Enable srcset support in HTMLImageElement

Source/core/html/parser/HTMLParserIdioms.cpp

 /*
  * Copyright (C) 2010 Apple 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:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 11 matching lines @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/html/parser/HTMLParserIdioms.h"
 
 #include <limits>
 #include "core/dom/QualifiedName.h"
 #include "core/html/parser/HTMLIdentifier.h"
 #include "core/platform/Decimal.h"
+#include "core/platform/text/DecodeEscapeSequences.h"
 #include "wtf/MathExtras.h"
 #include "wtf/text/AtomicString.h"
 #include "wtf/text/StringBuilder.h"
 
 namespace WebCore {
 
 template <typename CharType>
 static String stripLeadingAndTrailingHTMLSpaces(String string, CharType characters, unsigned length)
 {
     unsigned numLeadingSpaces = 0;
@@ skipping 247 matching lines @@
 bool threadSafeMatch(const QualifiedName& a, const QualifiedName& b)
 {
     return threadSafeEqual(a.localName().impl(), b.localName().impl());
 }
 
 bool threadSafeMatch(const HTMLIdentifier& localName, const QualifiedName& qName)
 {
     return threadSafeEqual(localName.asStringImpl(), qName.localName().impl());
 }
 
+struct ImageWithScale {
+    String imageURL;
+    float scaleFactor;
+    bool operator==(const ImageWithScale& image) const
+    {
+        return scaleFactor == image.scaleFactor && imageURL == image.imageURL;
+    }
+};
+typedef Vector<ImageWithScale> ImageCandidates;
+
+static inline bool compareByScaleFactor(const ImageWithScale& first, const ImageWithScale& second)
+{
+    return first.scaleFactor < second.scaleFactor;
 }
+
+String bestFitSourceForImageAttributes(float deviceScaleFactor, const String& srcAttribute, const String& srcSetAttribute)

[abarth-chromium, 2013/09/13 05:54:07]

This is too much code for this file.  Please put this function in it's own file.

Parsing the srcset attribute isn't really an idiom of the HTML parser.  It's
something specific to HTMLImageElement.  Perhaps it should be a static function
in HTMLImageElement?

[Yoav Weiss, 2013/09/13 07:50:58]

Agreed that it's not an idiom. Since this is also used by HTMLPreloadScanner, I
believe it would be better if this code in in its own file (e.g.
HTMLSrcsetParser). If you disagree, I can add it to HTMLImageElement as a static
function.

[abarth-chromium, 2013/09/13 17:58:12]

Sounds great.

+{
+    ImageCandidates imageCandidates;
+
+    const String srcSetAttributeValue = srcSetAttribute.simplifyWhiteSpace(isHTMLSpace);

[abarth-chromium, 2013/09/13 05:54:07]

No need for |const|.  The constness of a string is basically meaningless.

+    Vector<String> srcSetTokens;
+
+    srcSetAttributeValue.split(',', srcSetTokens);
+    for (size_t i = 0; i < srcSetTokens.size(); ++i) {
+        Vector<String> data;
+        float imgScaleFactor = 1.0;
+        bool validScaleFactor = false;
+
+        srcSetTokens[i].stripWhiteSpace().split(' ', data);
+        // There must be at least one candidate descriptor, and the last one must
+        // be a scale factor. Since we don't support descriptors other than scale,
+        // it's better to discard any rule with such descriptors rather than accept
+        // only the scale data.
+        if (data.size() != 2)
+            continue;
+        if (!data.last().endsWith('x'))
+            continue;
+
+        imgScaleFactor = data.last().substring(0, data.last().length() - 1).toFloat(&validScaleFactor);

[abarth-chromium, 2013/09/13 05:54:07]

This whole implementation is really inefficient.  You're creating a ton of
temporary strings just to parse out a few floating point numbers.  Maybe it
doesn't matter for performance.

[Yoav Weiss, 2013/09/13 07:50:58]

There's work being done by Romain Perier on the WebKit implementation to improve
the parsing efficiency[1]. I was thinking of porting that work as an improvement
CL once it's done. If you deem this critical, I can port the existing work to
this CL.

[1]https://bugs.webkit.org/show_bug.cgi?id=119423

+        if (!validScaleFactor)
+            continue;
+
+        ImageWithScale image;
+        image.imageURL = decodeEscapeSequences<URLEscapeSequence>(data[0], UTF8Encoding());
+        image.scaleFactor = imgScaleFactor;
+
+        imageCandidates.append(image);

[abarth-chromium, 2013/09/13 05:54:07]

Please call imageCandidates.reserveInitialCapacity(srcSetTokens.size()) before
this loop starts to at least avoid reallocing that vector.

+    }
+
+    const String src =  srcAttribute.simplifyWhiteSpace(isHTMLSpace);

[abarth-chromium, 2013/09/13 05:54:07]

No const, please.  Also, you've got an extra space after the =

+    if (!src.isEmpty()) {
+        ImageWithScale image;
+        image.imageURL = decodeEscapeSequences<URLEscapeSequence>(src, UTF8Encoding());
+        image.scaleFactor = 1.0;
+
+        imageCandidates.append(image);
+    }
+
+    if (imageCandidates.isEmpty())
+        return String();
+
+    std::stable_sort(imageCandidates.begin(), imageCandidates.end(), compareByScaleFactor);
+
+    for (size_t i = 0; i < imageCandidates.size() - 1; ++i) {
+        if (imageCandidates[i].scaleFactor >= deviceScaleFactor)
+            return imageCandidates[i].imageURL;
+    }
+    return imageCandidates.last().imageURL;
+}
+
+}