feat: check image sources in picture and srcset

Message changes:

- new `MED-007` (`ERROR`) message is reported when an image source
  references a foreign resource (unless on `source` elements which
  `type` attribute specify non-core media type).
- reworded `MED-003`; it now includes the resource path.

Internal changes:

- add a `SourceSet` class to represent an image source set, along with
  a parser folloing the "parsing a srcset attribute" algorithm from HTML.
  See https://html.spec.whatwg.org/#parsing-a-srcset-attribute
- add to new reference types to the `XRefChecker` class, `PICTURE_SOURCE`
  and `PICTURE_SOURCE_FOREIGN`, to identify image source references in
  `picture` elements.
- add tests, for both the `srcset` parser and the new fallback rules.

Fix #781

src/main/java/com/adobe/epubcheck/messages/DefaultSeverities.java

@@ -144,6 +144,7 @@ private void initialize()
     severities.put(MessageId.MED_004, Severity.ERROR);
     severities.put(MessageId.MED_005, Severity.ERROR);
     severities.put(MessageId.MED_006, Severity.USAGE);
+    severities.put(MessageId.MED_007, Severity.ERROR);
 
     // NAV
     severities.put(MessageId.NAV_001, Severity.ERROR);

src/main/java/com/adobe/epubcheck/messages/MessageId.java

@@ -137,6 +137,7 @@ public enum MessageId implements Comparable<MessageId>
   MED_004("MED-004"),
   MED_005("MED-005"),
   MED_006("MED_006"),
+  MED_007("MED_007"),
 
   // Epub3 based table of content errors
   NAV_001("NAV-001"),

src/main/java/com/adobe/epubcheck/opf/XRefChecker.java

@@ -68,7 +68,9 @@ public static enum Type
     REGION_BASED_NAV,
     SEARCH_KEY,
     NAV_TOC_LINK,
-    NAV_PAGELIST_LINK;
+    NAV_PAGELIST_LINK,
+    PICTURE_SOURCE,
+    PICTURE_SOURCE_FOREIGN;
   }
 
   private static class Reference
@@ -98,6 +100,7 @@ public Reference(String srcResource, int srcLineNumber, int srcColumnNumber, Str
   private static class Anchor
   {
 
+    @SuppressWarnings("unused")
     public final String id;
     public final Type type;
     public final int position;
@@ -383,19 +386,28 @@ else if (!undeclared.contains(ref.refResource)
       }
       break;
     case IMAGE:
+    case PICTURE_SOURCE:
+    case PICTURE_SOURCE_FOREIGN:
       if (ref.fragment != null && !res.item.getMimeType().equals("image/svg+xml"))
       {
         report.message(MessageId.RSC_009, EPUBLocation.create(ref.source, ref.lineNumber,
             ref.columnNumber, ref.refResource + "#" + ref.fragment));
         return;
       }
       // if mimeType is null, we should have reported an error already
-      if (!OPFChecker.isBlessedImageType(res.item.getMimeType()) && !res.hasValidImageFallback)
+      if (!OPFChecker.isBlessedImageType(res.item.getMimeType()))
       {
-        report.message(MessageId.MED_003,
-            EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
-            res.item.getMimeType());
-        return;
+        if (version == EPUBVersion.VERSION_3 && ref.type == Type.PICTURE_SOURCE) {
+          report.message(MessageId.MED_007,
+              EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
+              ref.refResource, res.item.getMimeType());
+          return;
+        }
+        else if (ref.type == Type.IMAGE && !res.hasValidImageFallback) {
+          report.message(MessageId.MED_003,
+              EPUBLocation.create(ref.source, ref.lineNumber, ref.columnNumber),
+              ref.refResource, res.item.getMimeType());
+        }
       }
       break;
     case SEARCH_KEY:

src/main/java/com/adobe/epubcheck/ops/OPSHandler30.java

@@ -6,6 +6,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.TreeSet;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -19,6 +20,7 @@
 import com.adobe.epubcheck.util.EpubConstants;
 import com.adobe.epubcheck.util.FeatureEnum;
 import com.adobe.epubcheck.util.PathUtil;
+import com.adobe.epubcheck.util.SourceSet;
 import com.adobe.epubcheck.vocab.AggregateVocab;
 import com.adobe.epubcheck.vocab.AltStylesheetVocab;
 import com.adobe.epubcheck.vocab.ComicsVocab;
@@ -70,6 +72,7 @@ public class OPSHandler30 extends OPSHandler
 
   protected boolean inVideo = false;
   protected boolean inAudio = false;
+  protected boolean inPicture = false;
   protected boolean hasValidFallback = false;
 
   protected int imbricatedObjects = 0;
@@ -134,6 +137,59 @@ public OPSHandler30(ValidationContext context, XMLParser parser)
         .contains(EpubCheckVocab.VOCAB.get(EpubCheckVocab.PROPERTIES.NON_LINEAR));
   }
 
+  protected void checkImage(XMLElement e, String attrNS, String attr)
+  {
+    // if it's an SVG image, fall back to super's logic
+    String ns = e.getNamespace();
+    if ("http://www.w3.org/2000/svg".equals(ns))
+    {
+      super.checkImage(e, attrNS, attr);
+    }
+    // else process image source sets in HTML
+    else if (xrefChecker.isPresent())
+    {
+      String src = e.getAttribute("src");
+      String srcset = e.getAttribute("srcset");
+      // if we're in a 'picture' element
+      if (inPicture)
+      {
+        String type = e.getAttribute("type");
+        // if in a 'source' element specifying a foreign MIME type,
+        // register as foreign picture source
+        if ("source".equals(e.getName()) && type != null && !OPFChecker.isBlessedImageType(type))
+        {
+          registerImageSources(src, srcset, XRefChecker.Type.PICTURE_SOURCE_FOREIGN);
+        }
+        // else register as regular picture source (must be a CMT)
+        else
+        // register as picture source
+        {
+          registerImageSources(src, srcset, XRefChecker.Type.PICTURE_SOURCE);
+        }
+      }
+      // register as regular image sources (must be a CMT or have a manifest fallback
+      else
+      {
+        registerImageSources(src, srcset, XRefChecker.Type.IMAGE);
+      }
+    }
+  }
+
+  protected void registerImageSources(String src, String srcset, XRefChecker.Type type)
+  {
+    // compute a list of URLs to register
+    Set<String> urls = new TreeSet<>();
+    if (src != null) urls.add(src);
+    urls.addAll(SourceSet.parse(srcset).getImageURLs());
+    // register all the URLs
+    for (String url : urls)
+    {
+      xrefChecker.get().registerReference(path, parser.getLineNumber(), parser.getColumnNumber(),
+          PathUtil.resolveRelativeReference(base, url), type);
+    }
+
+  }
+
   protected void checkType(XMLElement e, String type)
   {
     if (type == null)
@@ -305,6 +361,14 @@ else if (name.equals("annotation-xml"))
     {
       hasAltorAnnotation = true;
     }
+    else if (name.equals("picture"))
+    {
+      inPicture = true;
+    }
+    else if (name.equals("source"))
+    {
+      if (inPicture) checkImage(e, null, null);
+    }
     else if ("http://www.w3.org/2000/svg".equals(e.getNamespace()) && name.equals("title"))
     {
       hasTitle = true;
@@ -709,6 +773,10 @@ else if (name.equals("nav") && inRegionBasedNav)
     {
       inRegionBasedNav = false;
     }
+    else if (name.equals("picture"))
+    {
+      inPicture = false;
+    }
     else if (name.equals("svg"))
     {
       inSvg = false;