Bug 1044498 - Improve accuracy of WebM buffered parser. r=cajbir

Store the end offset of the block, rather than the start offset, since a
frame is only usable if we have all of it.  Also compute the start of the
buffered range based on the offset of the block's resync point, since we
can't decode frames unless we have their cluster to resync at and compute
the absolute timecode from.

content/media/webm/WebMBufferedParser.cpp

@@ -144,14 +144,16 @@ void WebMBufferedParser::Append(const unsigned char* aBuffer, uint32_t aLength,
         // duplicate WebMTimeDataOffset entries.
         {
           ReentrantMonitorAutoEnter mon(aReentrantMonitor);
-          uint32_t idx = aMapping.IndexOfFirstElementGt(mBlockOffset);
-          if (idx == 0 || !(aMapping[idx - 1] == mBlockOffset)) {
+          int64_t endOffset = mBlockOffset + mBlockSize +
+                              mElement.mID.mLength + mElement.mSize.mLength;
+          uint32_t idx = aMapping.IndexOfFirstElementGt(endOffset);
+          if (idx == 0 || aMapping[idx - 1] != endOffset) {
             // Don't insert invalid negative timecodes.
             if (mBlockTimecode >= 0 || mClusterTimecode >= uint16_t(abs(mBlockTimecode))) {
               MOZ_ASSERT(mGotTimecodeScale);
               uint64_t absTimecode = mClusterTimecode + mBlockTimecode;
               absTimecode *= mTimecodeScale;
-              WebMTimeDataOffset entry(mBlockOffset, absTimecode, mClusterOffset);
+              WebMTimeDataOffset entry(endOffset, absTimecode, mClusterOffset);
               aMapping.InsertElementAt(idx, entry);
             }
           }
@@ -182,19 +184,29 @@ void WebMBufferedParser::Append(const unsigned char* aBuffer, uint32_t aLength,
   mCurrentOffset += aLength;
 }
 
+struct SyncOffsetComparator {
+  bool Equals(const WebMTimeDataOffset& a, const int64_t& b) const {
+    return a.mSyncOffset == b;
+  }
+
+  bool LessThan(const WebMTimeDataOffset& a, const int64_t& b) const {
+    return a.mSyncOffset < b;
+  }
+};
+
 bool WebMBufferedState::CalculateBufferedForRange(int64_t aStartOffset, int64_t aEndOffset,
                                                   uint64_t* aStartTime, uint64_t* aEndTime)
 {
   ReentrantMonitorAutoEnter mon(mReentrantMonitor);
 
   // Find the first WebMTimeDataOffset at or after aStartOffset.
-  uint32_t start = mTimeMapping.IndexOfFirstElementGt(aStartOffset - 1);
+  uint32_t start = mTimeMapping.IndexOfFirstElementGt(aStartOffset - 1, SyncOffsetComparator());
   if (start == mTimeMapping.Length()) {
     return false;
   }
 
   // Find the first WebMTimeDataOffset at or before aEndOffset.
-  uint32_t end = mTimeMapping.IndexOfFirstElementGt(aEndOffset - 1);
+  uint32_t end = mTimeMapping.IndexOfFirstElementGt(aEndOffset);
   if (end > 0) {
     end -= 1;
   }
@@ -204,25 +216,21 @@ bool WebMBufferedState::CalculateBufferedForRange(int64_t aStartOffset, int64_t
     return false;
   }
 
-  NS_ASSERTION(mTimeMapping[start].mOffset >= aStartOffset &&
-               mTimeMapping[end].mOffset <= aEndOffset,
+  NS_ASSERTION(mTimeMapping[start].mSyncOffset >= aStartOffset &&
+               mTimeMapping[end].mEndOffset <= aEndOffset,
                "Computed time range must lie within data range.");
   if (start > 0) {
-    NS_ASSERTION(mTimeMapping[start - 1].mOffset <= aStartOffset,
+    NS_ASSERTION(mTimeMapping[start - 1].mSyncOffset < aStartOffset,
                  "Must have found least WebMTimeDataOffset for start");
   }
   if (end < mTimeMapping.Length() - 1) {
-    NS_ASSERTION(mTimeMapping[end + 1].mOffset >= aEndOffset,
+    NS_ASSERTION(mTimeMapping[end + 1].mEndOffset > aEndOffset,
                  "Must have found greatest WebMTimeDataOffset for end");
   }
 
-  // The timestamp of the first media sample, in ns. We must subtract this
-  // from the ranges' start and end timestamps, so that those timestamps are
-  // normalized in the range [0,duration].
-
+  uint64_t frameDuration = mTimeMapping[end].mTimecode - mTimeMapping[end - 1].mTimecode;
   *aStartTime = mTimeMapping[start].mTimecode;
-  *aEndTime = mTimeMapping[end].mTimecode;
-  *aEndTime += mTimeMapping[end].mTimecode - mTimeMapping[end - 1].mTimecode;
+  *aEndTime = mTimeMapping[end].mTimecode + frameDuration;
   return true;
 }
 

content/media/webm/WebMBufferedParser.h

@@ -20,19 +20,23 @@ class TimeRanges;
 // that offset.
 struct WebMTimeDataOffset
 {
-  WebMTimeDataOffset(int64_t aOffset, uint64_t aTimecode, int64_t aSyncOffset)
-    : mOffset(aOffset), mSyncOffset(aSyncOffset), mTimecode(aTimecode)
+  WebMTimeDataOffset(int64_t aEndOffset, uint64_t aTimecode, int64_t aSyncOffset)
+    : mEndOffset(aEndOffset), mSyncOffset(aSyncOffset), mTimecode(aTimecode)
   {}
 
-  bool operator==(int64_t aOffset) const {
-    return mOffset == aOffset;
+  bool operator==(int64_t aEndOffset) const {
+    return mEndOffset == aEndOffset;
   }
 
-  bool operator<(int64_t aOffset) const {
-    return mOffset < aOffset;
+  bool operator!=(int64_t aEndOffset) const {
+    return mEndOffset != aEndOffset;
   }
 
-  int64_t mOffset;
+  bool operator<(int64_t aEndOffset) const {
+    return mEndOffset < aEndOffset;
+  }
+
+  int64_t mEndOffset;
   int64_t mSyncOffset;
   uint64_t mTimecode;
 };