Okay went to merge this and doubled check it was mentioned in the docs.
Turns out there is no collection function for this one. In looking at
implementing I noticed that NearestVisitor was artificially limited to
working against PropertyName expressions. I will prep a fix and issue a
pull request for your review.
Jody Garnett
On Mon, Mar 10, 2014 at 9:43 AM, Jody Garnett <[email protected]>wrote:
> Looks clear to me, thanks for getting the headers correct (sigh!).
>
> I had one general question about the design of JDBCAggregateFunctionTest (
> https://github.com/aaime/geotools/blob/b7f66c845acc3ee456cdc2db174e5f1578ddf41d/modules/library/jdbc/src/test/java/org/geotools/jdbc/JDBCAggregateFunctionTest.java
> ).
>
> Why does the test cases create a subclass to test (i.e. MyNearestVisitor
> extends NearestVisitor)?
>
>
>
> Jody Garnett
>
>
> On Sun, Mar 9, 2014 at 10:27 PM, Andrea Aime <[email protected]
> > wrote:
>
>> Hi,
>> in a GeoServer pull request we have an implementation a NearestVisitor
>> whose objective was to find the closest value to a given reference value
>> in
>> the domain of a feature attribute whose value was a Number, a Date or
>> String:
>>
>> https://github.com/ilkkarinne/geoserver/blob/master/src/wms/src/main/java/org/geoserver/wms/dimension/NearestVisitorFactory.java
>>
>> In essence, this is similar to a max/min visitor, and not so similar
>> instead to
>> a KNN search (find the N closest features to a given point), in that it
>> searches
>> for the closest value, not for the closest features.
>>
>> I've started backporting that code to Geotools and adding unit tests, a
>> JDBC optimized
>> implementation, and then moved to make it more general, so that it could
>> work
>> against Geometry and generally speaking Comparable objects as well, with a
>> small twist there if you want, that if we are dealing wit a Comparable
>> that's not
>> a Number or a Date, we end up with the two closest values (unless the
>> reference is
>> above or below the domain), and then we basically arbitrarily pick one.
>>
>> I did not add a String based implementation, but if we had one, it would
>> probably have to
>> use some reliable string distance (e.g. the Levenstein distance for
>> example,
>> http://en.wikipedia.org/wiki/Levenshtein_distance)
>>
>> During the implementation I've also noticed that what we are searching a
>> value in
>> the attribute domain, which is not necessarily of the same type as the
>> reference
>> value (e.g., domain made of integers, with the reference being a double)
>> so
>> had the code make sure to handle that.
>> The overall visitor code is here:
>>
>> https://github.com/aaime/geotools/blob/nearest/modules/library/main/src/main/java/org/geotools/feature/visitor/NearestVisitor.java
>>
>> The JDBC optimized implementation turns the nearest visit into two visits,
>> basically finding the closest value to the reference in the half below
>> the reference (the max in that interval),
>> and then the closest value to the reference in the half above (the min in
>> that interval) and then
>> picks the closest of the two. If the domain is indexed this should be
>> quite a bit
>> faster than scanning the whole domain, if not, it's still going to be
>> faster than loading
>> all the features from the database in the in memory visitor:
>>
>> https://github.com/geotools/geotools/pull/386/files#diff-5e45fc0686088e1fbeea65365775a1fbL678
>>
>> The pull request is here, comments welcomed:
>> https://github.com/geotools/geotools/pull/386
>>
>> Cheers
>> Andrea
>>
>> --
>> == Our support, Your Success! Visit http://opensdi.geo-solutions.it for
>> more information ==
>>
>> Ing. Andrea Aime
>> @geowolf
>> Technical Lead
>>
>> GeoSolutions S.A.S.
>> Via Poggio alle Viti 1187
>> 55054 Massarosa (LU)
>> Italy
>> phone: +39 0584 962313
>> fax: +39 0584 1660272
>> mob: +39 339 8844549
>>
>> http://www.geo-solutions.it
>> http://twitter.com/geosolutions_it
>>
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>>
>>
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>>
>
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