Insertion by one sometimes gives drammatically better results than

components/omnibox/browser/url_index_private_data.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/omnibox/browser/url_index_private_data.h"
 
 #include <stdint.h>
 
 #include <functional>
 #include <iterator>
@@ skipping 572 matching lines @@
       return word_list_[word_id].find(term) == base::string16::npos;
     });
   } else {
     word_id_set = WordIDSetForTermChars(Char16SetFromString16(term));
   }
 
   // If any words resulted then we can compose a set of history IDs by unioning
   // the sets from each word.
   // We use |buffer| because it's more efficient to collect everything and then
   // construct a flat_set than to insert elements one by one.
+
+#define BUFFER 1
+#define BY_ONE 2
+#define MERGE 3
+
+#define METHOD MERGE
+
+#if METHOD == BUFFER
+
   HistoryIDVector buffer;
   for (WordID word_id : word_id_set) {
     WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
     if (word_iter != word_id_history_map_.end()) {
       HistoryIDSet& word_history_id_set(word_iter->second);
       buffer.insert(buffer.end(), word_history_id_set.begin(),
                     word_history_id_set.end());
     }
   }
   HistoryIDSet history_id_set(buffer.begin(), buffer.end(),
                               base::KEEP_FIRST_OF_DUPES);
 
+#elif METHOD == BY_ONE
+
+  HistoryIDSet history_id_set;
+
+  for (WordID word_id : word_id_set) {
+    WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
+    if (word_iter != word_id_history_map_.end()) {
+      HistoryIDSet& word_history_id_set(word_iter->second);
+      history_id_set.insert_by_one(word_history_id_set.begin(),
+                                   word_history_id_set.end());
+    }
+  }
+
+#else  // MERGE
+
+  HistoryIDSet history_id_set;
+
+  for (WordID word_id : word_id_set) {
+    WordIDHistoryMap::iterator word_iter = word_id_history_map_.find(word_id);
+    if (word_iter != word_id_history_map_.end()) {
+      HistoryIDSet& word_history_id_set(word_iter->second);
+      history_id_set.insert_merge(word_history_id_set.begin(),
+                                  word_history_id_set.end());
+    }
+  }
+
+#endif
+
   // Record a new cache entry for this word if the term is longer than
   // a single character.
   if (term_length > 1)
     search_term_cache_[term] = SearchTermCacheItem(word_id_set, history_id_set);
 
   return history_id_set;
 }
 
 WordIDSet URLIndexPrivateData::WordIDSetForTermChars(
     const Char16Set& term_chars) {
@@ skipping 651 matching lines @@
   // First cut: typed count, visit count, recency.
   // TODO(mrossetti): This is too simplistic. Consider an approach which ranks
   // recently visited (within the last 12/24 hours) as highly important. Get
   // input from mpearson.
   if (r1.typed_count() != r2.typed_count())
     return (r1.typed_count() > r2.typed_count());
   if (r1.visit_count() != r2.visit_count())
     return (r1.visit_count() > r2.visit_count());
   return (r1.last_visit() > r2.last_visit());
 }