Moves frecency score formula to shared location

This is done in order for Frecency score to be used outside of
LocalHistoryZeroSuggestProvider for repeatable quereis.

Bug: 1110077
Change-Id: I7c077d7c5215b88cf565c30e84556dffca55182b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422153Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809143}

components/history/core/browser/keyword_search_term.cc

@@ -8,6 +8,17 @@ namespace history {
 
 NormalizedKeywordSearchTermVisit::~NormalizedKeywordSearchTermVisit() = default;
 
+double NormalizedKeywordSearchTermVisit::GetFrecency(
+    base::Time now,
+    int recency_decay_unit_sec,
+    double frequency_exponent) const {
+  const double recency_sec =
+      base::TimeDelta(now - most_recent_visit_time).InSeconds();
+  const double recency_decayed =
+      recency_decay_unit_sec / (recency_sec + recency_decay_unit_sec);
+  const double frequency_powered = pow(visits, frequency_exponent);
+  return frequency_powered * recency_decayed;
+}
 
 KeywordSearchTermVisit::KeywordSearchTermVisit() : visits(0) {}
 

components/history/core/browser/keyword_search_term.h

@@ -20,6 +20,19 @@ struct NormalizedKeywordSearchTermVisit {
   NormalizedKeywordSearchTermVisit() = default;
   ~NormalizedKeywordSearchTermVisit();
 
+  // Returns the frecency score of the visit based on the following formula:
+  //            (frequency ^ frequency_exponent) * recency_decay_unit_in_seconds
+  // frecency = ————————————————————————————————————————————————————————————————
+  //                   recency_in_seconds + recency_decay_unit_in_seconds
+  // This score combines frequency and recency of the visit favoring ones that
+  // are more frequent and more recent (see go/local-zps-frecency-ranking).
+  // |recency_decay_unit_sec| is the number of seconds until the recency
+  // component of the score decays to half. |frequency_exponent| is factor by
+  // which the frequency of the visit is exponentiated.
+  double GetFrecency(base::Time now,
+                     int recency_decay_unit_sec,
+                     double frequency_exponent) const;
+
   base::string16 normalized_term;     // The search term, in lower case and with
                                       // extra whitespaces collapsed.
   int visits{0};                      // The visit count.

components/omnibox/browser/local_history_zero_suggest_provider.cc

@@ -94,21 +94,6 @@ bool AllowLocalHistoryZeroSuggestSuggestions(const AutocompleteInput& input) {
 #endif
 }
 
-// Helper function for calculating frecency of a visit based on this formula:
-//            (frequency ^ frequency_exponent) * recency_decay_unit_in_seconds
-// frecency = ————————————————————————————————————————————————————————————————
-//                   recency_in_seconds + recency_decay_unit_in_seconds
-// a frecency score combines frequency and recency of occurrences favoring ones
-// that are more frequent and more recent (see go/local-zps-frecency-ranking).
-double CalculateFrecency(const history::NormalizedKeywordSearchTermVisit& visit,
-                         base::Time now) {
-  const double recency_sec =
-      base::TimeDelta(now - visit.most_recent_visit_time).InSeconds();
-  const double recency_decayed = 60 / (recency_sec + 60);
-  const double frequency_powered = pow(visit.visits, 1.15);
-  return frequency_powered * recency_decayed;
-}
-
 }  // namespace
 
 // static
@@ -249,12 +234,16 @@ void LocalHistoryZeroSuggestProvider::QueryURLDatabase(
   bool frecency_ranking = base::FeatureList::IsEnabled(
       omnibox::kOmniboxLocalZeroSuggestFrecencyRanking);
   const base::Time now = base::Time::Now();
-  std::sort(results.begin(), results.end(),
-            [frecency_ranking, now](const auto& a, const auto& b) {
+  const int kRecencyDecayUnitSec = 60;
+  const double kFrequencyExponent = 1.15;
+  auto CompareByFrecency = [&](const auto& a, const auto& b) {
     return frecency_ranking
-                         ? CalculateFrecency(a, now) > CalculateFrecency(b, now)
+               ? a.GetFrecency(now, kRecencyDecayUnitSec, kFrequencyExponent) >
+                     b.GetFrecency(now, kRecencyDecayUnitSec,
+                                   kFrequencyExponent)
                : a.most_recent_visit_time > b.most_recent_visit_time;
-            });
+  };
+  std::sort(results.begin(), results.end(), CompareByFrecency);
 
   int relevance = kLocalHistoryZeroSuggestRelevance;
   for (const auto& result : results) {