[DASH] Support signalling of last segment number (#713)

Per https://dashif.org/docs/DASH-IF-IOP-v4.3.pdf 4.4.3.6 Signalling the last segment number in Period, there are three ways to signal the last segment number:

  a. Use the lmsg signalling as defined in clause 4.4.3.5.
  b. Use the Segment Timeline with @r value greater or equal to 0.
  c. Add a Supplemental Descriptor with @schemeIdUri set to http://dashif.org/guidelines/last-segment-number with the @value set to the last segment number.

We do not support (a). This change adds support for (c) when Segment Timeline (b) is not used, i.e. when Representation has constant duration (could be approximate).

Under flag --dash_add_last_segment_number_when_needed (disabled by default).

packager/mpd/base/xml/xml_node.cc

@@ -24,6 +24,12 @@ DEFINE_bool(segment_template_constant_duration,
             "Generates SegmentTemplate@duration if all segments except the "
             "last one has the same duration if this flag is set to true.");
 
+DEFINE_bool(dash_add_last_segment_number_when_needed,
+            false,
+            "Adds a Supplemental Descriptor with @schemeIdUri "
+            "set to http://dashif.org/guidelines/last-segment-number with "
+            "the @value set to the last segment number.");
+
 namespace shaka {
 
 using xml::XmlNode;
@@ -429,6 +435,15 @@ bool RepresentationXmlNode::AddLiveOnlyInfo(
     if (IsTimelineConstantDuration(segment_infos, start_number)) {
       segment_template.SetIntegerAttribute("duration",
                                            segment_infos.front().duration);
+      if (FLAGS_dash_add_last_segment_number_when_needed) {
+        uint32_t last_segment_number = start_number - 1;
+        for (const auto& segment_info_element : segment_infos) 
+          last_segment_number += segment_info_element.repeat + 1;
+	
+        AddSupplementalProperty(
+          "http://dashif.org/guidelines/last-segment-number",
+          std::to_string(last_segment_number));	
+      }
     } else {
       XmlNode segment_timeline("SegmentTimeline");
       if (!PopulateSegmentTimeline(segment_infos, &segment_timeline) ||

packager/mpd/base/xml/xml_node_unittest.cc

@@ -18,6 +18,7 @@
 #include "packager/mpd/test/xml_compare.h"
 
 DECLARE_bool(segment_template_constant_duration);
+DECLARE_bool(dash_add_last_segment_number_when_needed);
 
 using ::testing::ElementsAre;
 
@@ -397,5 +398,31 @@ TEST_F(LiveSegmentTimelineTest, TwoSegmentInfoWithGap) {
                   "</Representation>"));
 }
 
+TEST_F(LiveSegmentTimelineTest, LastSegmentNumberSupplementalProperty) {        
+  const uint32_t kStartNumber = 1;                                              
+  const uint64_t kStartTime = 0;                                                
+  const uint64_t kDuration = 100;                                               
+  const uint64_t kRepeat = 9;                                                   
+                                                                                
+  std::list<SegmentInfo> segment_infos = {                                      
+      {kStartTime, kDuration, kRepeat},                                         
+  };                                                                            
+  RepresentationXmlNode representation;                                         
+  FLAGS_dash_add_last_segment_number_when_needed = true;                       
+                                                                                
+  ASSERT_TRUE(                                                                  
+      representation.AddLiveOnlyInfo(media_info_, segment_infos, kStartNumber));
+                                                                                
+  EXPECT_THAT(                                                                  
+      representation.GetRawPtr(),                                               
+      XmlNodeEqual("<Representation>"                                           
+                   "<SupplementalProperty schemeIdUri=\"http://dashif.org/"     
+                   "guidelines/last-segment-number\" value=\"10\"/>"            
+                   "  <SegmentTemplate media=\"$Number$.m4s\" "                 
+                   "                   startNumber=\"1\" duration=\"100\"/>"    
+                   "</Representation>"));                                       
+  FLAGS_dash_add_last_segment_number_when_needed = false;                                                                                                      
+}                      
+
 }  // namespace xml
 }  // namespace shaka