fix(WebVTT): Re-add rollover logic (#7104)

Fixes: #6448 Revert 9ab54d1
shaka-project · Aug 19, 2024 · 4546606 · 4546606
1 parent b7e543f
commit 4546606
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diff --git a/lib/text/vtt_text_parser.js b/lib/text/vtt_text_parser.js
@@ -160,9 +160,17 @@ shaka.text.VttTextParser = class {
           shaka.util.Error.Code.INVALID_TEXT_HEADER);
     }
 
-    const mpegTime = Number(mpegTimeMatch[1]);
+    let mpegTime = Number(mpegTimeMatch[1]);
     const mpegTimescale = shaka.text.VttTextParser.MPEG_TIMESCALE_;
 
+    const rolloverSeconds =
+        shaka.text.VttTextParser.TS_ROLLOVER_ / mpegTimescale;
+    let segmentStart = time.segmentStart - time.periodStart;
+    while (segmentStart >= rolloverSeconds) {
+      segmentStart -= rolloverSeconds;
+      mpegTime += shaka.text.VttTextParser.TS_ROLLOVER_;
+    }
+
     return time.periodStart + mpegTime / mpegTimescale - cueTime;
   }
 
@@ -1007,6 +1015,13 @@ shaka.text.VttTextParser = class {
  */
 shaka.text.VttTextParser.MPEG_TIMESCALE_ = 90000;
 
+/**
+ * At this value, timestamps roll over in TS content.
+ * @const {number}
+ * @private
+ */
+shaka.text.VttTextParser.TS_ROLLOVER_ = 0x200000000;
+
 /**
  * @const
  * @private {!RegExp}

diff --git a/test/text/vtt_text_parser_unit.js b/test/text/vtt_text_parser_unit.js
@@ -583,6 +583,59 @@ describe('VttTextParser', () => {
         /* hls= */ true, /* sequenceMode= */ true);
   });
 
+  it('handles timestamp rollover with X-TIMESTAMP-MAP header', () => {
+    verifyHelper(
+        [
+          {startTime: 95443, endTime: 95445, payload: 'Test'},
+        ],
+        // 8589870000/90000 = 95443 sec, so expect every timestamp to be 95443
+        // seconds ahead of what is specified.
+        'WEBVTT\n' +
+        'X-TIMESTAMP-MAP=MPEGTS:8589870000,LOCAL:00:00:00.000\n\n' +
+        '00:00:00.000 --> 00:00:02.000 line:0\n' +
+        'Test',
+        // Non-null segmentStart takes precedence over X-TIMESTAMP-MAP.
+        // This protects us from rollover in the MPEGTS field.
+        {periodStart: 0, segmentStart: 95440, segmentEnd: 95550, vttOffset: 0},
+        /* hls= */ true, /* sequenceMode= */ true);
+
+    verifyHelper(
+        [
+          {startTime: 95552, endTime: 95554, payload: 'Test2'},
+        ],
+        // 95550 is larger than the roll over timestamp, so the timestamp offset
+        // gets rolled over.
+        // (9745408 + 0x200000000) / 90000 = 95552 sec
+        'WEBVTT\n' +
+        'X-TIMESTAMP-MAP=MPEGTS:9745408,LOCAL:00:00:00.000\n\n' +
+        '00:00:00.000 --> 00:00:02.000 line:0\n' +
+        'Test2',
+        {periodStart: 0, segmentStart: 95550, segmentEnd: 95560, vttOffset: 0},
+        /* hls= */ true, /* sequenceMode= */ true);
+  });
+
+  // A mock-up of HLS live subs as seen in b/253104251.
+  it('handles timestamp rollover and negative offset in HLS live', () => {
+    // Similar to values seen in b/253104251, for a realistic regression test.
+    // When using sequence mode on live HLS, we get negative offsets that
+    // represent the timestamp of our first append in sequence mode.
+    verifyHelper(
+        [
+          {startTime: 3600, endTime: 3602, payload: 'Test'},
+        ],
+        'WEBVTT\n' +
+        'X-TIMESTAMP-MAP=MPEGTS:8355814896,LOCAL:00:00:00.000\n\n' +
+        '00:00:00.000 --> 00:00:02.000 line:0\n' +
+        'Test',
+        {
+          periodStart: -1234567,
+          segmentStart: 3600,
+          segmentEnd: 3610,
+          vttOffset: -1234567,
+        },
+        /* hls= */ true, /* sequenceMode= */ true);
+  });
+
   it('supports global style blocks', () => {
     const textShadow = '-1px 0 black, 0 1px black, 1px 0 black, 0 -1px black';
     verifyHelper(