feat: get start number from muxer and specify initial sequence number (#879)

Set the start number in representation to the segment index that is sent by muxer.

With this enhancement, you can now specify the initial sequence number
to be used on the generated segments when calling the packager.
With the old implementation, it was always starting with "1".

---------

Co-authored-by: Cosmin Stejerean <[email protected]>

Author: sr1990

docs/source/options/chunking_options.rst

@@ -21,3 +21,7 @@ Chunking options
 
    Force fragments to begin with stream access points. This flag implies
    *segment_sap_aligned*. Default enabled.
+
+--start_segment_number
+
+   Indicates the startNumber in DASH SegmentTemplate and HLS segment name.

include/packager/chunking_params.h

@@ -31,6 +31,9 @@ struct ChunkingParams {
   /// and mdat atom. Each chunk is uploaded immediately upon creation,
   /// decoupling latency from segment duration.
   bool low_latency_dash_mode = false;
+
+  /// Indicates the startNumber in DASH SegmentTemplate and HLS segment name.
+  int64_t start_segment_number = 1;
 };
 
 }  // namespace shaka

packager/app/muxer_flags.cc

@@ -73,3 +73,9 @@ ABSL_FLAG(
     "If the first sample comes after default_text_zero_bias_ms then the start "
     "of the stream will not be padded as we cannot assume the start time of "
     "the stream.");
+
+ABSL_FLAG(int64_t,
+          start_segment_number,
+          1,
+          "Indicates the startNumber in DASH SegmentTemplate and HLS "
+          "segment name.");

packager/app/muxer_flags.h

@@ -22,5 +22,6 @@ ABSL_DECLARE_FLAG(std::string, temp_dir);
 ABSL_DECLARE_FLAG(bool, mp4_include_pssh_in_stream);
 ABSL_DECLARE_FLAG(int32_t, transport_stream_timestamp_offset_ms);
 ABSL_DECLARE_FLAG(int32_t, default_text_zero_bias_ms);
+ABSL_DECLARE_FLAG(int64_t, start_segment_number);
 
 #endif  // APP_MUXER_FLAGS_H_

packager/app/packager_main.cc

@@ -360,6 +360,8 @@ std::optional<PackagingParams> GetPackagingParams() {
       absl::GetFlag(FLAGS_segment_sap_aligned);
   chunking_params.subsegment_sap_aligned =
       absl::GetFlag(FLAGS_fragment_sap_aligned);
+  chunking_params.start_segment_number =
+      absl::GetFlag(FLAGS_start_segment_number);
 
   int num_key_providers = 0;
   EncryptionParams& encryption_params = packaging_params.encryption_params;

packager/app/test/packager_test.py

@@ -485,8 +485,8 @@ def _GetFlags(self,
                 use_fake_clock=True,
                 allow_codec_switching=False,
                 dash_force_segment_list=False,
-                force_cl_index=None):
-
+                force_cl_index=None,
+                start_segment_number=None):
     flags = ['--single_threaded']
 
     if not strip_parameter_set_nalus:
@@ -575,6 +575,9 @@ def _GetFlags(self,
     elif force_cl_index is False:
       flags += ['--noforce_cl_index']
 
+    if start_segment_number is not None:
+      flags += ['--start_segment_number', str(start_segment_number)]
+
     if ad_cues:
       flags += ['--ad_cues', ad_cues]
 
@@ -823,6 +826,13 @@ def testForcedSubtitle(self):
                                                       output_hls=True))
     self._CheckTestResults('forced-subtitle')
 
+  def testDashStartNumber(self):
+    audio_video_streams = self._GetStreams(['audio', 'video'], segmented=True)
+    streams = audio_video_streams
+    self.assertPackageSuccess(streams, self._GetFlags(output_dash=True,
+                              start_segment_number=0))
+    self._CheckTestResults('dash-start-number')
+
   def testAudioVideoWithLanguageOverride(self):
     self.assertPackageSuccess(
         self._GetStreams(['audio', 'video'], language='por', hls=True),