Matroska: Use seek head for faster element lookup (#1321)

Limit scan for Matroska seek head to 512 KB in ReadStyle::Fast

---------

Co-authored-by: tolriq <git@leetzone.org>

Author: ufleisch

examples/tagreader.cpp

@@ -117,4 +117,3 @@ int main(int argc, char *argv[])
   }
   return 0;
 }
-

taglib/fileref.cpp

@@ -225,7 +225,7 @@ namespace
 #endif
 #ifdef TAGLIB_WITH_MATROSKA
     else if(ext == "MKA" || ext == "MKV" || ext == "WEBM")
-      file = new Matroska::File(stream, readAudioProperties);
+      file = new Matroska::File(stream, readAudioProperties, audioPropertiesStyle);
 #endif
 
     // if file is not valid, leave it to content-based detection.

taglib/matroska/ebml/ebmlmksegment.cpp

@@ -30,6 +30,32 @@
 
 using namespace TagLib;
 
+namespace {
+
+template <EBML::Element::Id Id, typename ElementType>
+std::unique_ptr<ElementType> readElementAt(File &file,
+                                           offset_t offset,
+                                           offset_t maxOffset)
+{
+  if(offset < 0 || offset >= maxOffset) {
+    return nullptr;
+  }
+
+  file.seek(offset);
+  auto element = EBML::Element::factory(file);
+  if(!element || element->getId() != Id) {
+    return nullptr;
+  }
+
+  auto typed = EBML::element_cast<Id>(std::move(element));
+  if(!typed || !typed->read(file)) {
+    return nullptr;
+  }
+  return typed;
+}
+
+} // namespace
+
 EBML::MkSegment::MkSegment(int sizeLength, offset_t dataSize, offset_t offset):
   MasterElement(Id::MkSegment, sizeLength, dataSize, offset)
 {
@@ -49,16 +75,64 @@ offset_t EBML::MkSegment::segmentDataOffset() const
 
 bool EBML::MkSegment::read(File &file)
 {
-  const offset_t maxOffset = file.tell() + dataSize;
+  return readLimited(file, dataSize);
+}
+
+bool EBML::MkSegment::readLimited(File &file, offset_t scanLimit)
+{
+  const offset_t filePos = file.tell();
+  const offset_t maxOffset = filePos + dataSize;
+  const offset_t maxScanOffset = filePos + std::min(scanLimit, dataSize);
   std::unique_ptr<Element> element;
-  int i = 0;
-  int seekHeadIndex = -1;
-  while((element = findNextElement(file, maxOffset))) {
+  while((element = findNextElement(file, maxScanOffset))) {
     if(const Id id = element->getId(); id == Id::MkSeekHead) {
-      seekHeadIndex = i;
       seekHead = element_cast<Id::MkSeekHead>(std::move(element));
       if(!seekHead->read(file))
         return false;
+      // We have a seek head, let's use it for faster access to the other elements
+      if(const auto elementAfterSeekHead = findNextElement(file, maxScanOffset);
+         elementAfterSeekHead && elementAfterSeekHead->getId() == Id::VoidElement)
+        seekHead->setPadding(elementAfterSeekHead->getSize());
+      const offset_t segDataOffset = segmentDataOffset();
+      const auto matroskaSeekHead = parseSeekHead();
+      for(const auto &[idValue, relativeOffset] : matroskaSeekHead->entryList()) {
+        const offset_t absoluteOffset = segDataOffset + relativeOffset;
+        switch(static_cast<Id>(idValue)) {
+        case Id::MkCues:
+          if(!((cues = readElementAt<Id::MkCues, MkCues>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        case Id::MkInfo:
+          if(!((info = readElementAt<Id::MkInfo, MkInfo>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        case Id::MkTracks:
+          if(!((tracks = readElementAt<Id::MkTracks, MkTracks>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        case Id::MkTags:
+          if(!((tags = readElementAt<Id::MkTags, MkTags>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        case Id::MkAttachments:
+          if(!((attachments = readElementAt<Id::MkAttachments, MkAttachments>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        case Id::MkChapters:
+          if(!((chapters = readElementAt<Id::MkChapters, MkChapters>(
+            file, absoluteOffset, maxOffset))))
+            return false;
+          break;
+        default:
+          break;
+        }
+      }
+      return true;
     }
     else if(id == Id::MkCues) {
       cues = element_cast<Id::MkCues>(std::move(element));
@@ -91,14 +165,8 @@ bool EBML::MkSegment::read(File &file)
         return false;
     }
     else {
-      if(id == Id::VoidElement
-         && seekHead
-         && seekHeadIndex == i - 1)
-        seekHead->setPadding(element->getSize());
-
       element->skipData(file);
     }
-    i++;
   }
   return true;
 }

taglib/matroska/ebml/ebmlmksegment.h

@@ -51,6 +51,7 @@ namespace TagLib {
 
       offset_t segmentDataOffset() const;
       bool read(File &file) override;
+      bool readLimited(File &file, offset_t scanLimit);
       std::unique_ptr<Matroska::Tag> parseTag() const;
       std::unique_ptr<Matroska::Attachments> parseAttachments() const;
       std::unique_ptr<Matroska::Chapters> parseChapters() const;

taglib/matroska/matroskafile.cpp

@@ -144,6 +144,8 @@ PropertyMap Matroska::File::setProperties(const PropertyMap &properties)
 
 namespace {
 
+  constexpr offset_t FAST_SCAN_LIMIT = static_cast<offset_t>(512 * 1024);
+
   String keyForAttachedFile(const Matroska::AttachedFile &attachedFile)
   {
     if(attachedFile.mediaType().startsWith("image/")) {
@@ -376,10 +378,15 @@ void Matroska::File::read(bool readProperties, Properties::ReadStyle readStyle)
     head->skipData(*this);
   }
 
+  offset_t maxOffset = fileLength - tell();
+  if (readStyle == Properties::ReadStyle::Fast && maxOffset > FAST_SCAN_LIMIT) {
+    maxOffset = FAST_SCAN_LIMIT;
+  }
+
   // Find the Matroska segment in the file
   const std::unique_ptr<EBML::MkSegment> segment(
     EBML::element_cast<EBML::Element::Id::MkSegment>(
-      EBML::findElement(*this, EBML::Element::Id::MkSegment, fileLength - tell())
+      EBML::findElement(*this, EBML::Element::Id::MkSegment, maxOffset)
     )
   );
   if(!segment) {
@@ -389,14 +396,18 @@ void Matroska::File::read(bool readProperties, Properties::ReadStyle readStyle)
   }
 
   // Read the segment into memory from file
-  if(!segment->read(*this)) {
+  d->segment = segment->parseSegment();
+  maxOffset = segment->getDataSize();
+  if (readStyle == Properties::ReadStyle::Fast && maxOffset > FAST_SCAN_LIMIT) {
+    maxOffset = FAST_SCAN_LIMIT;
+  }
+  if(!segment->readLimited(*this, maxOffset)) {
     debug("Failed to read segment");
     setValid(false);
     return;
   }
 
   // Parse the elements
-  d->segment = segment->parseSegment();
   d->seekHead = segment->parseSeekHead();
   d->cues = segment->parseCues();
   d->tag = segment->parseTag();

taglib/matroska/matroskaseekhead.cpp

@@ -54,7 +54,6 @@ bool Matroska::SeekHead::isValid(TagLib::File &file) const
 void Matroska::SeekHead::addEntry(const Element &element)
 {
   entries.append({element.id(), element.offset()});
-  debug("adding to seekhead");
   setNeedsRender(true);
 }
 
@@ -64,6 +63,11 @@ void Matroska::SeekHead::addEntry(ID id, offset_t offset)
   setNeedsRender(true);
 }
 
+const List<std::pair<unsigned int, offset_t>> &Matroska::SeekHead::entryList() const
+{
+  return entries;
+}
+
 ByteVector Matroska::SeekHead::renderInternal()
 {
   const auto beforeSize = sizeRenderedOrWritten();

taglib/matroska/matroskaseekhead.h

@@ -39,6 +39,7 @@ namespace TagLib {
       bool isValid(TagLib::File &file) const;
       void addEntry(const Element &element);
       void addEntry(ID id, offset_t offset);
+      const List<std::pair<unsigned int, offset_t>> &entryList() const;
       void write(TagLib::File &file) override;
       void sort();
       bool sizeChanged(Element &caller, offset_t delta) override;