The problem comes when your float value is encoded into that 8 bit
field norm, the 3 length and 4 length both become the same 8 bit
value. Call Similarity.encodeNorm on the values you calculate for the
different numbers of terms and make sure they return different byte
values.
Andrew
On 4/5/07, Otis Gospodnetic <[EMAIL PROTECTED]> wrote:
As far as I know, this is the case where you want your custom Similarity that
knows how to deal with a small number of terms.
public float lengthNorm(String fieldName, int numTerms) {
if (numTerms < N)
// return something smart
return (float)(1.0 / Math.sqrt(numTerms));
}
I think the rest of what you said is correct. Look at this piece of Similarity
javadoc:
* However the resulted <i>norm</i> value is [EMAIL PROTECTED]
#encodeNorm(float) encoded} as a single byte
* before being stored.
* At search time, the norm byte value is read from the index
* [EMAIL PROTECTED] org.apache.lucene.store.Directory directory} and
* [EMAIL PROTECTED] #decodeNorm(byte) decoded} back to a float
<i>norm</i> value.
* This encoding/decoding, while reducing index size, comes with the price
of
* precision loss - it is not guaranteed that decode(encode(x)) = x.
* For instance, decode(encode(0.89)) = 0.75.
* Also notice that search time is too late to modify this <i>norm</i>
part of scoring, e.g. by
* using a different [EMAIL PROTECTED] Similarity} for search.
If you come up with a more generic lengthNorm that dels with "overly short"
documents/fields well, I'd love to know! :)
Otis
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Simpy -- http://www.simpy.com/ - Tag - Search - Share
----- Original Message ----
From: John Kleven <[EMAIL PROTECTED]>
To: java-user@lucene.apache.org
Sent: Thursday, April 5, 2007 1:45:34 PM
Subject: Re: short documents = help me tweak Similarity??
Sorry to re-post -- is this the correct forum for questions like this? I
think that writing a new encode/decode operation should help alleviate my
problem, but thought that this must be fairly widespread issue for people
using lucene for "non-web-page" searches (i.e., shorter documents)
Thanks again,
John
On 4/2/07, John Kleven <[EMAIL PROTECTED]> wrote:
>
> My documents are cars...
> i.e.,
> Nissan Altima Sports Package
> Nissan Altima Standard
>
> The problem I have is when i search "Nissan Altima", I want to get the 2nd
> hit back first, i.e. "Nissan Altima Standard", because it is shorter.
> However, this doesn't happen. They are both scored the exact same.
>
> I know that the lengthNorm in Similarity is using 1/sqrt(numTerms), and
> you would think that would be enuff to make sure the order is correct.
> However, it is not, and I assume this is because of the encode/decode
> functions that pack this value into a single byte do not have the
> granularity to represent differences between numbers like 1/sqrt(3) vs
> 1/sqrt(4)??
>
> Is the suggested approach here to re-write the encode/decode operations,
> or is there any easier way?
>
> Thanks kindly -
> John
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