Omnibox UI Experiments: Add flag to change max autocomplete matches.

components/omnibox/browser/autocomplete_result.cc

 // Copyright 2014 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/autocomplete_result.h"
 
 #include <algorithm>
 #include <iterator>
 
 #include "base/command_line.h"
 #include "base/logging.h"
+#include "base/metrics/field_trial_params.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/metrics/proto/omnibox_event.pb.h"
 #include "components/metrics/proto/omnibox_input_type.pb.h"
 #include "components/omnibox/browser/autocomplete_input.h"
 #include "components/omnibox/browser/autocomplete_match.h"
 #include "components/omnibox/browser/autocomplete_provider.h"
 #include "components/omnibox/browser/match_compare.h"
 #include "components/omnibox/browser/omnibox_field_trial.h"
 #include "components/omnibox/browser/omnibox_switches.h"
 #include "components/url_formatter/url_fixer.h"
 
 // static
-const size_t AutocompleteResult::kMaxMatches = 6;
+size_t AutocompleteResult::GetMaxMatches() {
+  return base::GetFieldTrialParamByFeatureAsInt(
+      omnibox::kUIExperimentMaxAutocompleteMatches,
+      OmniboxFieldTrial::kUIMaxAutocompleteMatchesParam, 6);

[Peter Kasting, 2017/05/10 22:08:05]

Nit: For clarity, pull this 6 out as a named constexpr

[tommycli, 2017/05/10 22:44:29]

Done.

+}
 
 void AutocompleteResult::Selection::Clear() {
   destination_url = GURL();
   provider_affinity = NULL;
   is_history_what_you_typed_match = false;
 }
 
 AutocompleteResult::AutocompleteResult() {
   // Reserve space for the max number of matches we'll show.
-  matches_.reserve(kMaxMatches);
+  matches_.reserve(GetMaxMatches());
 
   // It's probably safe to do this in the initializer list, but there's little
   // penalty to doing it here and it ensures our object is fully constructed
   // before calling member functions.
   default_match_ = end();
 }
 
 AutocompleteResult::~AutocompleteResult() {}
 
 void AutocompleteResult::CopyOldMatches(
@@ skipping 77 matching lines @@
 }
 
 void AutocompleteResult::SortAndCull(
     const AutocompleteInput& input,
     TemplateURLService* template_url_service) {
   for (ACMatches::iterator i(matches_.begin()); i != matches_.end(); ++i)
     i->ComputeStrippedDestinationURL(input, template_url_service);
 
   SortAndDedupMatches(input.current_page_classification(), &matches_);
 
-  // Sort and trim to the most relevant kMaxMatches matches.
-  size_t max_num_matches = std::min(kMaxMatches, matches_.size());
+  // Sort and trim to the most relevant GetMaxMatches() matches.
+  size_t max_num_matches = std::min(GetMaxMatches(), matches_.size());
   CompareWithDemoteByType<AutocompleteMatch> comparing_object(
       input.current_page_classification());
   std::sort(matches_.begin(), matches_.end(), comparing_object);
   // Top match is not allowed to be the default match.  Find the most
   // relevant legal match and shift it to the front.
   ACMatches::iterator it = FindTopMatch(&matches_);
   if (it != matches_.end())
     std::rotate(matches_.begin(), it, it + 1);
   // In the process of trimming, drop all matches with a demoted relevance
   // score of 0.
@@ skipping 254 matching lines @@
        i != old_matches.rend() && delta > 0; ++i) {
     if (!HasMatchByDestination(*i, new_matches)) {
       AutocompleteMatch match = *i;
       match.relevance = std::min(max_relevance, match.relevance);
       match.from_previous = true;
       matches_.push_back(match);
       delta--;
     }
   }
 }