LUCENE-7493: FacetCollector.search now accepts limit=0, for getting facets but not search hits

lucene/CHANGES.txt

@@ -89,6 +89,9 @@ Bug Fixes
 * LUCENE-6914: Fixed DecimalDigitFilter in case of supplementary code points.
   (Hossman)
 
+* LUCENE-7493: FacetCollector.search threw an unexpected exception if
+  you asked for zero hits but wanted facets (Mahesh via Mike McCandless)
+
 Improvements
 
 * LUCENE-7439: FuzzyQuery now matches all terms within the specified

lucene/facet/src/java/org/apache/lucene/facet/FacetsCollector.java

@@ -36,6 +36,7 @@ import org.apache.lucene.search.TopDocsCollector;
 import org.apache.lucene.search.TopFieldCollector;
 import org.apache.lucene.search.TopFieldDocs;
 import org.apache.lucene.search.TopScoreDocCollector;
+import org.apache.lucene.search.TotalHitCountCollector;
 import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.BitDocIdSet;
 import org.apache.lucene.util.FixedBitSet;
@@ -251,6 +252,12 @@ public class FacetsCollector extends SimpleCollector implements Collector {
                                          + after.doc + " limit=" + limit);
     }
 
+    TopDocs topDocs = null;
+    if (n==0) {
+      TotalHitCountCollector totalHitCountCollector = new TotalHitCountCollector();
+      searcher.search(q, MultiCollector.wrap(totalHitCountCollector, fc));
+      topDocs = new TopDocs(totalHitCountCollector.getTotalHits(), new ScoreDoc[0], Float.NaN);
+    } else {
       TopDocsCollector<?> hitsCollector;
       if (sort != null) {
         if (after != null && !(after instanceof FieldDoc)) {
@@ -268,6 +275,9 @@ public class FacetsCollector extends SimpleCollector implements Collector {
         hitsCollector = TopScoreDocCollector.create(n, after);
       }
       searcher.search(q, MultiCollector.wrap(hitsCollector, fc));
-    return hitsCollector.topDocs();
+    
+      topDocs = hitsCollector.topDocs();
+    }
+    return topDocs;
   }
 }

lucene/facet/src/test/org/apache/lucene/facet/TestDrillDownQuery.java

@@ -182,6 +182,17 @@ public class TestDrillDownQuery extends FacetTestCase {
     assertEquals(10, docs.totalHits);
   }
   
+  public void testZeroLimit() throws IOException {
+    IndexSearcher searcher = newSearcher(reader);
+    DrillDownQuery q = new DrillDownQuery(config);
+    q.add("b", "1");
+    int limit = 0;
+    FacetsCollector facetCollector = new FacetsCollector();
+    FacetsCollector.search(searcher, q, limit, facetCollector);
+    Facets facets = getTaxonomyFacetCounts(taxo, config, facetCollector, config.getDimConfig("b").indexFieldName);
+    assertNotNull(facets.getTopChildren(10, "b"));
+  }
+  
   public void testScoring() throws IOException {
     // verify that drill-down queries do not modify scores
     IndexSearcher searcher = newSearcher(reader);