Improve highlighting perf (a bit) by reusing some constructs across hits, closes elastic#1513.

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/search/fetch/FetchPhase.java

@@ -202,7 +202,7 @@ public void execute(SearchContext context) {
             for (FetchSubPhase fetchSubPhase : fetchSubPhases) {
                 FetchSubPhase.HitContext hitContext = new FetchSubPhase.HitContext();
                 if (fetchSubPhase.hitExecutionNeeded(context)) {
-                    hitContext.reset(searchHit, subReader, subDoc, doc);
+                    hitContext.reset(searchHit, subReader, subDoc, context.searcher().getIndexReader(), docId, doc);
                     fetchSubPhase.hitExecute(context, hitContext);
                 }
             }

modules/elasticsearch/src/main/java/org/elasticsearch/search/fetch/FetchSubPhase.java

@@ -22,6 +22,7 @@
 import org.apache.lucene.document.Document;
 import org.apache.lucene.index.IndexReader;
 import org.elasticsearch.ElasticSearchException;
+import org.elasticsearch.common.collect.Maps;
 import org.elasticsearch.search.SearchParseElement;
 import org.elasticsearch.search.internal.InternalSearchHit;
 import org.elasticsearch.search.internal.SearchContext;
@@ -35,14 +36,19 @@ public interface FetchSubPhase {
 
     public static class HitContext {
         private InternalSearchHit hit;
+        private IndexReader topLevelReader;
+        private int topLevelDocId;
         private IndexReader reader;
         private int docId;
         private Document doc;
+        private Map<String, Object> cache;
 
-        public void reset(InternalSearchHit hit, IndexReader reader, int docId, Document doc) {
+        public void reset(InternalSearchHit hit, IndexReader reader, int docId, IndexReader topLevelReader, int topLevelDocId, Document doc) {
             this.hit = hit;
             this.reader = reader;
             this.docId = docId;
+            this.topLevelReader = topLevelReader;
+            this.topLevelDocId = topLevelDocId;
             this.doc = doc;
         }
 
@@ -58,9 +64,24 @@ public int docId() {
             return docId;
         }
 
+        public IndexReader topLevelReader() {
+            return topLevelReader;
+        }
+
+        public int topLevelDocId() {
+            return topLevelDocId;
+        }
+
         public Document doc() {
             return doc;
         }
+
+        public Map<String, Object> cache() {
+            if (cache == null) {
+                cache = Maps.newHashMap();
+            }
+            return cache;
+        }
     }
 
     Map<String, ? extends SearchParseElement> parseElements();