MySQL's field() function in Solr.

build.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="custom_field_jar" default="jar" basedir="." >
    <target name="jar">
        <jar basedir="./bin" jarfile="custom_field.jar" />
    </target>
</project>

CustomFieldFunction.java

package jp.co.hogehoge.function;

import java.io.IOException;
import java.util.*;
import org.apache.lucene.index.AtomicReaderContext;
import org.apache.lucene.queries.function.FunctionValues;
import org.apache.lucene.queries.function.ValueSource;
import org.apache.lucene.queries.function.docvalues.IntDocValues;

public class CustomFieldFunction extends ValueSource
{
    protected final ValueSource source;
    protected final String[] strArr;

    public CustomFieldFunction(ValueSource source, String strArr[]) {
        this.source = source;
        this.strArr = strArr;
    }

    @Override
    public boolean equals(Object o) {
        return false;
    }

    @Override
    public int hashCode() {
        return 0;
    }

    @Override
    public String description() {
        return null;
    }

    @Override
    public FunctionValues getValues(Map context, AtomicReaderContext readerContext) throws IOException {
        final FunctionValues sval = source.getValues(context, readerContext);
        final String[] haystack = strArr;

        return new IntDocValues(this) {
            @Override
            public int intVal(int doc) {
                int result = 0;
                String needles = sval.strVal(doc);
                for (int i = 0; i < haystack.length; i++) {
                    if (haystack[i].equals(needles)) {
                        result = haystack.length - i;
                        break;
                    }
                }
                return result;
            }
        };
    }
}

CustomFieldValueSourceParser.java

package jp.co.hogehoge.parser;

import java.util.*;
import jp.co.hogehoge.function.CustomFieldFunction;
import org.apache.lucene.queries.function.ValueSource;
import org.apache.solr.common.util.NamedList;
import org.apache.solr.search.FunctionQParser;
import org.apache.solr.search.SyntaxError;
import org.apache.solr.search.ValueSourceParser;

public class CustomFieldValueSourceParser extends ValueSourceParser
{
    public void init(NamedList namedList) {
    }

    public ValueSource parse(FunctionQParser fqp) throws SyntaxError {
        ValueSource source = fqp.parseValueSource();
        List<String> list = new ArrayList<String>();

        while (fqp.hasMoreArguments()) {
            list.add(fqp.parseArg());
        }
        String [] strArr = list.toArray(new String[list.size()]);

        return new CustomFieldFunction(source, strArr);
    }
}

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