How to properly use a scoring function in Solrj

I'm trying to boost the score of a document according to the value of a floating number field from the document.

For example, the user may search for "Oliphaunts named ron, with height between 6 and 10 meters" and I'd like to query the height field in a manner akin to a DecayFunction.

In the following simplified query I expect to retrieve oliphaunts scored by name and height, where height closer to 8 meters is better -

q=name:"ron" _val_:"div(1,abs(sub(height,8)))"

I've drafted my exponential-decay scoring function using a combination of math operands and regular functions -

exp(sub(0,div(pow(max(0,sub(abs(sub(value,origin)),offset)),2),mul(2,sub(0,div(pow(scale,2),mul(2,ln(decay))))))))

Now I'd like to incorporate this function into the query score using the _val_ magic field.

How can I accomplish this in Solrj?

What other ways are there (instead of _val_) to do this?

{p.s. - I'm using the standard query parser, in Solr 5.3.1}

Solution

I ended up Accomplishing this by implementing a custom lucene.search.Query. The following is a summary of the class and its usage -

package org.example.ronvisbord.solr.queries.custom;

import org.apache.lucene.search.Query;

public class HeightFunctionQuery extends Query {

    private final String queryTemplate = "(_val_:\"%s\")^%f";
    private final String functionTemplate = "div(1,abs(sub(%s,%d)))";
    private double boost;

    @Override
    public String toString(String field) {
        return String.format(queryTemplate, createFunction(field), boost);
    }

    public HeightFunctionQuery(double boost, int targetHeight) {
        this.boost = boost;
        this.targetHeight = targetHeight;
    }

    private String createFunction(String field) {
        return String.format(functionTemplate, field, targetHeight);
    }
}

I used the class by putting its toString(field) in the "q" parameter of a solrj.SolrQuery -

import org.apache.solr.client.solrj.impl.HttpSolrClient;
import org.apache.solr.client.solrj.SolrClient;
import org.example.ronvisbord.solr.queries.custom.HeightFunctionQuery;
import org.apache.lucene.search.Query;
import org.apache.solr.client.solrj.SolrQuery;
import org.apache.solr.client.solrj.SolrRequest;
import org.apache.solr.client.solrj.response.QueryResponse;

...

    double boost = 10.0;
    double targetHeight = 8;
    String heightField = "height";

    Query heightQuery = new HeightFunctionQuery(targetHeight, boost);
    SolrQuery solrQuery = new SolrQuery();
    solrQuery.set("q", heightQuery.toString(heightField));
    // ... further configure the solrQuery ...

    SolrClient client = new HttpSolrClient("http://solr_host:8983/solr/core")
    QueryResponse response = client.query(query, SolrRequest.METHOD.POST)
    // ... process results ...