mkvmuxer: fix DiscardPadding

DiscardPadding is a signed int.
this change fixes 2 things:
- allows negative values for discard padding
- fixes cases where an unsigned value would be written such that on read
  the sign would be flipped

Change-Id: I9418da7a22c09768e02d5b61da8d01c2bccb5dee

mkvmuxer.cpp

@@ -1636,7 +1636,7 @@ bool Cluster::DoWriteBlockWithDiscardPadding(
     const uint8* frame, uint64 length, int64 discard_padding,
     uint64 track_number, uint64 abs_timecode, uint64 generic_arg,
     WriteBlockDiscardPadding write_block) {
-  if (frame == NULL || length == 0 || discard_padding <= 0)
+  if (frame == NULL || length == 0)
     return false;
 
   if (!IsValidTrackNumber(track_number))
@@ -2430,7 +2430,7 @@ bool Segment::AddFrameWithDiscardPadding(const uint8* frame, uint64 length,
                                          int64 discard_padding,
                                          uint64 track_number, uint64 timestamp,
                                          bool is_key) {
-  if (frame == NULL || discard_padding <= 0)
+  if (frame == NULL)
     return false;
 
   if (!CheckHeaderInfo())
@@ -2547,7 +2547,7 @@ bool Segment::AddGenericFrame(const Frame* frame) {
         frame->frame(), frame->length(), frame->additional(),
         frame->additional_length(), frame->add_id(), frame->track_number(),
         frame->timestamp(), frame->is_key());
-  } else if (frame->discard_padding() > 0) {
+  } else if (frame->discard_padding() != 0) {
     return AddFrameWithDiscardPadding(
         frame->frame(), frame->length(), frame->discard_padding(),
         frame->track_number(), frame->timestamp(), frame->is_key());
@@ -3025,7 +3025,7 @@ int Segment::WriteFramesAll() {
     const uint64 frame_timestamp = frame->timestamp();  // ns
     const uint64 frame_timecode = frame_timestamp / timecode_scale;
 
-    if (frame->discard_padding() > 0) {
+    if (frame->discard_padding() != 0) {
       if (!cluster->AddFrameWithDiscardPadding(
               frame->frame(), frame->length(), frame->discard_padding(),
               frame->track_number(), frame_timecode, frame->is_key())) {
@@ -3085,7 +3085,7 @@ bool Segment::WriteFramesLessThan(uint64 timestamp) {
       const uint64 frame_timecode = frame_timestamp / timecode_scale;
       const int64 discard_padding = frame_prev->discard_padding();
 
-      if (discard_padding > 0) {
+      if (discard_padding != 0) {
         if (!cluster->AddFrameWithDiscardPadding(
                 frame_prev->frame(), frame_prev->length(), discard_padding,
                 frame_prev->track_number(), frame_timecode,

mkvmuxerutil.cpp

@@ -72,6 +72,13 @@ int32 GetUIntSize(uint64 value) {
   return 8;
 }
 
+int32 GetIntSize(int64 value) {
+  // Doubling the requested value ensures positive values with their high bit
+  // set are written with 0-padding to avoid flipping the signedness.
+  const uint64 v = (value < 0) ? value ^ -1LL : value;
+  return GetUIntSize(2 * v);
+}
+
 uint64 EbmlMasterElementSize(uint64 type, uint64 value) {
   // Size of EBML ID
   int32 ebml_size = GetUIntSize(type);
@@ -83,7 +90,16 @@ uint64 EbmlMasterElementSize(uint64 type, uint64 value) {
 }
 
 uint64 EbmlElementSize(uint64 type, int64 value) {
-  return EbmlElementSize(type, static_cast<uint64>(value));
+  // Size of EBML ID
+  int32 ebml_size = GetUIntSize(type);
+
+  // Datasize
+  ebml_size += GetIntSize(value);
+
+  // Size of Datasize
+  ebml_size++;
+
+  return ebml_size;
 }
 
 uint64 EbmlElementSize(uint64 type, uint64 value) {
@@ -592,7 +608,7 @@ uint64 WriteBlockWithDiscardPadding(IMkvWriter* writer, const uint8* data,
                                     uint64 length, int64 discard_padding,
                                     uint64 track_number, int64 timecode,
                                     uint64 is_key) {
-  if (!data || length < 1 || discard_padding <= 0)
+  if (!data || length < 1)
     return 0;
 
   const uint64 block_payload_size = 4 + length;
@@ -630,7 +646,7 @@ uint64 WriteBlockWithDiscardPadding(IMkvWriter* writer, const uint8* data,
   if (WriteID(writer, kMkvDiscardPadding))
     return 0;
 
-  const uint64 size = GetUIntSize(discard_padding);
+  const uint64 size = GetIntSize(discard_padding);
   if (WriteUInt(writer, size))
     return false;