m2ts.js

/**
 * mux.js
 *
 * Copyright (c) 2015 Brightcove
 * All rights reserved.
 *
 * A stream-based mp2t to mp4 converter. This utility can be used to
 * deliver mp4s to a SourceBuffer on platforms that support native
 * Media Source Extensions.
 */
'use strict';
var Stream = require('../utils/stream.js'),
  CaptionStream = require('./caption-stream'),
  StreamTypes = require('./stream-types');

var m2tsStreamTypes = require('./stream-types.js');

// object types
var TransportPacketStream, TransportParseStream, ElementaryStream;

// constants
var
  MP2T_PACKET_LENGTH = 188, // bytes
  SYNC_BYTE = 0x47;

/**
 * Splits an incoming stream of binary data into MPEG-2 Transport
 * Stream packets.
 */
TransportPacketStream = function() {
  var
    buffer = new Uint8Array(MP2T_PACKET_LENGTH),
    bytesInBuffer = 0;

  TransportPacketStream.prototype.init.call(this);

   // Deliver new bytes to the stream.

  this.push = function(bytes) {
    var
      startIndex = 0,
      endIndex = MP2T_PACKET_LENGTH,
      everything;

    // If there are bytes remaining from the last segment, prepend them to the
    // bytes that were pushed in
    if (bytesInBuffer) {
      everything = new Uint8Array(bytes.byteLength + bytesInBuffer);
      everything.set(buffer.subarray(0, bytesInBuffer));
      everything.set(bytes, bytesInBuffer);
      bytesInBuffer = 0;
    } else {
      everything = bytes;
    }

    // While we have enough data for a packet
    while (endIndex < everything.byteLength) {
      // Look for a pair of start and end sync bytes in the data..
      if (everything[startIndex] === SYNC_BYTE && everything[endIndex] === SYNC_BYTE) {
        // We found a packet so emit it and jump one whole packet forward in
        // the stream
        this.trigger('data', everything.subarray(startIndex, endIndex));
        startIndex += MP2T_PACKET_LENGTH;
        endIndex += MP2T_PACKET_LENGTH;
        continue;
      }
      // If we get here, we have somehow become de-synchronized and we need to step
      // forward one byte at a time until we find a pair of sync bytes that denote
      // a packet
      startIndex++;
      endIndex++;
    }

    // If there was some data left over at the end of the segment that couldn't
    // possibly be a whole packet, keep it because it might be the start of a packet
    // that continues in the next segment
    if (startIndex < everything.byteLength) {
      buffer.set(everything.subarray(startIndex), 0);
      bytesInBuffer = everything.byteLength - startIndex;
    }
  };

  this.flush = function() {
    // If the buffer contains a whole packet when we are being flushed, emit it
    // and empty the buffer. Otherwise hold onto the data because it may be
    // important for decoding the next segment
    if (bytesInBuffer === MP2T_PACKET_LENGTH && buffer[0] === SYNC_BYTE) {
      this.trigger('data', buffer);
      bytesInBuffer = 0;
    }
    this.trigger('done');
  };
};
TransportPacketStream.prototype = new Stream();

/**
 * Accepts an MP2T TransportPacketStream and emits data events with parsed
 * forms of the individual transport stream packets.
 */
TransportParseStream = function() {
  var parsePsi, parsePat, parsePmt, self;
  TransportParseStream.prototype.init.call(this);
  self = this;

  this.packetsWaitingForPmt = [];
  this.programMapTable = undefined;

  parsePsi = function(payload, psi) {
    var offset = 0;

    // PSI packets may be split into multiple sections and those
    // sections may be split into multiple packets. If a PSI
    // section starts in this packet, the payload_unit_start_indicator
    // will be true and the first byte of the payload will indicate
    // the offset from the current position to the start of the
    // section.
    if (psi.payloadUnitStartIndicator) {
      offset += payload[offset] + 1;
    }

    if (psi.type === 'pat') {
      parsePat(payload.subarray(offset), psi);
    } else {
      parsePmt(payload.subarray(offset), psi);
    }
  };

  parsePat = function(payload, pat) {
    pat.section_number = payload[7]; // eslint-disable-line camelcase
    pat.last_section_number = payload[8]; // eslint-disable-line camelcase

    // skip the PSI header and parse the first PMT entry
    self.pmtPid = (payload[10] & 0x1F) << 8 | payload[11];
    pat.pmtPid = self.pmtPid;
  };

  /**
   * Parse out the relevant fields of a Program Map Table (PMT).
   * @param payload {Uint8Array} the PMT-specific portion of an MP2T
   * packet. The first byte in this array should be the table_id
   * field.
   * @param pmt {object} the object that should be decorated with
   * fields parsed from the PMT.
   */
  parsePmt = function(payload, pmt) {
    var sectionLength, tableEnd, programInfoLength, offset;

    // PMTs can be sent ahead of the time when they should actually
    // take effect. We don't believe this should ever be the case
    // for HLS but we'll ignore "forward" PMT declarations if we see
    // them. Future PMT declarations have the current_next_indicator
    // set to zero.
    if (!(payload[5] & 0x01)) {
      return;
    }

    // overwrite any existing program map table
    self.programMapTable = {};

    // the mapping table ends at the end of the current section
    sectionLength = (payload[1] & 0x0f) << 8 | payload[2];
    tableEnd = 3 + sectionLength - 4;

    // to determine where the table is, we have to figure out how
    // long the program info descriptors are
    programInfoLength = (payload[10] & 0x0f) << 8 | payload[11];

    // advance the offset to the first entry in the mapping table
    offset = 12 + programInfoLength;
    while (offset < tableEnd) {
      // add an entry that maps the elementary_pid to the stream_type
      self.programMapTable[(payload[offset + 1] & 0x1F) << 8 | payload[offset + 2]] = payload[offset];

      // move to the next table entry
      // skip past the elementary stream descriptors, if present
      offset += ((payload[offset + 3] & 0x0F) << 8 | payload[offset + 4]) + 5;
    }

    // record the map on the packet as well
    pmt.programMapTable = self.programMapTable;

    // if there are any packets waiting for a PMT to be found, process them now
    while (self.packetsWaitingForPmt.length) {
      self.processPes_.apply(self, self.packetsWaitingForPmt.shift());
    }
  };

  /**
   * Deliver a new MP2T packet to the stream.
   */
  this.push = function(packet) {
    var
      result = {},
      offset = 4;

    result.payloadUnitStartIndicator = !!(packet[1] & 0x40);

    // pid is a 13-bit field starting at the last bit of packet[1]
    result.pid = packet[1] & 0x1f;
    result.pid <<= 8;
    result.pid |= packet[2];

    // if an adaption field is present, its length is specified by the
    // fifth byte of the TS packet header. The adaptation field is
    // used to add stuffing to PES packets that don't fill a complete
    // TS packet, and to specify some forms of timing and control data
    // that we do not currently use.
    if (((packet[3] & 0x30) >>> 4) > 0x01) {
      offset += packet[offset] + 1;
    }

    // parse the rest of the packet based on the type
    if (result.pid === 0) {
      result.type = 'pat';
      parsePsi(packet.subarray(offset), result);
      this.trigger('data', result);
    } else if (result.pid === this.pmtPid) {
      result.type = 'pmt';
      parsePsi(packet.subarray(offset), result);
      this.trigger('data', result);
    } else if (this.programMapTable === undefined) {
      // When we have not seen a PMT yet, defer further processing of
      // PES packets until one has been parsed
      this.packetsWaitingForPmt.push([packet, offset, result]);
    } else {
      this.processPes_(packet, offset, result);
    }
  };

  this.processPes_ = function(packet, offset, result) {
    result.streamType = this.programMapTable[result.pid];
    result.type = 'pes';
    result.data = packet.subarray(offset);

    this.trigger('data', result);
  };

};
TransportParseStream.prototype = new Stream();
TransportParseStream.STREAM_TYPES  = {
  h264: 0x1b,
  adts: 0x0f
};

/**
 * Reconsistutes program elementary stream (PES) packets from parsed
 * transport stream packets. That is, if you pipe an
 * mp2t.TransportParseStream into a mp2t.ElementaryStream, the output
 * events will be events which capture the bytes for individual PES
 * packets plus relevant metadata that has been extracted from the
 * container.
 */
ElementaryStream = function() {
  var
    self = this,
    // PES packet fragments
    video = {
      data: [],
      size: 0
    },
    audio = {
      data: [],
      size: 0
    },
    timedMetadata = {
      data: [],
      size: 0
    },
    parsePes = function(payload, pes) {
      var ptsDtsFlags;

      // find out if this packets starts a new keyframe
      pes.dataAlignmentIndicator = (payload[6] & 0x04) !== 0;
      // PES packets may be annotated with a PTS value, or a PTS value
      // and a DTS value. Determine what combination of values is
      // available to work with.
      ptsDtsFlags = payload[7];

      // PTS and DTS are normally stored as a 33-bit number.  Javascript
      // performs all bitwise operations on 32-bit integers but javascript
      // supports a much greater range (52-bits) of integer using standard
      // mathematical operations.
      // We construct a 31-bit value using bitwise operators over the 31
      // most significant bits and then multiply by 4 (equal to a left-shift
      // of 2) before we add the final 2 least significant bits of the
      // timestamp (equal to an OR.)
      if (ptsDtsFlags & 0xC0) {
        // the PTS and DTS are not written out directly. For information
        // on how they are encoded, see
        // http://dvd.sourceforge.net/dvdinfo/pes-hdr.html
        pes.pts = (payload[9] & 0x0E) << 27 |
          (payload[10] & 0xFF) << 20 |
          (payload[11] & 0xFE) << 12 |
          (payload[12] & 0xFF) <<  5 |
          (payload[13] & 0xFE) >>>  3;
        pes.pts *= 4; // Left shift by 2
        pes.pts += (payload[13] & 0x06) >>> 1; // OR by the two LSBs
        pes.dts = pes.pts;
        if (ptsDtsFlags & 0x40) {
          pes.dts = (payload[14] & 0x0E) << 27 |
            (payload[15] & 0xFF) << 20 |
            (payload[16] & 0xFE) << 12 |
            (payload[17] & 0xFF) << 5 |
            (payload[18] & 0xFE) >>> 3;
          pes.dts *= 4; // Left shift by 2
          pes.dts += (payload[18] & 0x06) >>> 1; // OR by the two LSBs
        }
      }

      // the data section starts immediately after the PES header.
      // pes_header_data_length specifies the number of header bytes
      // that follow the last byte of the field.
      pes.data = payload.subarray(9 + payload[8]);
    },
    flushStream = function(stream, type) {
      var
        packetData = new Uint8Array(stream.size),
        event = {
          type: type
        },
        i = 0,
        fragment;

      // do nothing if there is no buffered data
      if (!stream.data.length) {
        return;
      }
      event.trackId = stream.data[0].pid;

      // reassemble the packet
      while (stream.data.length) {
        fragment = stream.data.shift();

        packetData.set(fragment.data, i);
        i += fragment.data.byteLength;
      }

      // parse assembled packet's PES header
      parsePes(packetData, event);

      stream.size = 0;

      self.trigger('data', event);
    };

  ElementaryStream.prototype.init.call(this);

  this.push = function(data) {
    ({
      pat: function() {
        // we have to wait for the PMT to arrive as well before we
        // have any meaningful metadata
      },
      pes: function() {
        var stream, streamType;

        switch (data.streamType) {
        case StreamTypes.H264_STREAM_TYPE:
        case m2tsStreamTypes.H264_STREAM_TYPE:
          stream = video;
          streamType = 'video';
          break;
        case StreamTypes.ADTS_STREAM_TYPE:
          stream = audio;
          streamType = 'audio';
          break;
        case StreamTypes.METADATA_STREAM_TYPE:
          stream = timedMetadata;
          streamType = 'timed-metadata';
          break;
        default:
          // ignore unknown stream types
          return;
        }

        // if a new packet is starting, we can flush the completed
        // packet
        if (data.payloadUnitStartIndicator) {
          flushStream(stream, streamType);
        }

        // buffer this fragment until we are sure we've received the
        // complete payload
        stream.data.push(data);
        stream.size += data.data.byteLength;
      },
      pmt: function() {
        var
          event = {
            type: 'metadata',
            tracks: []
          },
          programMapTable = data.programMapTable,
          k,
          track;

        // translate streams to tracks
        for (k in programMapTable) {
          if (programMapTable.hasOwnProperty(k)) {
            track = {
              timelineStartInfo: {
                baseMediaDecodeTime: 0
              }
            };
            track.id = +k;
            if (programMapTable[k] === m2tsStreamTypes.H264_STREAM_TYPE) {
              track.codec = 'avc';
              track.type = 'video';
            } else if (programMapTable[k] === m2tsStreamTypes.ADTS_STREAM_TYPE) {
              track.codec = 'adts';
              track.type = 'audio';
            }
            event.tracks.push(track);
          }
        }
        self.trigger('data', event);
      }
    })[data.type]();
  };

  /**
   * Flush any remaining input. Video PES packets may be of variable
   * length. Normally, the start of a new video packet can trigger the
   * finalization of the previous packet. That is not possible if no
   * more video is forthcoming, however. In that case, some other
   * mechanism (like the end of the file) has to be employed. When it is
   * clear that no additional data is forthcoming, calling this method
   * will flush the buffered packets.
   */
  this.flush = function() {
    // !!THIS ORDER IS IMPORTANT!!
    // video first then audio
    flushStream(video, 'video');
    flushStream(audio, 'audio');
    flushStream(timedMetadata, 'timed-metadata');
    this.trigger('done');
  };
};
ElementaryStream.prototype = new Stream();

var m2ts = {
  PAT_PID: 0x0000,
  MP2T_PACKET_LENGTH: MP2T_PACKET_LENGTH,
  TransportPacketStream: TransportPacketStream,
  TransportParseStream: TransportParseStream,
  ElementaryStream: ElementaryStream,
  CaptionStream: CaptionStream.CaptionStream,
  Cea608Stream: CaptionStream.Cea608Stream,
  MetadataStream: require('./metadata-stream')
};

for (var type in StreamTypes) {
  if (StreamTypes.hasOwnProperty(type)) {
    m2ts[type] = StreamTypes[type];
  }
}

module.exports = m2ts;