extract multi-features for one solr feature extractor in solr learning to rank

[Jianxiong Dong]

Hi,
    I found that solr learning-to-rank (LTR) supports only ONE feature
for a given feature extractor.

See interface:

https://github.com/apache/lucene-solr/blob/master/solr/contrib/ltr/src/java/org/apache/solr/ltr/feature/Feature.java

Line (281, 282) (in FeatureScorer)
@Override
      public abstract float score() throws IOException;

I have a user case: given a <query, doc>, I like to extract multiple
features (e.g.  100 features.  In the current framework,  I have to
define 100 features in feature.json. Also more cost for scored doc
iterations).

I would like to have an interface:

public abstract Map<String, Float> score() throws IOException;

It helps support sparse vector feature.

Can anybody provide an insight?

Thanks

Jianxiong