way to get this kind of flexibility from an approach that
> insists on turning everything into BigInteger.
>
> -Jonathan
>
> On Mon, Mar 30, 2009 at 2:16 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
> > Avinash,
> >
> > You mentioned that you have a new order-preserving hash function that
> > you think will be more generally useful.  Can you post it?
> >
> > thanks,
> >
> > -Jonathan
> >
>

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