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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: OPHF
Date Wed, 01 Apr 2009 21:43:38 GMT
For that aspect no difference between a String ring based on compareTo
and a BigInteger one.  The only difference (and it is an important one
for the reasons I gave!) is how the compare works.  But for the p2p
aspect it does not matter.

On Wed, Apr 1, 2009 at 3:40 PM, Avinash Lakshman
<avinash.lakshman@gmail.com> wrote:
> 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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