Avoid srcset resource download when higher density resource is in cache.

Source/core/html/parser/HTMLSrcsetParser.cpp

 /*
  * Copyright (C) 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 15 matching lines @@
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/html/parser/HTMLSrcsetParser.h"
 
 #include "core/dom/Document.h"
+#include "core/fetch/MemoryCache.h"
 #include "core/frame/FrameConsole.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/UseCounter.h"
 #include "core/html/parser/HTMLParserIdioms.h"
 #include "core/inspector/ConsoleMessage.h"
 #include "platform/ParsingUtilities.h"
 #include "platform/RuntimeEnabledFeatures.h"
 
 namespace blink {
 
@@ skipping 301 matching lines @@
 
         currentDensity = imageCandidates[i].density();
         geometricMean = sqrt(currentDensity * nextDensity);
         if (deviceScaleFactor >= geometricMean)
             return next;
         break;
     }
     return i;
 }
 
-static ImageCandidate pickBestImageCandidate(float deviceScaleFactor, float sourceSize, Vector<ImageCandidate>& imageCandidates)
+static unsigned avoidDownloadIfHigherDensityResourceIsInCache(Vector<ImageCandidate>& imageCandidates, unsigned winner, Document* document)
+{
+    if (!document)
+        return winner;
+    for (unsigned i = imageCandidates.size() - 1; i > winner; --i) {
+        KURL url = document->completeURL(stripLeadingAndTrailingHTMLSpaces(imageCandidates[i].url()));
+        if (memoryCache()->resourceForURL(url))
+            return i;
+    }
+    return winner;
+}
+
+static ImageCandidate pickBestImageCandidate(float deviceScaleFactor, float sourceSize, Vector<ImageCandidate>& imageCandidates, Document* document = nullptr)
 {
     const float defaultDensityValue = 1.0;
     bool ignoreSrc = false;
     if (imageCandidates.isEmpty())
         return ImageCandidate();
 
     // http://picture.responsiveimages.org/#normalize-source-densities
     for (Vector<ImageCandidate>::iterator it = imageCandidates.begin(); it != imageCandidates.end(); ++it) {
         if (it->resourceWidth() > 0) {
             it->setDensity((float)it->resourceWidth() / sourceSize);
             ignoreSrc = true;
         } else if (it->density() < 0) {
             it->setDensity(defaultDensityValue);
         }
     }
 
     std::stable_sort(imageCandidates.begin(), imageCandidates.end(), compareByDensity);
 
-    unsigned i = selectionLogic(imageCandidates, deviceScaleFactor, ignoreSrc);
-    ASSERT(i < imageCandidates.size());
+    unsigned winner = selectionLogic(imageCandidates, deviceScaleFactor, ignoreSrc);
+    ASSERT(winner < imageCandidates.size());

[Mike West, 2014/10/24 08:03:13]

&& >= 0?

+    winner = avoidDownloadIfHigherDensityResourceIsInCache(imageCandidates, winner, document);
 
-    float winningDensity = imageCandidates[i].density();
-
-    unsigned winner = i;
+    float winningDensity = imageCandidates[winner].density();
     // 16. If an entry b in candidates has the same associated ... pixel density as an earlier entry a in candidates,
     // then remove entry b
-    while ((i > 0) && (imageCandidates[--i].density() == winningDensity))
-        winner = i;
+    while ((winner > 0) && (imageCandidates[winner - 1].density() == winningDensity))
+        --winner;
 
     return imageCandidates[winner];
 }
 
 ImageCandidate bestFitSourceForSrcsetAttribute(float deviceScaleFactor, float sourceSize, const String& srcsetAttribute, Document* document)
 {
     Vector<ImageCandidate> imageCandidates;
 
     parseImageCandidatesFromSrcsetAttribute(srcsetAttribute, imageCandidates, document);
 
-    return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates);
+    return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates, document);
 }
 
 ImageCandidate bestFitSourceForImageAttributes(float deviceScaleFactor, float sourceSize, const String& srcAttribute, const String& srcsetAttribute, Document* document)
 {
     if (srcsetAttribute.isNull()) {
         if (srcAttribute.isNull())
             return ImageCandidate();
         return ImageCandidate(srcAttribute, 0, srcAttribute.length(), DescriptorParsingResult(), ImageCandidate::SrcOrigin);
     }
 
     Vector<ImageCandidate> imageCandidates;
 
     parseImageCandidatesFromSrcsetAttribute(srcsetAttribute, imageCandidates, document);
 
     if (!srcAttribute.isEmpty())
         imageCandidates.append(ImageCandidate(srcAttribute, 0, srcAttribute.length(), DescriptorParsingResult(), ImageCandidate::SrcOrigin));
 
-    return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates);
+    return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates, document);
 }
 
 String bestFitSourceForImageAttributes(float deviceScaleFactor, float sourceSize, const String& srcAttribute, ImageCandidate& srcsetImageCandidate)
 {
     if (srcsetImageCandidate.isEmpty())
         return srcAttribute;
 
     Vector<ImageCandidate> imageCandidates;
     imageCandidates.append(srcsetImageCandidate);
 
     if (!srcAttribute.isEmpty())
         imageCandidates.append(ImageCandidate(srcAttribute, 0, srcAttribute.length(), DescriptorParsingResult(), ImageCandidate::SrcOrigin));
 
     return pickBestImageCandidate(deviceScaleFactor, sourceSize, imageCandidates).toString();
 }
 
 }