feat: support emsg box parsing

lib/mp4/emsg.js

@@ -0,0 +1,76 @@
+var parseStringUntilNullChar = function(data) {
+    var index = 0;
+    var curChar = String.fromCharCode(data[index]);
+    var retString = '';
+    while (curChar !== '\0') {
+        retString += curChar;
+        index++;
+        curChar = String.fromCharCode(data[index]);
+    }
+    // Add nullChar
+    retString += curChar;
+    return retString;
+};
+
+var parseEmsgBox = function(boxData) {
+    var offset = 0
+    var version = boxData[offset];
+    var scheme_id_uri, value, timescale, presentation_time, presentation_time_delta, event_duration, id;
+    // version 0 vs version 1 have different formats.
+    if (version === 0) {
+        // Parse id and value strings first.
+        scheme_id_uri = parseStringUntilNullChar(boxData);
+        offset += scheme_id_uri.length - 1;
+        value = parseStringUntilNullChar(boxData.subarray(offset));
+
+        var dv = new DataView(boxData);
+        timescale = dv.getUint32(12);
+        presentation_time_delta = dv.getUint32(16);
+        event_duration = dv.getUint32(20);
+        id = dv.getUint32(24);
+        offset = 28;
+    } else if (version === 1) {
+        var dv = new DataView(boxData);
+        timescale = dv.getUint32(0);
+        presentation_time = dv.getBigUint64(4);
+        event_duration = dv.getUint32(12);
+        id = dv.getUint32(16);
+
+        offset = 20;
+        scheme_id_uri = parseStringUntilNullChar(boxData.subarray(offset));
+        offset += scheme_id_uri.length - 1;
+        value = parseStringUntilNullChar(boxData.subarray(offset));
+        offset += value.length - 1;
+
+    }
+    var message_data = boxData.subarray(offset, boxData.byteLength);
+    return { scheme_id_uri, value, 
+            timescale, presentation_time, 
+            presentation_time_delta, 
+            event_duration, id, 
+            message_data };
+};
+
+var scaleTime = function(presentationTime, timescale, timeDelta, offset) {
+    return presentationTime ? presentationTime / timescale : offset + timeDelta / timescale;
+};
+
+// TODO: This may need to return more ID3 specific data rather than just data, pts, duration.
+var getEmsgData = function(data, ptOffset) {
+    var emsgBoxes = findBox(data, ['emsg']);
+    var parsedBoxes = [];
+    emsgBoxes.forEach(function(boxData) {
+        // TODO: We may need to check for ID3 specific schemeIdURI before returning data.
+        var parsedBox = parseEmsgBox(boxData);
+        var pts = scaleTime(parsedBox.presentation_time, parsedBox.timescale, parsedBox.presentation_time_delta, ptOffset);
+        var duration = scaleTime(parsedBox.duration, parsedBox.timescale);
+        parsedBoxes.push({
+            data: parsedBox.message_data,
+            pts,
+            duration,
+        });
+    });
+    return parsedBoxes;
+};
+
+module.exports = getEmsgData;

lib/mp4/probe.js

@@ -16,8 +16,9 @@ var parseTfhd = require('../tools/parse-tfhd.js');
 var parseTrun = require('../tools/parse-trun.js');
 var parseTfdt = require('../tools/parse-tfdt.js');
 var getUint64 = require('../utils/numbers.js').getUint64;
+var getEmsgData = require('../mp4/emsg.js');
 var timescale, startTime, compositionStartTime, getVideoTrackIds, getTracks,
-  getTimescaleFromMediaHeader;
+  getTimescaleFromMediaHeader, getEmsg;
 var window = require('global/window');
 
 
@@ -370,6 +371,11 @@ getTracks = function(init) {
   return tracks;
 };
 
+// TODO: If we are returning specific ID3 emsg data lets modify the function signature.
+getEmsg = function(segmentData, ptOffset) {
+  return getEmsgData(segmentData, ptOffset);
+};
+
 module.exports = {
   // export mp4 inspector's findBox and parseType for backwards compatibility
   findBox: findBox,
@@ -379,5 +385,6 @@ module.exports = {
   compositionStartTime: compositionStartTime,
   videoTrackIds: getVideoTrackIds,
   tracks: getTracks,
-  getTimescaleFromMediaHeader: getTimescaleFromMediaHeader
+  getTimescaleFromMediaHeader: getTimescaleFromMediaHeader,
+  getEmsg: getEmsg,
 };