Handle negative PTS values on the TS segments

Overwrite setNextTimeStamp after the first invocation, capturing the first PTS value. Write all FLV PTS and DTS values as offsets from that.

 /*
  * aac-stream
- * 
+ *
  *
  * Copyright (c) 2013 Brightcove
  * All rights reserved.
@@ -18,9 +18,9 @@ var
   ];
 
 window.videojs.hls.AacStream = function() {
-  var 
+  var
     next_pts, // :uint
-    pts_delta = -1, // :int
+    pts_offset, // :int
     state, // :uint
     pes_length, // :int
 
@@ -39,18 +39,22 @@ window.videojs.hls.AacStream = function() {
 
   // (pts:uint, pes_size:int, dataAligned:Boolean):void
   this.setNextTimeStamp = function(pts, pes_size, dataAligned) {
-    if (0 > pts_delta) {
-      // We assume the very first pts is less than 0x80000000
-      pts_delta = pts;
-    }
 
-    next_pts = pts - pts_delta;
-    pes_length = pes_size;
+    // on the first invocation, capture the starting PTS value
+    pts_offset = pts;
 
-    // If data is aligned, flush all internal buffers
-    if (dataAligned) {
-      state = 0;
-    }
+    // on subsequent invocations, calculate the PTS based on the starting offset
+    this.setNextTimeStamp = function(pts, pes_size, dataAligned) {
+      next_pts = pts - pts_offset;
+      pes_length = pes_size;
+
+      // If data is aligned, flush all internal buffers
+      if (dataAligned) {
+        state = 0;
+      }
+    };
+
+    this.setNextTimeStamp(pts, pes_size, dataAligned);
   };
 
   // (data:ByteArray, o:int = 0, l:int = 0):void
@@ -114,7 +118,7 @@ window.videojs.hls.AacStream = function() {
         offset += 1;
         state = 3;
         break;
-      case 3: 
+      case 3:
         if (offset >= end) {
           return;
         }
@@ -124,7 +128,7 @@ window.videojs.hls.AacStream = function() {
         offset += 1;
         state = 4;
         break;
-      case 4: 
+      case 4:
         if (offset >= end) {
           return;
         }
@@ -143,7 +147,7 @@ window.videojs.hls.AacStream = function() {
         offset += 1;
         state = 6;
         break;
-      case 6: 
+      case 6:
         if (offset >= end) {
           return;
         }
@@ -159,11 +163,11 @@ window.videojs.hls.AacStream = function() {
           aacFrame.dts = next_pts;
 
           // AAC is always 10
-          aacFrame.writeMetaDataDouble("audiocodecid", 10); 
+          aacFrame.writeMetaDataDouble("audiocodecid", 10);
           aacFrame.writeMetaDataBoolean("stereo", 2 === adtsChanelConfig);
           aacFrame.writeMetaDataDouble ("audiosamplerate", adtsSampleingRates[adtsSampleingIndex]);
           // Is AAC always 16 bit?
-          aacFrame.writeMetaDataDouble ("audiosamplesize", 16); 
+          aacFrame.writeMetaDataDouble ("audiosamplesize", 16);
 
           this.tags.push(aacFrame);
 
@@ -173,7 +177,7 @@ window.videojs.hls.AacStream = function() {
           // For audio, DTS is always the same as PTS. We want to set the DTS
           // however so we can compare with video DTS to determine approximate
           // packet order
-          aacFrame.pts = next_pts; 
+          aacFrame.pts = next_pts;
           aacFrame.view.setUint16(aacFrame.position, newExtraData);
           aacFrame.position += 2;
           aacFrame.length = Math.max(aacFrame.length, aacFrame.position);

@@ -253,7 +253,7 @@
     var
       next_pts, // :uint;
       next_dts, // :uint;
-      pts_delta = -1, // :int
+      pts_offset, // :int
 
       h264Frame, // :FlvTag
 
@@ -268,20 +268,22 @@
 
     //(pts:uint, dts:uint, dataAligned:Boolean):void
     this.setNextTimeStamp = function(pts, dts, dataAligned) {
-      if (pts_delta < 0) {
-        // We assume the very first pts is less than 0x8FFFFFFF (max signed
-        // int32)
-        pts_delta = pts;
-      }
-
-      // We could end up with a DTS less than 0 here. We need to deal with that!
-      next_pts = pts - pts_delta;
-      next_dts = dts - pts_delta;
+      // on the first invocation, capture the starting PTS value
+      pts_offset = pts;
+
+      // on subsequent invocations, calculate the PTS based on the starting offset
+      this.setNextTimeStamp = function(pts, dts, dataAligned) {
+        // We could end up with a DTS less than 0 here. We need to deal with that!
+        next_pts = pts - pts_offset;
+        next_dts = dts - pts_offset;
+
+        // If data is aligned, flush all internal buffers
+        if (dataAligned) {
+          this.finishFrame();
+        }
+      };
 
-      // If data is aligned, flush all internal buffers
-      if (dataAligned) {
-        this.finishFrame();
-      }
+      this.setNextTimeStamp(pts, dts, dataAligned);
     };
 
     this.finishFrame = function() {

@@ -18,8 +18,7 @@
       streamBuffer = new Uint8Array(MP2T_PACKET_LENGTH),
       streamBufferByteCount = 0,
       h264Stream = new H264Stream(),
-      aacStream = new AacStream(),
-      seekToKeyFrame = false;
+      aacStream = new AacStream();
 
     // expose the stream metadata
     self.stream = {
@@ -84,60 +83,35 @@
     self.flushTags = function() {
       h264Stream.finishFrame();
     };
-    self.doSeek = function() {
-      self.flushTags();
-      aacStream.tags.length = 0;
-      h264Stream.tags.length = 0;
-      seekToKeyFrame = true;
-    };
 
+    /**
+     * Returns whether a call to `getNextTag()` will be successful.
+     * @return {boolean} whether there is at least one transmuxed FLV
+     * tag ready
+     */
     self.tagsAvailable = function() { // :int {
-      var i, pts; // :uint
-
-      if (seekToKeyFrame) {
-        for (i = 0 ; i < h264Stream.tags.length && seekToKeyFrame; ++i) {
-          if (h264Stream.tags[i].keyFrame) {
-            seekToKeyFrame = false; // We found, a keyframe, stop seeking
-          }
-        }
-
-        if (seekToKeyFrame) {
-          // we didnt find a keyframe. yet
-          h264Stream.tags.length = 0;
-          return 0;
-        }
-
-        // TODO we MAY need to use dts, not pts
-        h264Stream.tags = h264Stream.tags.slice(i);
-        pts = h264Stream.tags[0].pts;
-
-        // Remove any audio before the found keyframe
-        while( 0 < aacStream.tags.length && pts > aacStream.tags[0].pts ) {
-          aacStream.tags.shift();
-        }
-      }
-
       return h264Stream.tags.length + aacStream.tags.length;
     };
 
-    self.getNextTag = function() { // :ByteArray {
-      var tag; // :FlvTag; // return tags in approximate dts order
-
-      if (0 === self.tagsAvailable()) {
-        throw new Error("getNextTag() called when 0 == tagsAvailable()");
-      }
+    /**
+     * Returns the next tag in decoder-timestamp (DTS) order.
+     * @returns {object} the next tag to decoded.
+     */
+    self.getNextTag = function() {
+      var tag;
 
-      if (0 < h264Stream.tags.length) {
-        if (0 < aacStream.tags.length && aacStream.tags[0].dts < h264Stream.tags[0].dts) {
-          tag = aacStream.tags.shift();
-        } else {
-          tag = h264Stream.tags.shift();
-        }
-      } else if ( 0 < aacStream.tags.length ) {
+      if (!h264Stream.tags.length) {
+        // only audio tags remain
+        tag = aacStream.tags.shift();
+      } else if (!aacStream.tags.length) {
+        // only video tags remain
+        tag = h264Stream.tags.shift();
+      } else if (aacStream.tags[0].dts < h264Stream.tags[0].dts) {
+        // audio should be decoded next
         tag = aacStream.tags.shift();
       } else {
-        // We dont have any tags available to return
-        return new Uint8Array();
+        // video should be decoded next
+        tag = h264Stream.tags.shift();
       }
 
       return tag.finalize();

@@ -59,4 +59,34 @@ test('metadata is generated for IDRs after a full NAL unit is written', function
      'picture parameter set is written');
   ok(h264Stream.tags[2].keyFrame, 'key frame is written');
 });
+
+test('starting PTS values can be negative', function() {
+  var
+    h264Stream = new videojs.hls.H264Stream(),
+    accessUnitDelimiter = new Uint8Array([
+      0x00,
+      0x00,
+      0x01,
+      nalUnitTypes.access_unit_delimiter_rbsp
+    ]);
+
+  h264Stream.setNextTimeStamp(-100, -100, true);
+  h264Stream.writeBytes(accessUnitDelimiter, 0, accessUnitDelimiter.byteLength);
+  h264Stream.setNextTimeStamp(-99, -99, true);
+  h264Stream.writeBytes(accessUnitDelimiter, 0, accessUnitDelimiter.byteLength);
+  h264Stream.setNextTimeStamp(0, 0, true);
+  h264Stream.writeBytes(accessUnitDelimiter, 0, accessUnitDelimiter.byteLength);
+  // flush out the last tag
+  h264Stream.writeBytes(accessUnitDelimiter, 0, accessUnitDelimiter.byteLength);
+
+  strictEqual(h264Stream.tags.length, 3, 'three tags are ready');
+  strictEqual(h264Stream.tags[0].pts, 0, 'the first PTS is zero');
+  strictEqual(h264Stream.tags[0].dts, 0, 'the first DTS is zero');
+  strictEqual(h264Stream.tags[1].pts, 1, 'the second PTS is one');
+  strictEqual(h264Stream.tags[1].dts, 1, 'the second DTS is one');
+
+  strictEqual(h264Stream.tags[2].pts, 100, 'the third PTS is 100');
+  strictEqual(h264Stream.tags[2].dts, 100, 'the third DTS is 100');
+});
+
 })(window.videojs);