text: // 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/history_url_provider.h"
  
  #include <algorithm>
  #include <memory>
  #include <utility>
  
  #include "base/bind.h"
  #include "base/command_line.h"
  #include "base/feature_list.h"
  #include "base/location.h"
  #include "base/macros.h"
  #include "base/metrics/histogram_macros.h"
  #include "base/single_thread_task_runner.h"
  #include "base/strings/string_util.h"
  #include "base/strings/utf_string_conversions.h"
  #include "base/threading/sequenced_task_runner_handle.h"
  #include "base/time/time.h"
  #include "base/trace_event/memory_usage_estimator.h"
  #include "base/trace_event/trace_event.h"
  #include "components/bookmarks/browser/bookmark_utils.h"
  #include "components/history/core/browser/history_backend.h"
  #include "components/history/core/browser/history_database.h"
  #include "components/history/core/browser/history_service.h"
  #include "components/history/core/browser/history_types.h"
  #include "components/omnibox/browser/autocomplete_match.h"
  #include "components/omnibox/browser/autocomplete_provider_listener.h"
  #include "components/omnibox/browser/autocomplete_result.h"
  #include "components/omnibox/browser/in_memory_url_index_types.h"
  #include "components/omnibox/browser/omnibox_field_trial.h"
  #include "components/omnibox/browser/scored_history_match.h"
  #include "components/omnibox/browser/url_prefix.h"
  #include "components/prefs/pref_service.h"
  #include "components/search_engines/search_terms_data.h"
  #include "components/search_engines/template_url_service.h"
  #include "components/url_formatter/url_fixer.h"
  #include "components/url_formatter/url_formatter.h"
  #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
  #include "third_party/metrics_proto/omnibox_input_type.pb.h"
  #include "url/gurl.h"
  #include "url/third_party/mozilla/url_parse.h"
  #include "url/url_util.h"
  
  namespace {
  
  // Acts like the > operator for URLInfo classes.
  bool CompareHistoryMatch(const history::HistoryMatch& a,
                           const history::HistoryMatch& b) {
    // A URL that has been typed at all is better than one that has never been
    // typed.  (Note "!"s on each side)
    if (!a.url_info.typed_count() != !b.url_info.typed_count())
      return a.url_info.typed_count() > b.url_info.typed_count();
  
    // Innermost matches (matches after any scheme or "www.") are better than
    // non-innermost matches.
    if (a.innermost_match != b.innermost_match)
      return a.innermost_match;
  
    // URLs that have been typed more often are better.
    if (a.url_info.typed_count() != b.url_info.typed_count())
      return a.url_info.typed_count() > b.url_info.typed_count();
  
    // For URLs that have each been typed once, a host (alone) is better than a
    // page inside.
    if ((a.url_info.typed_count() == 1) && (a.IsHostOnly() != b.IsHostOnly()))
      return a.IsHostOnly();
  
    // URLs that have been visited more often are better.
    if (a.url_info.visit_count() != b.url_info.visit_count())
      return a.url_info.visit_count() > b.url_info.visit_count();
  
    // URLs that have been visited more recently are better.
    if (a.url_info.last_visit() != b.url_info.last_visit())
      return a.url_info.last_visit() > b.url_info.last_visit();
  
    // Use alphabetical order on the url spec as a tie-breaker.
    return a.url_info.url().spec() > b.url_info.url().spec();
  }
  
  // Sorts and dedups the given list of matches.
  void SortAndDedupMatches(history::HistoryMatches* matches) {
    // Sort by quality, best first.
    std::sort(matches->begin(), matches->end(), &CompareHistoryMatch);
  
    // Remove duplicate matches (caused by the search string appearing in one of
    // the prefixes as well as after it).  Consider the following scenario:
    //
    // User has visited "http://http.com" once and "http://htaccess.com" twice.
    // User types "http".  The autocomplete search with prefix "http://" returns
    // the first host, while the search with prefix "" returns both hosts.  Now
    // we sort them into rank order:
    //   http://http.com     (innermost_match)
    //   http://htaccess.com (!innermost_match, url_info.visit_count == 2)
    //   http://http.com     (!innermost_match, url_info.visit_count == 1)
    //
    // The above scenario tells us we can't use std::unique(), since our
    // duplicates are not always sequential.  It also tells us we should remove
    // the lower-quality duplicate(s), since otherwise the returned results won't
    // be ordered correctly.  This is easy to do: we just always remove the later
    // element of a duplicate pair.
    // Be careful!  Because the vector contents may change as we remove elements,
    // we use an index instead of an iterator in the outer loop, and don't
    // precalculate the ending position.
    for (size_t i = 0; i < matches->size(); ++i) {
      for (history::HistoryMatches::iterator j(matches->begin() + i + 1);
           j != matches->end(); ) {
        if ((*matches)[i].url_info.url() == j->url_info.url())
          j = matches->erase(j);
        else
          ++j;
      }
    }
  }
  
  // Calculates a new relevance score applying half-life time decaying to |count|
  // using |time_since_last_visit| and |score_buckets|.  This function will never
  // return a score higher than |undecayed_relevance|; in other words, it can only
  // demote the old score.
  double CalculateRelevanceUsingScoreBuckets(
      const HUPScoringParams::ScoreBuckets& score_buckets,
      const base::TimeDelta& time_since_last_visit,
      int undecayed_relevance,
      int undecayed_count) {
    // Back off if above relevance cap.
    if ((score_buckets.relevance_cap() != -1) &&
        (undecayed_relevance >= score_buckets.relevance_cap()))
      return undecayed_relevance;
  
    // Time based decay using half-life time.
    double decayed_count = undecayed_count;
    double decay_factor = score_buckets.HalfLifeTimeDecay(time_since_last_visit);
    if (decayed_count > 0)
      decayed_count *= decay_factor;
  
    const HUPScoringParams::ScoreBuckets::CountMaxRelevance* score_bucket =
        nullptr;
    const double factor = (score_buckets.use_decay_factor() ?
        decay_factor : decayed_count);
    for (size_t i = 0; i < score_buckets.buckets().size(); ++i) {
      score_bucket = &score_buckets.buckets()[i];
      if (factor >= score_bucket->first)
        break;
    }
  
    return (score_bucket && (undecayed_relevance > score_bucket->second)) ?
        score_bucket->second : undecayed_relevance;
  }
  
  // Returns a new relevance score for the given |match| based on the
  // |old_relevance| score and |scoring_params|.  The new relevance score is
  // guaranteed to be less than or equal to |old_relevance|.  In other words, this
  // function can only demote a score, never boost it.  Returns |old_relevance| if
  // experimental scoring is disabled.
  int CalculateRelevanceScoreUsingScoringParams(
      const history::HistoryMatch& match,
      int old_relevance,
      const HUPScoringParams& scoring_params) {
    const base::TimeDelta time_since_last_visit =
        base::Time::Now() - match.url_info.last_visit();
  
    int relevance = CalculateRelevanceUsingScoreBuckets(
        scoring_params.typed_count_buckets, time_since_last_visit, old_relevance,
        match.url_info.typed_count());
  
    // Additional demotion (on top of typed_count demotion) of URLs that were
    // never typed.
    if (match.url_info.typed_count() == 0) {
      relevance = CalculateRelevanceUsingScoreBuckets(
          scoring_params.visited_count_buckets, time_since_last_visit, relevance,
          match.url_info.visit_count());
    }
  
    DCHECK_LE(relevance, old_relevance);
    return relevance;
  }
  
  // Extracts typed_count, visit_count, and last_visited time from the URLRow and
  // puts them in the additional info field of the |match| for display in
  // about:omnibox.
  void RecordAdditionalInfoFromUrlRow(const history::URLRow& info,
                                      AutocompleteMatch* match) {
    match->RecordAdditionalInfo("typed count", info.typed_count());
    match->RecordAdditionalInfo("visit count", info.visit_count());
    match->RecordAdditionalInfo("last visit", info.last_visit());
  }
  
  // If |create_if_necessary| is true, ensures that |matches| contains an entry
  // for |info|, creating a new such entry if necessary (using |match_template|
  // to get all the other match data).
  //
  // If |promote| is true, this also ensures the entry is the first element in
  // |matches|, moving or adding it to the front as appropriate.  When |promote|
  // is false, existing matches are left in place, and newly added matches are
  // placed at the back.
  //
  // It's OK to call this function with both |create_if_necessary| and |promote|
  // false, in which case we'll do nothing.
  //
  // Returns whether the match exists regardless if it was promoted/created.
  bool CreateOrPromoteMatch(const history::URLRow& info,
                            const history::HistoryMatch& match_template,
                            history::HistoryMatches* matches,
                            bool create_if_necessary,
                            bool promote) {
    // |matches| may already have an entry for this.
    for (history::HistoryMatches::iterator i(matches->begin());
         i != matches->end(); ++i) {
      if (i->url_info.url() == info.url()) {
        // Rotate it to the front if the caller wishes.
        if (promote)
          std::rotate(matches->begin(), i, i + 1);
        return true;
      }
    }
  
    if (!create_if_necessary)
      return false;
  
    // No entry, so create one using |match_template| as a basis.
    history::HistoryMatch match = match_template;
    match.url_info = info;
    if (promote)
      matches->push_front(match);
    else
      matches->push_back(match);
  
    return true;
  }
  
  // Returns whether |match| is suitable for inline autocompletion.
  bool CanPromoteMatchForInlineAutocomplete(const history::HistoryMatch& match) {
    // We can promote this match if it's been typed at least n times, where n == 1
    // for "simple" (host-only) URLs and n == 2 for others.  We set a higher bar
    // for these long URLs because it's less likely that users will want to visit
    // them again.  Even though we don't increment the typed_count for pasted-in
    // URLs, if the user manually edits the URL or types some long thing in by
    // hand, we wouldn't want to immediately start autocompleting it.
    return match.url_info.typed_count() &&
        ((match.url_info.typed_count() > 1) || match.IsHostOnly());
  }
  
  // Given the user's |input| and a |match| created from it, reduce the match's
  // URL to just a host.  If this host still matches the user input, return it.
  // Returns the empty string on failure.
  GURL ConvertToHostOnly(const history::HistoryMatch& match,
                         const base::string16& input) {
    // See if we should try to do host-only suggestions for this URL. Nonstandard
    // schemes means there's no authority section, so suggesting the host name
    // is useless. File URLs are standard, but host suggestion is not useful for
    // them either.
    const GURL& url = match.url_info.url();
    if (!url.is_valid() || !url.IsStandard() || url.SchemeIsFile())
      return GURL();
  
    // Transform to a host-only match.  Bail if the host no longer matches the
    // user input (e.g. because the user typed more than just a host).
    GURL host = url.GetWithEmptyPath();
    if ((host.spec().length() < (match.input_location + input.length())))
      return GURL();  // User typing is longer than this host suggestion.
  
    const base::string16 spec = base::UTF8ToUTF16(host.spec());
    if (spec.compare(match.input_location, input.length(), input))
      return GURL();  // User typing is no longer a prefix.
  
    return host;
  }
  
  }  // namespace
  
  // -----------------------------------------------------------------
  // SearchTermsDataSnapshot
  
  // Implementation of SearchTermsData that takes a snapshot of another
  // SearchTermsData by copying all the responses to the different getters into
  // member strings, then returning those strings when its own getters are called.
  // This will typically be constructed on the UI thread from
  // UIThreadSearchTermsData but is subsequently safe to use on any thread.
  class SearchTermsDataSnapshot : public SearchTermsData {
   public:
    explicit SearchTermsDataSnapshot(const SearchTermsData& search_terms_data);
    ~SearchTermsDataSnapshot() override;
  
    std::string GoogleBaseURLValue() const override;
    std::string GetApplicationLocale() const override;
    base::string16 GetRlzParameterValue(bool from_app_list) const override;
    std::string GetSearchClient() const override;
    std::string GoogleImageSearchSource() const override;
  
    // Estimates dynamic memory usage.
    // See base/trace_event/memory_usage_estimator.h for more info.
    size_t EstimateMemoryUsage() const override;
  
   private:
    std::string google_base_url_value_;
    std::string application_locale_;
    base::string16 rlz_parameter_value_;
    std::string search_client_;
    std::string google_image_search_source_;
  
    DISALLOW_COPY_AND_ASSIGN(SearchTermsDataSnapshot);
  };
  
  SearchTermsDataSnapshot::SearchTermsDataSnapshot(
      const SearchTermsData& search_terms_data)
      : google_base_url_value_(search_terms_data.GoogleBaseURLValue()),
        application_locale_(search_terms_data.GetApplicationLocale()),
        rlz_parameter_value_(search_terms_data.GetRlzParameterValue(false)),
        search_client_(search_terms_data.GetSearchClient()),
        google_image_search_source_(search_terms_data.GoogleImageSearchSource()) {
  }
  
  SearchTermsDataSnapshot::~SearchTermsDataSnapshot() {
  }
  
  std::string SearchTermsDataSnapshot::GoogleBaseURLValue() const {
    return google_base_url_value_;
  }
  
  std::string SearchTermsDataSnapshot::GetApplicationLocale() const {
    return application_locale_;
  }
  
  base::string16 SearchTermsDataSnapshot::GetRlzParameterValue(
      bool from_app_list) const {
    return rlz_parameter_value_;
  }
  
  std::string SearchTermsDataSnapshot::GetSearchClient() const {
    return search_client_;
  }
  
  std::string SearchTermsDataSnapshot::GoogleImageSearchSource() const {
    return google_image_search_source_;
  }
  
  size_t SearchTermsDataSnapshot::EstimateMemoryUsage() const {
    size_t res = 0;
  
    res += base::trace_event::EstimateMemoryUsage(google_base_url_value_);
    res += base::trace_event::EstimateMemoryUsage(application_locale_);
    res += base::trace_event::EstimateMemoryUsage(rlz_parameter_value_);
    res += base::trace_event::EstimateMemoryUsage(search_client_);
    res += base::trace_event::EstimateMemoryUsage(google_image_search_source_);
  
    return res;
  }
  
  // -----------------------------------------------------------------
  // HistoryURLProvider
  
  // These ugly magic numbers will go away once we switch all scoring
  // behavior (including URL-what-you-typed) to HistoryQuick provider.
  const int HistoryURLProvider::kScoreForBestInlineableResult = 1413;
  const int HistoryURLProvider::kScoreForUnvisitedIntranetResult = 1403;
  const int HistoryURLProvider::kScoreForWhatYouTypedResult = 1203;
  const int HistoryURLProvider::kBaseScoreForNonInlineableResult = 900;
  
  // VisitClassifier is used to classify the type of visit to a particular url.
  class HistoryURLProvider::VisitClassifier {
   public:
    enum Type {
      INVALID,             // Navigations to the URL are not allowed.
      UNVISITED_INTRANET,  // A navigable URL for which we have no visit data but
                           // which is known to refer to a visited intranet host.
      VISITED,             // The site has been previously visited.
    };
  
    VisitClassifier(HistoryURLProvider* provider,
                    const AutocompleteInput& input,
                    history::URLDatabase* db);
  
    // Returns the type of visit for the specified input.
    Type type() const { return type_; }
  
    // Returns the URLRow for the visit.
    // If the type of the visit is UNVISITED_INTRANET, the return value of this
    // function does not have any visit data; only the URL field is set.
    const history::URLRow& url_row() const { return url_row_; }
  
   private:
    HistoryURLProvider* provider_;
    history::URLDatabase* db_;
    Type type_;
    history::URLRow url_row_;
  
    DISALLOW_COPY_AND_ASSIGN(VisitClassifier);
  };
  
  HistoryURLProvider::VisitClassifier::VisitClassifier(
      HistoryURLProvider* provider,
      const AutocompleteInput& input,
      history::URLDatabase* db)
      : provider_(provider),
        db_(db),
        type_(INVALID) {
    // Detect email addresses.  These cases will look like "http://user@site/",
    // and because the history backend strips auth creds, we'll get a bogus exact
    // match below if the user has visited "site".
    if ((input.type() == metrics::OmniboxInputType::UNKNOWN) &&
        input.parts().username.is_nonempty() &&
        !input.parts().password.is_nonempty() &&
        !input.parts().path.is_nonempty())
      return;
  
    // If the input can be canonicalized to a valid URL, look up all
    // prefix+input combinations in the URL database to determine if the input
    // corresponds to any visited URL.
    if (!input.canonicalized_url().is_valid())
      return;
  
    // Iterate over all prefixes in ascending number of components (i.e. from the
    // empty prefix to those that have most components).
    const std::string& desired_tld = input.desired_tld();
    const URLPrefixes& url_prefixes = URLPrefix::GetURLPrefixes();
    for (auto prefix_it = url_prefixes.rbegin(); prefix_it != url_prefixes.rend();
         ++prefix_it) {
      const GURL url_with_prefix = url_formatter::FixupURL(
          base::UTF16ToUTF8(prefix_it->prefix + input.text()), desired_tld);
      if (url_with_prefix.is_valid() &&
          db_->GetRowForURL(url_with_prefix, &url_row_) && !url_row_.hidden()) {
        type_ = VISITED;
        return;
      }
    }
  
    // If the input does not correspond to a visited URL, we check if the
    // canonical URL has an intranet hostname that the user visited (albeit with a
    // different port and/or path) before. If this is true, |url_row_| will be
    // mostly empty: the URL field will be set to an unvisited URL with the same
    // scheme and host as some visited URL in the db.
    const GURL as_known_intranet_url = provider_->AsKnownIntranetURL(db_, input);
    if (as_known_intranet_url.is_valid()) {
      url_row_ = history::URLRow(as_known_intranet_url);
      type_ = UNVISITED_INTRANET;
    }
  }
  
  HistoryURLProviderParams::HistoryURLProviderParams(
      const AutocompleteInput& input,
      bool trim_http,
      const AutocompleteMatch& what_you_typed_match,
      const TemplateURL* default_search_provider,
      const SearchTermsData& search_terms_data)
      : origin_task_runner(base::SequencedTaskRunnerHandle::Get()),
        input(input),
        prevent_inline_autocomplete(input.prevent_inline_autocomplete()),
        trim_http(trim_http),
        what_you_typed_match(what_you_typed_match),
        failed(false),
        exact_suggestion_is_in_history(false),
        promote_type(NEITHER),
        default_search_provider(
            default_search_provider
                ? new TemplateURL(default_search_provider->data())
                : nullptr),
        search_terms_data(new SearchTermsDataSnapshot(search_terms_data)) {}
  
  HistoryURLProviderParams::~HistoryURLProviderParams() {
  }
  
  size_t HistoryURLProviderParams::EstimateMemoryUsage() const {
    size_t res = 0;
  
    res += base::trace_event::EstimateMemoryUsage(input);
    res += base::trace_event::EstimateMemoryUsage(what_you_typed_match);
    res += base::trace_event::EstimateMemoryUsage(matches);
    res += base::trace_event::EstimateMemoryUsage(default_search_provider);
    res += base::trace_event::EstimateMemoryUsage(search_terms_data);
  
    return res;
  }
  
  HistoryURLProvider::HistoryURLProvider(AutocompleteProviderClient* client,
                                         AutocompleteProviderListener* listener)
      : HistoryProvider(AutocompleteProvider::TYPE_HISTORY_URL, client),
        listener_(listener),
        params_(nullptr),
        search_url_database_(OmniboxFieldTrial::HUPSearchDatabase()) {
    // Initialize the default HUP scoring params.
    OmniboxFieldTrial::GetDefaultHUPScoringParams(&scoring_params_);
    // Initialize HUP scoring params based on the current experiment.
    OmniboxFieldTrial::GetExperimentalHUPScoringParams(&scoring_params_);
  }
  
  void HistoryURLProvider::Start(const AutocompleteInput& input,
                                 bool minimal_changes) {
    TRACE_EVENT0("omnibox", "HistoryURLProvider::Start");
    // NOTE: We could try hard to do less work in the |minimal_changes| case
    // here; some clever caching would let us reuse the raw matches from the
    // history DB without re-querying.  However, we'd still have to go back to
    // the history thread to mark these up properly, and if pass 2 is currently
    // running, we'd need to wait for it to return to the main thread before
    // doing this (we can't just write new data for it to read due to thread
    // safety issues).  At that point it's just as fast, and easier, to simply
    // re-run the query from scratch and ignore |minimal_changes|.
  
    // Cancel any in-progress query.
    Stop(false, false);
  
    matches_.clear();
  
    if (input.from_omnibox_focus() ||
        (input.type() == metrics::OmniboxInputType::INVALID))
      return;
  
    // Do some fixup on the user input before matching against it, so we provide
    // good results for local file paths, input with spaces, etc.
    const FixupReturn fixup_return(FixupUserInput(input));
    if (!fixup_return.first)
      return;
    url::Parsed parts;
    url_formatter::SegmentURL(fixup_return.second, &parts);
    AutocompleteInput fixed_up_input(input);
    fixed_up_input.UpdateText(fixup_return.second, base::string16::npos, parts);
  
    // Create a match for what the user typed.
    const bool trim_http = !AutocompleteInput::HasHTTPScheme(input.text());
    AutocompleteMatch what_you_typed_match(SuggestExactInput(
        fixed_up_input, fixed_up_input.canonicalized_url(), trim_http));
    what_you_typed_match.relevance = CalculateRelevance(WHAT_YOU_TYPED, 0);
  
    // Add the what-you-typed match as a fallback in case we can't get the history
    // service or URL DB; otherwise, we'll replace this match lower down.  Don't
    // do this for queries, though -- while we can sometimes mark up a match for
    // them, it's not what the user wants, and just adds noise.
    if (fixed_up_input.type() != metrics::OmniboxInputType::QUERY)
      matches_.push_back(what_you_typed_match);
  
    // We'll need the history service to run both passes, so try to obtain it.
    history::HistoryService* const history_service =
        client()->GetHistoryService();
    if (!history_service)
      return;
  
    // Get the default search provider and search terms data now since we have to
    // retrieve these on the UI thread, and the second pass runs on the history
    // thread. |template_url_service| can be null when testing.
    TemplateURLService* template_url_service = client()->GetTemplateURLService();
    const TemplateURL* default_search_provider = template_url_service ?
        template_url_service->GetDefaultSearchProvider() : nullptr;
  
    // Create the data structure for the autocomplete passes.  We'll save this off
    // onto the |params_| member for later deletion below if we need to run pass
    // 2.
    std::unique_ptr<HistoryURLProviderParams> params(new HistoryURLProviderParams(
        fixed_up_input, trim_http, what_you_typed_match, default_search_provider,
        client()->GetSearchTermsData()));
    // Note that we use the non-fixed-up input here, since fixup may strip
    // trailing whitespace.
    params->prevent_inline_autocomplete = PreventInlineAutocomplete(input);
  
    // Pass 1: Get the in-memory URL database, and use it to find and promote
    // the inline autocomplete match, if any.
    history::URLDatabase* url_db = history_service->InMemoryDatabase();
    // url_db can be null if it hasn't finished initializing (or failed to
    // initialize).  In this case all we can do is fall back on the second pass.
    //
    // TODO(pkasting): We should just block here until this loads.  Any time
    // someone unloads the history backend, we'll get inconsistent inline
    // autocomplete behavior here.
    if (url_db) {
      DoAutocomplete(nullptr, url_db, params.get());
      matches_.clear();
      PromoteMatchesIfNecessary(*params);
      // NOTE: We don't reset |params| here since at least the |promote_type|
      // field on it will be read by the second pass -- see comments in
      // DoAutocomplete().
    }
  
    // Pass 2: Ask the history service to call us back on the history thread,
    // where we can read the full on-disk DB.
    if (search_url_database_ && input.want_asynchronous_matches()) {
      done_ = false;
      params_ = params.release();  // This object will be destroyed in
                                   // QueryComplete() once we're done with it.
      history_service->ScheduleAutocomplete(
          base::Bind(&HistoryURLProvider::ExecuteWithDB, this, params_));
    }
  }
  
  void HistoryURLProvider::Stop(bool clear_cached_results,
                                bool due_to_user_inactivity) {
    done_ = true;
  
    if (params_)
      params_->cancel_flag.Set();
  }
  
  size_t HistoryURLProvider::EstimateMemoryUsage() const {
    size_t res = HistoryProvider::EstimateMemoryUsage();
  
    if (params_)
      res += base::trace_event::EstimateMemoryUsage(*params_);
    res += base::trace_event::EstimateMemoryUsage(scoring_params_);
  
    return res;
  }
  
  AutocompleteMatch HistoryURLProvider::SuggestExactInput(
      const AutocompleteInput& input,
      const GURL& destination_url,
      bool trim_http) {
    // The FormattedStringWithEquivalentMeaning() call below requires callers to
    // be on the main thread.
    DCHECK(thread_checker_.CalledOnValidThread());
  
    AutocompleteMatch match(this, 0, false,
                            AutocompleteMatchType::URL_WHAT_YOU_TYPED);
  
    if (destination_url.is_valid()) {
      match.destination_url = destination_url;
  
      // If the input explicitly contains "http://", callers must set |trim_http|
      // to false. Otherwise, |trim_http| may be either true or false.
      DCHECK(!(trim_http && AutocompleteInput::HasHTTPScheme(input.text())));
      base::string16 display_string(url_formatter::FormatUrl(
          destination_url,
          url_formatter::kFormatUrlOmitDefaults &
              ~url_formatter::kFormatUrlOmitHTTP,
          net::UnescapeRule::SPACES, nullptr, nullptr, nullptr));
      const size_t offset = trim_http ? TrimHttpPrefix(&display_string) : 0;
      match.fill_into_edit =
          AutocompleteInput::FormattedStringWithEquivalentMeaning(
              destination_url, display_string, client()->GetSchemeClassifier(),
              nullptr);
      // The what-you-typed match is generally only allowed to be default for
      // URL inputs or when there is no default search provider.  (It's also
      // allowed to be default for UNKNOWN inputs where the destination is a known
      // intranet site.  In this case, |allowed_to_be_default_match| is revised in
      // FixupExactSuggestion().)
      const bool has_default_search_provider =
         client()->GetTemplateURLService() &&
         client()->GetTemplateURLService()->GetDefaultSearchProvider();
      match.allowed_to_be_default_match =
          (input.type() == metrics::OmniboxInputType::URL) ||
          !has_default_search_provider;
      // NOTE: Don't set match.inline_autocompletion to something non-empty here;
      // it's surprising and annoying.
  
      // Try to highlight "innermost" match location.  If we fix up "w" into
      // "www.w.com", we want to highlight the fifth character, not the first.
      // This relies on match.destination_url being the non-prefix-trimmed version
      // of match.contents.
      match.contents = display_string;
      const URLPrefix* best_prefix = URLPrefix::BestURLPrefix(
          base::UTF8ToUTF16(destination_url.spec()), input.text());
      // It's possible for match.destination_url to not contain the user's input
      // at all (so |best_prefix| is null), for example if the input is
      // "view-source:x" and |destination_url| has an inserted "http://" in the
      // middle.
      if (!best_prefix) {
        AutocompleteMatch::ClassifyMatchInString(input.text(),
                                                 match.contents,
                                                 ACMatchClassification::URL,
                                                 &match.contents_class);
      } else {
        AutocompleteMatch::ClassifyLocationInString(
            best_prefix->prefix.length() - offset, input.text().length(),
            match.contents.length(), ACMatchClassification::URL,
            &match.contents_class);
      }
    }
  
    return match;
  }
  
  void HistoryURLProvider::ExecuteWithDB(HistoryURLProviderParams* params,
                                         history::HistoryBackend* backend,
                                         history::URLDatabase* db) {
    // We may get called with a null database if it couldn't be properly
    // initialized.
    if (!db) {
      params->failed = true;
    } else if (!params->cancel_flag.IsSet()) {
      base::TimeTicks beginning_time = base::TimeTicks::Now();
  
      DoAutocomplete(backend, db, params);
  
      UMA_HISTOGRAM_TIMES("Autocomplete.HistoryAsyncQueryTime",
                          base::TimeTicks::Now() - beginning_time);
    }
  
    // Return the results (if any) to the originating sequence.
    params->origin_task_runner->PostTask(
        FROM_HERE, base::Bind(&HistoryURLProvider::QueryComplete, this, params));
  }
  
  HistoryURLProvider::~HistoryURLProvider() {
    // Note: This object can get leaked on shutdown if there are pending
    // requests on the database (which hold a reference to us). Normally, these
    // messages get flushed for each thread. We do a round trip from main, to
    // history, back to main while holding a reference. If the main thread
    // completes before the history thread, the message to delegate back to the
    // main thread will not run and the reference will leak. Therefore, don't do
    // anything on destruction.
  }
  
  // static
  int HistoryURLProvider::CalculateRelevance(MatchType match_type,
                                             int match_number) {
    switch (match_type) {
      case INLINE_AUTOCOMPLETE:
        return kScoreForBestInlineableResult;
  
      case UNVISITED_INTRANET:
        return kScoreForUnvisitedIntranetResult;
  
      case WHAT_YOU_TYPED:
        return kScoreForWhatYouTypedResult;
  
      default:  // NORMAL
        return kBaseScoreForNonInlineableResult + match_number;
    }
  }
  
  // static
  ACMatchClassifications HistoryURLProvider::ClassifyDescription(
      const base::string16& input_text,
      const base::string16& description) {
    base::string16 clean_description =
        bookmarks::CleanUpTitleForMatching(description);
    TermMatches description_matches(DeoverlapMatches(SortMatches(
        MatchTermInString(input_text, clean_description, 0))));
    WordStarts description_word_starts;
    String16VectorFromString16(clean_description, false,
                               &description_word_starts);
    // If HistoryURL retrieves any matches (and hence we reach this code), we
    // are guaranteed that the beginning of input_text must be a word break.
    WordStarts offsets(1, 0u);
    description_matches = ScoredHistoryMatch::FilterTermMatchesByWordStarts(
        description_matches, offsets, description_word_starts, 0,
        std::string::npos);
    return SpansFromTermMatch(
        description_matches, clean_description.length(), false);
  }
  
  void HistoryURLProvider::DoAutocomplete(history::HistoryBackend* backend,
                                          history::URLDatabase* db,
                                          HistoryURLProviderParams* params) {
    // Get the matching URLs from the DB.
    params->matches.clear();
    history::URLRows url_matches;
  
    if (search_url_database_) {
      const URLPrefixes& prefixes = URLPrefix::GetURLPrefixes();
      for (URLPrefixes::const_iterator i(prefixes.begin()); i != prefixes.end();
           ++i) {
        if (params->cancel_flag.IsSet())
          return;  // Canceled in the middle of a query, give up.
  
        // We only need kMaxMatches results in the end, but before we get there we
        // need to promote lower-quality matches that are prefixes of higher-
        // quality matches, and remove lower-quality redirects.  So we ask for
        // more results than we need, of every prefix type, in hopes this will
        // give us far more than enough to work with.  CullRedirects() will then
        // reduce the list to the best kMaxMatches results.
        std::string prefixed_input =
            base::UTF16ToUTF8(i->prefix + params->input.text());
        db->AutocompleteForPrefix(prefixed_input, kMaxMatches * 2, !backend,
                                  &url_matches);
        for (history::URLRows::const_iterator j(url_matches.begin());
             j != url_matches.end(); ++j) {
          const GURL& row_url = j->url();
          const URLPrefix* best_prefix = URLPrefix::BestURLPrefix(
              base::UTF8ToUTF16(row_url.spec()), base::string16());
          DCHECK(best_prefix);
          history::HistoryMatch match;
          match.url_info = *j;
          match.input_location = i->prefix.length();
          match.innermost_match =
              i->num_components >= best_prefix->num_components;
  
          AutocompleteMatch::GetMatchComponents(
              row_url, {{match.input_location, prefixed_input.length()}},
              &match.match_in_scheme, &match.match_in_subdomain,
              &match.match_after_host);
  
          params->matches.push_back(std::move(match));
        }
      }
  
      // Create sorted list of suggestions.
      CullPoorMatches(params);
      SortAndDedupMatches(&params->matches);
    }
  
    // Try to create a shorter suggestion from the best match.
    // We consider the what you typed match eligible for display when it's
    // navigable and there's a reasonable chance the user intended to do
    // something other than search.  We use a variety of heuristics to determine
    // this, e.g. whether the user explicitly typed a scheme, or if omnibox
    // searching has been disabled by policy. In the cases where we've parsed as
    // UNKNOWN, we'll still show an accidental search infobar if need be.
    VisitClassifier classifier(this, params->input, db);
    params->have_what_you_typed_match =
        (params->input.type() != metrics::OmniboxInputType::QUERY) &&
        ((params->input.type() != metrics::OmniboxInputType::UNKNOWN) ||
         (classifier.type() == VisitClassifier::UNVISITED_INTRANET) ||
         !params->trim_http ||
         (AutocompleteInput::NumNonHostComponents(params->input.parts()) > 0) ||
         !params->default_search_provider);
    const bool have_shorter_suggestion_suitable_for_inline_autocomplete =
        PromoteOrCreateShorterSuggestion(db, params);
  
    // Check whether what the user typed appears in history.
    const bool can_check_history_for_exact_match =
        // Checking what_you_typed_match.destination_url.is_valid() tells us
        // whether SuggestExactInput() succeeded in constructing a valid match.
        params->what_you_typed_match.destination_url.is_valid() &&
        // Additionally, in the case where the user has typed "foo.com" and
        // visited (but not typed) "foo/", and the input is "foo", the first pass
        // will fall into the FRONT_HISTORY_MATCH case for "foo.com" but the
        // second pass can suggest the exact input as a better URL.  Since we need
        // both passes to agree, and since during the first pass there's no way to
        // know about "foo/", ensure that if the promote type was set to
        // FRONT_HISTORY_MATCH during the first pass, the second pass will not
        // consider the exact suggestion to be in history and therefore will not
        // suggest the exact input as a better match.  (Note that during the first
        // pass, this conditional will always succeed since |promote_type| is
        // initialized to NEITHER.)
        (params->promote_type != HistoryURLProviderParams::FRONT_HISTORY_MATCH);
    params->exact_suggestion_is_in_history = can_check_history_for_exact_match &&
        FixupExactSuggestion(db, classifier, params);
  
    // If we succeeded in fixing up the exact match based on the user's history,
    // we should treat it as the best match regardless of input type.  If not,
    // then we check whether there's an inline autocompletion we can create from
    // this input, so we can promote that as the best match.
    if (params->exact_suggestion_is_in_history) {
      params->promote_type = HistoryURLProviderParams::WHAT_YOU_TYPED_MATCH;
    } else if (!params->matches.empty() &&
               (have_shorter_suggestion_suitable_for_inline_autocomplete ||
                CanPromoteMatchForInlineAutocomplete(params->matches[0]))) {
      // Note that we promote this inline-autocompleted match even when
      // params->prevent_inline_autocomplete is true.  This is safe because in
      // this case the match will be marked as "not allowed to be default", and
      // a non-inlined match that is "allowed to be default" will be reordered
      // above it by the controller/AutocompleteResult.  We ensure there is such
      // a match in two ways:
      //   * If params->have_what_you_typed_match is true, we force the
      //     what-you-typed match to be added in this case.  See comments in
      //     PromoteMatchesIfNecessary().
      //   * Otherwise, we should have some sort of QUERY or UNKNOWN input that
      //     the SearchProvider will provide a defaultable what-you-typed match
      //     for.
      params->promote_type = HistoryURLProviderParams::FRONT_HISTORY_MATCH;
    } else {
      // Failed to promote any URLs.  Use the What You Typed match, if we have it.
      params->promote_type = params->have_what_you_typed_match ?
          HistoryURLProviderParams::WHAT_YOU_TYPED_MATCH :
          HistoryURLProviderParams::NEITHER;
    }
  
    const size_t max_results =
        kMaxMatches + (params->exact_suggestion_is_in_history ? 1 : 0);
    if (backend) {
      // Remove redirects and trim list to size.  We want to provide up to
      // kMaxMatches results plus the What You Typed result, if it was added to
      // params->matches above.
      CullRedirects(backend, &params->matches, max_results);
    } else if (params->matches.size() > max_results) {
      // Simply trim the list to size.
      params->matches.resize(max_results);
    }
  }
  
  void HistoryURLProvider::PromoteMatchesIfNecessary(
      const HistoryURLProviderParams& params) {
    if (params.promote_type == HistoryURLProviderParams::NEITHER)
      return;
    if (params.promote_type == HistoryURLProviderParams::FRONT_HISTORY_MATCH) {
      matches_.push_back(
          HistoryMatchToACMatch(params, 0,
                                CalculateRelevance(INLINE_AUTOCOMPLETE, 0)));
    }
    // There are two cases where we need to add the what-you-typed-match:
    //   * If params.promote_type is WHAT_YOU_TYPED_MATCH, we're being explicitly
    //     directed to.
    //   * If params.have_what_you_typed_match is true, then params.promote_type
    //     can't be NEITHER (see code near the end of DoAutocomplete()), so if
    //     it's not WHAT_YOU_TYPED_MATCH, it must be FRONT_HISTORY_MATCH, and
    //     we'll have promoted the history match above.  If
    //     params.prevent_inline_autocomplete is also true, then this match
    //     will be marked "not allowed to be default", and we need to add the
    //     what-you-typed match to ensure there's a legal default match for the
    //     controller/AutocompleteResult to promote.  (If
    //     params.have_what_you_typed_match is false, the SearchProvider should
    //     take care of adding this defaultable match.)
    if ((params.promote_type == HistoryURLProviderParams::WHAT_YOU_TYPED_MATCH) ||
        (params.prevent_inline_autocomplete &&
         params.have_what_you_typed_match)) {
      matches_.push_back(params.what_you_typed_match);
    }
  }
  
  void HistoryURLProvider::QueryComplete(
      HistoryURLProviderParams* params_gets_deleted) {
    TRACE_EVENT0("omnibox", "HistoryURLProvider::QueryComplete");
    // Ensure |params_gets_deleted| gets deleted on exit.
    std::unique_ptr<HistoryURLProviderParams> params(params_gets_deleted);
  
    // If the user hasn't already started another query, clear our member pointer
    // so we can't write into deleted memory.
    if (params_ == params_gets_deleted)
      params_ = nullptr;
  
    // Don't send responses for queries that have been canceled.
    if (params->cancel_flag.IsSet())
      return;  // Already set done_ when we canceled, no need to set it again.
  
    // Don't modify |matches_| if the query failed, since it might have a default
    // match in it, whereas |params->matches| will be empty.
    if (!params->failed) {
      matches_.clear();
      PromoteMatchesIfNecessary(*params);
  
      // Determine relevance of highest scoring match, if any.
      int relevance = matches_.empty() ?
          CalculateRelevance(NORMAL,
                             static_cast<int>(params->matches.size() - 1)) :
          matches_[0].relevance;
  
      // Convert the history matches to autocomplete matches.  If we promoted the
      // first match, skip over it.
      const size_t first_match =
          (params->exact_suggestion_is_in_history ||
           (params->promote_type ==
               HistoryURLProviderParams::FRONT_HISTORY_MATCH)) ? 1 : 0;
      for (size_t i = first_match; i < params->matches.size(); ++i) {
        // All matches score one less than the previous match.
        --relevance;
        // The experimental scoring must not change the top result's score.
        if (!matches_.empty()) {
          relevance = CalculateRelevanceScoreUsingScoringParams(
              params->matches[i], relevance, scoring_params_);
        }
        matches_.push_back(HistoryMatchToACMatch(*params, i, relevance));
      }
    }
  
    done_ = true;
    listener_->OnProviderUpdate(true);
  }
  
  bool HistoryURLProvider::FixupExactSuggestion(
      history::URLDatabase* db,
      const VisitClassifier& classifier,
      HistoryURLProviderParams* params) const {
    MatchType type = INLINE_AUTOCOMPLETE;
  
    switch (classifier.type()) {
      case VisitClassifier::INVALID:
        return false;
      case VisitClassifier::UNVISITED_INTRANET:
        params->what_you_typed_match.destination_url = classifier.url_row().url();
        type = UNVISITED_INTRANET;
        break;
      default:
        DCHECK_EQ(VisitClassifier::VISITED, classifier.type());
        // We have data for this match, use it.
        params->what_you_typed_match.deletable = true;
        params->what_you_typed_match.description = classifier.url_row().title();
        params->what_you_typed_match.destination_url = classifier.url_row().url();
        RecordAdditionalInfoFromUrlRow(classifier.url_row(),
                                       &params->what_you_typed_match);
        params->what_you_typed_match.description_class = ClassifyDescription(
            params->input.text(), params->what_you_typed_match.description);
        if (!classifier.url_row().typed_count()) {
          // If we reach here, we must be in the second pass, and we must not have
          // this row's data available during the first pass.  That means we
          // either scored it as WHAT_YOU_TYPED or UNVISITED_INTRANET, and to
          // maintain the ordering between passes consistent, we need to score it
          // the same way here.
          const GURL as_known_intranet_url =
              AsKnownIntranetURL(db, params->input);
          type = as_known_intranet_url.is_valid() ? UNVISITED_INTRANET
                                                  : WHAT_YOU_TYPED;
        }
        break;
    }
  
    const GURL& url = params->what_you_typed_match.destination_url;
    const url::Parsed& parsed = url.parsed_for_possibly_invalid_spec();
    // If the what-you-typed result looks like a single word (which can be
    // interpreted as an intranet address) followed by a pound sign ("#"), leave
    // the score for the url-what-you-typed result as is and also don't mark it
    // as allowed to be the default match.  It will likely be outscored by a
    // search query from the SearchProvider or, if not, the search query default
    // match will in any case--which is allowed to be the default match--will be
    // reordered to be first.  This test fixes cases such as "c#" and "c# foo"
    // where the user has visited an intranet site "c".  We want the search-what-
    // you-typed score to beat the URL-what-you-typed score in this case.  Most
    // of the below test tries to make sure that this code does not trigger if
    // the user did anything to indicate the desired match is a URL.  For
    // instance, "c/# foo" will not pass the test because that will be classified
    // as input type URL.  The parsed.CountCharactersBefore() in the test looks
    // for the presence of a reference fragment in the URL by checking whether
    // the position differs included the delimiter (pound sign) versus not
    // including the delimiter.  (One cannot simply check url.ref() because it
    // will not distinguish between the input "c" and the input "c#", both of
    // which will have empty reference fragments.)
    if ((type == UNVISITED_INTRANET) &&
        (params->input.type() != metrics::OmniboxInputType::URL) &&
        url.username().empty() && url.password().empty() && url.port().empty() &&
        (url.path_piece() == "/") && url.query().empty() &&
        (parsed.CountCharactersBefore(url::Parsed::REF, true) !=
         parsed.CountCharactersBefore(url::Parsed::REF, false))) {
      return false;
    }
  
    params->what_you_typed_match.allowed_to_be_default_match = true;
    params->what_you_typed_match.relevance = CalculateRelevance(type, 0);
  
    // If there are any other matches, then don't promote this match here, in
    // hopes the caller will be able to inline autocomplete a better suggestion.
    // DoAutocomplete() will fall back on this match if inline autocompletion
    // fails.  This matches how we react to never-visited URL inputs in the non-
    // intranet case.
    if (type == UNVISITED_INTRANET && !params->matches.empty())
      return false;
  
    // Put it on the front of the HistoryMatches for redirect culling.
    CreateOrPromoteMatch(classifier.url_row(), history::HistoryMatch(),
                         &params->matches, true, true);
    return true;
  }
  
  GURL HistoryURLProvider::AsKnownIntranetURL(
      history::URLDatabase* db,
      const AutocompleteInput& input) const {
    // Normally passing the first two conditions below ought to guarantee the
    // third condition, but because FixupUserInput() can run and modify the
    // input's text and parts between Parse() and here, it seems better to be
    // paranoid and check.
    if ((input.type() != metrics::OmniboxInputType::UNKNOWN) ||
        !base::LowerCaseEqualsASCII(input.scheme(), url::kHttpScheme) ||
        !input.parts().host.is_nonempty())
      return GURL();
  
    const std::string host(base::UTF16ToUTF8(
        input.text().substr(input.parts().host.begin, input.parts().host.len)));
  
    // Check if the host has registry domain.
    if (net::registry_controlled_domains::HostHasRegistryControlledDomain(
            host, net::registry_controlled_domains::EXCLUDE_UNKNOWN_REGISTRIES,
            net::registry_controlled_domains::EXCLUDE_PRIVATE_REGISTRIES))
      return GURL();
  
    const GURL& url = input.canonicalized_url();
    // Check if the host of the canonical URL can be found in the database.
    std::string scheme_in_db;
    if (db->IsTypedHost(host, &scheme_in_db)) {
      GURL::Replacements replace_scheme;
      replace_scheme.SetSchemeStr(scheme_in_db);
      return url.ReplaceComponents(replace_scheme);
    }
  
    // Check if appending "www." to the canonicalized URL generates a URL found in
    // the database.
    const std::string alternative_host = "www." + host;
    if (!base::StartsWith(host, "www.", base::CompareCase::INSENSITIVE_ASCII) &&
        db->IsTypedHost(alternative_host, &scheme_in_db)) {
      GURL::Replacements replace_scheme_and_host;
      replace_scheme_and_host.SetHostStr(alternative_host);
      replace_scheme_and_host.SetSchemeStr(scheme_in_db);
      return url.ReplaceComponents(replace_scheme_and_host);
    }
  
    return GURL();
  }
  
  bool HistoryURLProvider::PromoteOrCreateShorterSuggestion(
      history::URLDatabase* db,
      HistoryURLProviderParams* params) {
    if (params->matches.empty())
      return false;  // No matches, nothing to do.
  
    // Determine the base URL from which to search, and whether that URL could
    // itself be added as a match.  We can add the base iff it's not "effectively
    // the same" as any "what you typed" match.
    const history::HistoryMatch& match = params->matches[0];
    GURL search_base = ConvertToHostOnly(match, params->input.text());
    bool can_add_search_base_to_matches = !params->have_what_you_typed_match;
    if (search_base.is_empty()) {
      // Search from what the user typed when we couldn't reduce the best match
      // to a host.  Careful: use a substring of |match| here, rather than the
      // first match in |params|, because they might have different prefixes.  If
      // the user typed "google.com", params->what_you_typed_match will hold
      // "http://google.com/", but |match| might begin with
      // "http://www.google.com/".
      // TODO: this should be cleaned up, and is probably incorrect for IDN.
      std::string new_match = match.url_info.url().possibly_invalid_spec().
          substr(0, match.input_location + params->input.text().length());
      search_base = GURL(new_match);
      if (search_base.is_empty())
        return false;  // Can't construct a URL from which to start a search.
    } else if (!can_add_search_base_to_matches) {
      can_add_search_base_to_matches =
          (search_base != params->what_you_typed_match.destination_url);
    }
    if (search_base == match.url_info.url())
      return false;  // Couldn't shorten |match|, so no URLs to search over.
  
    // Search the DB for short URLs between our base and |match|.
    history::URLRow info(search_base);
    bool promote = true;
    // A short URL is only worth suggesting if it's been visited at least a third
    // as often as the longer URL.
    const int min_visit_count = ((match.url_info.visit_count() - 1) / 3) + 1;
    // For stability between the in-memory and on-disk autocomplete passes, when
    // the long URL has been typed before, only suggest shorter URLs that have
    // also been typed.  Otherwise, the on-disk pass could suggest a shorter URL
    // (which hasn't been typed) that the in-memory pass doesn't know about,
    // thereby making the top match, and thus the behavior of inline
    // autocomplete, unstable.
    const int min_typed_count = match.url_info.typed_count() ? 1 : 0;
    if (!db->FindShortestURLFromBase(search_base.possibly_invalid_spec(),
            match.url_info.url().possibly_invalid_spec(), min_visit_count,
            min_typed_count, can_add_search_base_to_matches, &info)) {
      if (!can_add_search_base_to_matches)
        return false;  // Couldn't find anything and can't add the search base.
  
      // Try to get info on the search base itself.  Promote it to the top if the
      // original best match isn't good enough to autocomplete.
      db->GetRowForURL(search_base, &info);
      promote = match.url_info.typed_count() <= 1;
    }
  
    // Promote or add the desired URL to the list of matches.
    const bool ensure_can_inline =
        promote && CanPromoteMatchForInlineAutocomplete(match);
    return CreateOrPromoteMatch(info, match, &params->matches, true, promote) &&
           ensure_can_inline;
  }
  
  void HistoryURLProvider::CullPoorMatches(
      HistoryURLProviderParams* params) const {
    const base::Time& threshold(history::AutocompleteAgeThreshold());
    for (history::HistoryMatches::iterator i(params->matches.begin());
         i != params->matches.end(); ) {
      if (RowQualifiesAsSignificant(i->url_info, threshold) &&
          (!params->default_search_provider ||
           !params->default_search_provider->IsSearchURL(
               i->url_info.url(), *params->search_terms_data))) {
        ++i;
      } else {
        i = params->matches.erase(i);
      }
    }
  }
  
  void HistoryURLProvider::CullRedirects(history::HistoryBackend* backend,
                                         history::HistoryMatches* matches,
                                         size_t max_results) const {
    for (size_t source = 0;
         (source < matches->size()) && (source < max_results); ) {
      const GURL& url = (*matches)[source].url_info.url();
      // TODO(brettw) this should go away when everything uses GURL.
      history::RedirectList redirects;
      backend->QueryRedirectsFrom(url, &redirects);
      if (!redirects.empty()) {
        // Remove all but the first occurrence of any of these redirects in the
        // search results. We also must add the URL we queried for, since it may
        // not be the first match and we'd want to remove it.
        //
        // For example, when A redirects to B and our matches are [A, X, B],
        // we'll get B as the redirects from, and we want to remove the second
        // item of that pair, removing B. If A redirects to B and our matches are
        // [B, X, A], we'll want to remove A instead.
        redirects.push_back(url);
        source = RemoveSubsequentMatchesOf(matches, source, redirects);
      } else {
        // Advance to next item.
        source++;
      }
    }
  
    if (matches->size() > max_results)
      matches->resize(max_results);
  }
  
  size_t HistoryURLProvider::RemoveSubsequentMatchesOf(
      history::HistoryMatches* matches,
      size_t source_index,
      const std::vector<GURL>& remove) const {
    size_t next_index = source_index + 1;  // return value = item after source
  
    // Find the first occurrence of any URL in the redirect chain. We want to
    // keep this one since it is rated the highest.
    history::HistoryMatches::iterator first(std::find_first_of(
        matches->begin(), matches->end(), remove.begin(), remove.end(),
        history::HistoryMatch::EqualsGURL));
    DCHECK(first != matches->end()) << "We should have always found at least the "
        "original URL.";
  
    // Find any following occurrences of any URL in the redirect chain, these
    // should be deleted.
    for (history::HistoryMatches::iterator next(std::find_first_of(first + 1,
             matches->end(), remove.begin(), remove.end(),
             history::HistoryMatch::EqualsGURL));
         next != matches->end(); next = std::find_first_of(next, matches->end(),
             remove.begin(), remove.end(), history::HistoryMatch::EqualsGURL)) {
      // Remove this item. When we remove an item before the source index, we
      // need to shift it to the right and remember that so we can return it.
      next = matches->erase(next);
      if (static_cast<size_t>(next - matches->begin()) < next_index)
        --next_index;
    }
    return next_index;
  }
  
  AutocompleteMatch HistoryURLProvider::HistoryMatchToACMatch(
      const HistoryURLProviderParams& params,
      size_t match_number,
      int relevance) {
    // The FormattedStringWithEquivalentMeaning() call below requires callers to
    // be on the main thread.
    DCHECK(thread_checker_.CalledOnValidThread());
  
    const history::HistoryMatch& history_match = params.matches[match_number];
    const history::URLRow& info = history_match.url_info;
    AutocompleteMatch match(this, relevance,
        !!info.visit_count(), AutocompleteMatchType::HISTORY_URL);
    match.typed_count = info.typed_count();
    match.destination_url = info.url();
    DCHECK(match.destination_url.is_valid());
    size_t inline_autocomplete_offset =
        history_match.input_location + params.input.text().length();
  
    auto fill_into_edit_format_types = url_formatter::kFormatUrlOmitDefaults;
    if (!params.trim_http || history_match.match_in_scheme)
      fill_into_edit_format_types &= ~url_formatter::kFormatUrlOmitHTTP;
    match.fill_into_edit =
        AutocompleteInput::FormattedStringWithEquivalentMeaning(
            info.url(),
            url_formatter::FormatUrl(info.url(), fill_into_edit_format_types,
                                     net::UnescapeRule::SPACES, nullptr, nullptr,
                                     &inline_autocomplete_offset),
            client()->GetSchemeClassifier(), &inline_autocomplete_offset);
    // |inline_autocomplete_offset| was guaranteed not to be npos before the call
    // to FormatUrl().  If it is npos now, that means the represented location no
    // longer exists as such in the formatted string, e.g. if the offset pointed
    // into the middle of a punycode sequence fixed up to Unicode.  In this case,
    // there can be no inline autocompletion, and the match must not be allowed to
    // be default.
    const bool autocomplete_offset_valid =
        inline_autocomplete_offset != base::string16::npos;
    if (!params.prevent_inline_autocomplete && autocomplete_offset_valid) {
      DCHECK(inline_autocomplete_offset <= match.fill_into_edit.length());
      match.inline_autocompletion =
          match.fill_into_edit.substr(inline_autocomplete_offset);
    }
    // The latter part of the test effectively asks "is the inline completion
    // empty?" (i.e., is this match effectively the what-you-typed match?).
    match.allowed_to_be_default_match = autocomplete_offset_valid &&
        (!params.prevent_inline_autocomplete ||
         (inline_autocomplete_offset >= match.fill_into_edit.length()));
  
    // Get the adjusted (for match contents) match start and end offsets.
    std::vector<size_t> offsets = {
        history_match.input_location,
        history_match.input_location + params.input.text().length()};
  
    const auto format_types = AutocompleteMatch::GetFormatTypes(
        params.input.parts().scheme.len > 0 || !params.trim_http ||
            history_match.match_in_scheme,
        history_match.match_in_subdomain, history_match.match_after_host);
    match.contents = url_formatter::FormatUrlWithOffsets(
        info.url(), format_types, net::UnescapeRule::SPACES, nullptr, nullptr,
        &offsets);
  
    size_t match_start = offsets[0];
    size_t match_end = offsets[1];
  
    if (match_start != base::string16::npos &&
        match_end != base::string16::npos && match_end != match_start) {
      DCHECK_GT(match_end, match_start);
      AutocompleteMatch::ClassifyLocationInString(
          match_start, match_end - match_start, match.contents.length(),
          ACMatchClassification::URL, &match.contents_class);
    } else {
      AutocompleteMatch::ClassifyLocationInString(base::string16::npos, 0,
          match.contents.length(), ACMatchClassification::URL,
          &match.contents_class);
    }
    match.description = info.title();
    match.description_class =
        ClassifyDescription(params.input.text(), match.description);
    RecordAdditionalInfoFromUrlRow(info, &match);
    return match;
  }
  