Changed the app list string matching formula.

ui/app_list/search/tokenized_string_match.cc

 // Copyright 2013 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 "ui/app_list/search/tokenized_string_match.h"
 
+#include <cmath>
+
 #include "base/i18n/string_search.h"
 #include "base/logging.h"
 #include "ui/app_list/search/tokenized_string_char_iterator.h"
 
 namespace app_list {
 
 namespace {
 
 // The factors below are applied when the current char of query matches
 // the current char of the text to be matched. Different factors are chosen
@@ skipping 194 matching lines @@
     if (base::i18n::StringSearchIgnoringCaseAndAccents(query.text(),
                                                        text.text(),
                                                        &substr_match_start,
                                                        &substr_match_length)) {
       relevance_ = kIsSubstringMultiplier * substr_match_length;
       hits_.push_back(gfx::Range(substr_match_start,
                                  substr_match_start + substr_match_length));
     }
   }
 
-  // Using length() for normalizing is not 100% correct but should be good
-  // enough compared with using real char count of the text.
-  if (text.text().length())
-    relevance_ /= text.text().length();
+  // Temper the relevance score with an exponential curve. Each point of
+  // relevance (roughly, each keystroke) is worth less than the last. This means
+  // that typing a few characters of a word is enough to promote matches very
+  // high, with any subsequent characters being worth comparatively less.
+  // TODO(mgiuca): This doesn't really play well with Omnibox results, since as
+  // you type more characters, the app/omnibox results tend to jump over each
+  // other.
+  relevance_ = 1.0 - std::pow(0.5, relevance_);
 
   return relevance_ > kNoMatchScore;
 }
 
 bool TokenizedStringMatch::Calculate(const base::string16& query,
                                      const base::string16& text) {
   const TokenizedString tokenized_query(query);
   const TokenizedString tokenized_text(text);
   return Calculate(tokenized_query, tokenized_text);
 }
 
 }  // namespace app_list