Add heuristics to skip thumbnail generation when it's unnecessary.

chrome/common/thumbnail_score.h

 // Copyright (c) 2009 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.
 
 #ifndef CHROME_COMMON_THUMBNAIL_SCORE_H_
 #define CHROME_COMMON_THUMBNAIL_SCORE_H_
 #pragma once
 
+#include <string>
 #include "base/time.h"
 
 // A set of metadata about a Thumbnail.
 struct ThumbnailScore {
   // Initializes the ThumbnailScore to the absolute worst possible values
   // except for time, which is set to Now(), and redirect_hops_from_dest which
   // is set to 0.
   ThumbnailScore();
 
   // Builds a ThumbnailScore with the passed in values, and sets the
   // thumbnail generation time to Now().
   ThumbnailScore(double score, bool clipping, bool top);
 
   // Builds a ThumbnailScore with the passed in values.
   ThumbnailScore(double score, bool clipping, bool top,
                  const base::Time& time);
   ~ThumbnailScore();
 
   // Tests for equivalence between two ThumbnailScore objects.
   bool Equals(const ThumbnailScore& rhs) const;
 
+  // Returns string representation of this object.
+  std::string ToString() const;
+
   // How "boring" a thumbnail is. The boring score is the 0,1 ranged
   // percentage of pixels that are the most common luma. Higher boring
   // scores indicate that a higher percentage of a bitmap are all the
   // same brightness (most likely the same color).
   double boring_score;
 
   // Whether the thumbnail was taken with height greater then
   // width. In cases where we don't have |good_clipping|, the
   // thumbnails are either clipped from the horizontal center of the
   // window, or are otherwise weirdly stretched.
@@ skipping 20 matching lines @@
   // by the comparison function.
   //
   // If "http://google.com/" redirected to "http://www.google.com/", then
   // a thumbnail for the first would have a redirect hop of 1, and the second
   // would have a redirect hop of 0.
   int redirect_hops_from_dest;
 
   // How bad a thumbnail needs to be before we completely ignore it.
   static const double kThumbnailMaximumBoringness;
 
+  // We consider a thumbnail interesting enough if the boring score is
+  // lower than this.
+  static const double kThumbnailInterestingEnoughBoringness;
+
   // Time before we take a worse thumbnail (subject to
   // kThumbnailMaximumBoringness) over what's currently in the database
   // for freshness.
   static const base::TimeDelta kUpdateThumbnailTime;
 
   // Penalty of how much more boring a thumbnail should be per hour.
   static const double kThumbnailDegradePerHour;
+
+  // Checks whether we should consider updating a new thumbnail based on
+  // this score. For instance, we don't have to update a new thumbnail
+  // if the current thumbnail is new and interesting enough.
+  bool ShouldConsiderUpdating();
 };
 
 // Checks whether we should replace one thumbnail with another.
 bool ShouldReplaceThumbnailWith(const ThumbnailScore& current,
                                 const ThumbnailScore& replacement);
 
 #endif  // CHROME_COMMON_THUMBNAIL_SCORE_H_