Changing confidence calculation to avoid div by zero

chrome/browser/autocomplete/network_action_predictor.cc

 // Copyright (c) 2011 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 "chrome/browser/autocomplete/network_action_predictor.h"
 
 #include <math.h>
 
 #include "base/i18n/case_conversion.h"
 #include "base/metrics/histogram.h"
@@ skipping 34 matching lines @@
     return ACTION_NONE;
 
   history::URLRow url_row;
   history::URLID url_id = url_db->GetRowForURL(match.destination_url, &url_row);
 
   if (url_id == 0)
     return ACTION_NONE;
 
   const double base_score = 1.0;
 
-  // This constant is ln(1/0.8) so we end up decaying to 80% of the base score
+  // This constant is ln(1/0.65) so we end up decaying to 65% of the base score
   // for each week that passes.
-  const double kLnDecayPercent = 0.2231435513142097;
+  const double kLnDecayPercent = 0.43078291609245;
   base::TimeDelta time_passed = base::Time::Now() - url_row.last_visit();
 
   // Clamp to 0.
   const double decay_exponent = std::max(0.0,
       kLnDecayPercent * static_cast<double>(time_passed.InMicroseconds()) /
           base::Time::kMicrosecondsPerWeek);
 
   const double kMaxDecaySpeedDivisor = 5.0;
-  const double kNumUsesPerDecaySpeedDivisorIncrement = 1.0;
+  const double kNumUsesPerDecaySpeedDivisorIncrement = 2.0;
   const double decay_divisor = std::min(kMaxDecaySpeedDivisor,
       (url_row.typed_count() + kNumUsesPerDecaySpeedDivisorIncrement - 1) /
           kNumUsesPerDecaySpeedDivisorIncrement);
 
   const double confidence = base_score / exp(decay_exponent / decay_divisor);
-
   CHECK(confidence >= 0.0 && confidence <= 1.0);
 
   UMA_HISTOGRAM_COUNTS_100("NetworkActionPredictor.Confidence",
                            confidence * 100);
 
   for (int i = 0; i < LAST_PREDICT_ACTION; ++i)
     if (confidence >= kConfidenceCutoff[i])
       return static_cast<Action>(i);
   return ACTION_NONE;
 }
 
 // Return true if the suggestion type warrants a TCP/IP preconnection.
 // i.e., it is now quite likely that the user will select the related domain.
 // static
 bool NetworkActionPredictor::IsPreconnectable(const AutocompleteMatch& match) {
   switch (match.type) {
     // Matches using the user's default search engine.
     case AutocompleteMatch::SEARCH_WHAT_YOU_TYPED:
     case AutocompleteMatch::SEARCH_HISTORY:
     case AutocompleteMatch::SEARCH_SUGGEST:
       // A match that uses a non-default search engine (e.g. for tab-to-search).
     case AutocompleteMatch::SEARCH_OTHER_ENGINE:
       return true;
 
     default:
       return false;
   }
 }