VuFind browse changes for Solr 6

Leech.java

import org.apache.lucene.store.*;
import org.apache.lucene.index.*;
import org.apache.lucene.search.*;
import java.io.*;
import java.util.*;

public class Leech
{
    protected CompositeReader reader;
    protected IndexSearcher searcher;

    protected List<LeafReaderContext> leafReaders;

    private String field;

    TermsEnum tenum = null;

    public Leech (String indexPath,
                  String field) throws Exception
    {
        // Solr6: Open our composite reader (a top-level DirectoryReader that
        // contains one reader per segment in our index).
        reader = DirectoryReader.open (FSDirectory.open (new File (indexPath).toPath ()));

        // Solr6: Open the searcher that we'll use to verify that terms are
        // being used by a non-deleted document.
        searcher = new IndexSearcher (reader);

        // Solr6: Extract the list of readers for our underlying segments.
        // We'll work through these one at a time until we've consumed them all.
        leafReaders = new ArrayList<>(reader.getContext().leaves());

        this.field = field;
    }


    public byte[] buildSortKey (String heading)
    {
        try {
            return heading.getBytes("UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        }
    }


    public void dropOff () throws IOException
    {
        reader.close ();
    }


    private boolean termExists (String t)
    {
        try {
            return (this.searcher.search (new ConstantScoreQuery(new TermQuery (new Term(this.field, t))),
                    1).totalHits > 0);
        } catch (IOException e) {
            return false;
        }
    }


    public String next () throws Exception
    {
        if (tenum == null && leafReaders.isEmpty()) {
            // Nothing left to do
            return null;
        }

        if (tenum == null) {
            // Solr6: Select our next LeafReader to work from
            LeafReader ir = leafReaders.remove(0).reader();
            Terms terms = ir.terms(this.field);

            if (terms == null) {
                // Solr6: Try the next reader
                return next();
            }

            tenum = terms.iterator();
        }

        if (tenum.next() != null) {
            String termText = tenum.term().utf8ToString();

            if (termExists(termText)) {
                return termText;
            } else {
                return this.next();
            }
        } else {
            // Solr6: Exhausted this reader
            tenum = null;
            return null;
        }
    }
}