caption-stream.js

/**
 * mux.js
 *
 * Copyright (c) Brightcove
 * Licensed Apache-2.0 https://github.com/videojs/mux.js/blob/master/LICENSE
 *
 * Reads in-band caption information from a video elementary
 * stream. Captions must follow the CEA-708 standard for injection
 * into an MPEG-2 transport streams.
 * @see https://en.wikipedia.org/wiki/CEA-708
 * @see https://www.gpo.gov/fdsys/pkg/CFR-2007-title47-vol1/pdf/CFR-2007-title47-vol1-sec15-119.pdf
 */
// -----------------
// Link To Transport
// -----------------

import Stream from '../utils/stream';
import {
  parseSei,
  USER_DATA_REGISTERED_ITU_T_T35,
  parseUserData,
  parseCaptionPackets
} from '../tools/caption-packet-parser';


// ----------------------
// Session to Application
// ----------------------

// This hash maps non-ASCII, special, and extended character codes to their
// proper Unicode equivalent. The first keys that are only a single byte
// are the non-standard ASCII characters, which simply map the CEA608 byte
// to the standard ASCII/Unicode. The two-byte keys that follow are the CEA608
// character codes, but have their MSB bitmasked with 0x03 so that a lookup
// can be performed regardless of the field and data channel on which the
// character code was received.
var CHARACTER_TRANSLATION = {
  0x2a: 0xe1,     // á
  0x5c: 0xe9,     // é
  0x5e: 0xed,     // í
  0x5f: 0xf3,     // ó
  0x60: 0xfa,     // ú
  0x7b: 0xe7,     // ç
  0x7c: 0xf7,     // ÷
  0x7d: 0xd1,     // Ñ
  0x7e: 0xf1,     // ñ
  0x7f: 0x2588,   // █
  0x0130: 0xae,   // ®
  0x0131: 0xb0,   // °
  0x0132: 0xbd,   // ½
  0x0133: 0xbf,   // ¿
  0x0134: 0x2122, // ™
  0x0135: 0xa2,   // ¢
  0x0136: 0xa3,   // £
  0x0137: 0x266a, // ♪
  0x0138: 0xe0,   // à
  0x0139: 0xa0,   //
  0x013a: 0xe8,   // è
  0x013b: 0xe2,   // â
  0x013c: 0xea,   // ê
  0x013d: 0xee,   // î
  0x013e: 0xf4,   // ô
  0x013f: 0xfb,   // û
  0x0220: 0xc1,   // Á
  0x0221: 0xc9,   // É
  0x0222: 0xd3,   // Ó
  0x0223: 0xda,   // Ú
  0x0224: 0xdc,   // Ü
  0x0225: 0xfc,   // ü
  0x0226: 0x2018, // ‘
  0x0227: 0xa1,   // ¡
  0x0228: 0x2a,   // *
  0x0229: 0x27,   // '
  0x022a: 0x2014, // —
  0x022b: 0xa9,   // ©
  0x022c: 0x2120, // ℠
  0x022d: 0x2022, // •
  0x022e: 0x201c, // “
  0x022f: 0x201d, // ”
  0x0230: 0xc0,   // À
  0x0231: 0xc2,   // Â
  0x0232: 0xc7,   // Ç
  0x0233: 0xc8,   // È
  0x0234: 0xca,   // Ê
  0x0235: 0xcb,   // Ë
  0x0236: 0xeb,   // ë
  0x0237: 0xce,   // Î
  0x0238: 0xcf,   // Ï
  0x0239: 0xef,   // ï
  0x023a: 0xd4,   // Ô
  0x023b: 0xd9,   // Ù
  0x023c: 0xf9,   // ù
  0x023d: 0xdb,   // Û
  0x023e: 0xab,   // «
  0x023f: 0xbb,   // »
  0x0320: 0xc3,   // Ã
  0x0321: 0xe3,   // ã
  0x0322: 0xcd,   // Í
  0x0323: 0xcc,   // Ì
  0x0324: 0xec,   // ì
  0x0325: 0xd2,   // Ò
  0x0326: 0xf2,   // ò
  0x0327: 0xd5,   // Õ
  0x0328: 0xf5,   // õ
  0x0329: 0x7b,   // {
  0x032a: 0x7d,   // }
  0x032b: 0x5c,   // \
  0x032c: 0x5e,   // ^
  0x032d: 0x5f,   // _
  0x032e: 0x7c,   // |
  0x032f: 0x7e,   // ~
  0x0330: 0xc4,   // Ä
  0x0331: 0xe4,   // ä
  0x0332: 0xd6,   // Ö
  0x0333: 0xf6,   // ö
  0x0334: 0xdf,   // ß
  0x0335: 0xa5,   // ¥
  0x0336: 0xa4,   // ¤
  0x0337: 0x2502, // │
  0x0338: 0xc5,   // Å
  0x0339: 0xe5,   // å
  0x033a: 0xd8,   // Ø
  0x033b: 0xf8,   // ø
  0x033c: 0x250c, // ┌
  0x033d: 0x2510, // ┐
  0x033e: 0x2514, // └
  0x033f: 0x2518  // ┘
};

var getCharFromCode = function(code) {
  if (code === null) {
    return '';
  }
  code = CHARACTER_TRANSLATION[code] || code;
  return String.fromCharCode(code);
};

// the index of the last row in a CEA-608 display buffer
var BOTTOM_ROW = 14;

// This array is used for mapping PACs -> row #, since there's no way of
// getting it through bit logic.
var ROWS = [0x1100, 0x1120, 0x1200, 0x1220, 0x1500, 0x1520, 0x1600, 0x1620,
            0x1700, 0x1720, 0x1000, 0x1300, 0x1320, 0x1400, 0x1420];

// CEA-608 captions are rendered onto a 34x15 matrix of character
// cells. The "bottom" row is the last element in the outer array.
var createDisplayBuffer = function() {
  var result = [], i = BOTTOM_ROW + 1;
  while (i--) {
    result.push('');
  }
  return result;
};

export class Cea608Stream extends Stream {
  constructor(field, dataChannel) {
    super();

    this.field_ = field || 0;
    this.dataChannel_ = dataChannel || 0;

    this.name_ = 'CC' + (((this.field_ << 1) | this.dataChannel_) + 1);

    this.setConstants();
    this.reset();
  }

  push(packet) {
    var data, swap, char0, char1, text;
    // remove the parity bits
    data = packet.ccData & 0x7f7f;

    // ignore duplicate control codes; the spec demands they're sent twice
    if (data === this.lastControlCode_) {
      this.lastControlCode_ = null;
      return;
    }

    // Store control codes
    if ((data & 0xf000) === 0x1000) {
      this.lastControlCode_ = data;
    } else if (data !== this.PADDING_) {
      this.lastControlCode_ = null;
    }

    char0 = data >>> 8;
    char1 = data & 0xff;

    if (data === this.PADDING_) {
      return;

    } else if (data === this.RESUME_CAPTION_LOADING_) {
      this.mode_ = 'popOn';

    } else if (data === this.END_OF_CAPTION_) {
      // If an EOC is received while in paint-on mode, the displayed caption
      // text should be swapped to non-displayed memory as if it was a pop-on
      // caption. Because of that, we should explicitly switch back to pop-on
      // mode
      this.mode_ = 'popOn';
      this.clearFormatting(packet.pts);
      // if a caption was being displayed, it's gone now
      this.flushDisplayed(packet.pts);

      // flip memory
      swap = this.displayed_;
      this.displayed_ = this.nonDisplayed_;
      this.nonDisplayed_ = swap;

      // start measuring the time to display the caption
      this.startPts_ = packet.pts;

    } else if (data === this.ROLL_UP_2_ROWS_) {
      this.rollUpRows_ = 2;
      this.setRollUp(packet.pts);
    } else if (data === this.ROLL_UP_3_ROWS_) {
      this.rollUpRows_ = 3;
      this.setRollUp(packet.pts);
    } else if (data === this.ROLL_UP_4_ROWS_) {
      this.rollUpRows_ = 4;
      this.setRollUp(packet.pts);
    } else if (data === this.CARRIAGE_RETURN_) {
      this.clearFormatting(packet.pts);
      this.flushDisplayed(packet.pts);
      this.shiftRowsUp_();
      this.startPts_ = packet.pts;

    } else if (data === this.BACKSPACE_) {
      if (this.mode_ === 'popOn') {
        this.nonDisplayed_[this.row_] = this.nonDisplayed_[this.row_].slice(0, -1);
      } else {
        this.displayed_[this.row_] = this.displayed_[this.row_].slice(0, -1);
      }
    } else if (data === this.ERASE_DISPLAYED_MEMORY_) {
      this.flushDisplayed(packet.pts);
      this.displayed_ = createDisplayBuffer();
    } else if (data === this.ERASE_NON_DISPLAYED_MEMORY_) {
      this.nonDisplayed_ = createDisplayBuffer();

    } else if (data === this.RESUME_DIRECT_CAPTIONING_) {
      if (this.mode_ !== 'paintOn') {
        // NOTE: This should be removed when proper caption positioning is
        // implemented
        this.flushDisplayed(packet.pts);
        this.displayed_ = createDisplayBuffer();
      }
      this.mode_ = 'paintOn';
      this.startPts_ = packet.pts;

      // Append special characters to caption text
    } else if (this.isSpecialCharacter(char0, char1)) {
      // Bitmask char0 so that we can apply character transformations
      // regardless of field and data channel.
      // Then byte-shift to the left and OR with char1 so we can pass the
      // entire character code to `getCharFromCode`.
      char0 = (char0 & 0x03) << 8;
      text = getCharFromCode(char0 | char1);
      this[this.mode_](packet.pts, text);
      this.column_++;

      // Append extended characters to caption text
    } else if (this.isExtCharacter(char0, char1)) {
      // Extended characters always follow their "non-extended" equivalents.
      // IE if a "è" is desired, you'll always receive "eè"; non-compliant
      // decoders are supposed to drop the "è", while compliant decoders
      // backspace the "e" and insert "è".
      // Delete the previous character
      if (this.mode_ === 'popOn') {
        this.nonDisplayed_[this.row_] = this.nonDisplayed_[this.row_].slice(0, -1);
      } else {
        this.displayed_[this.row_] = this.displayed_[this.row_].slice(0, -1);
      }

      // Bitmask char0 so that we can apply character transformations
      // regardless of field and data channel.
      // Then byte-shift to the left and OR with char1 so we can pass the
      // entire character code to `getCharFromCode`.
      char0 = (char0 & 0x03) << 8;
      text = getCharFromCode(char0 | char1);
      this[this.mode_](packet.pts, text);
      this.column_++;

      // Process mid-row codes
    } else if (this.isMidRowCode(char0, char1)) {
      // Attributes are not additive, so clear all formatting
      this.clearFormatting(packet.pts);

      // According to the standard, mid-row codes
      // should be replaced with spaces, so add one now
      this[this.mode_](packet.pts, ' ');
      this.column_++;

      if ((char1 & 0xe) === 0xe) {
        this.addFormatting(packet.pts, ['i']);
      }

      if ((char1 & 0x1) === 0x1) {
        this.addFormatting(packet.pts, ['u']);
      }

      // Detect offset control codes and adjust cursor
    } else if (this.isOffsetControlCode(char0, char1)) {
      // Cursor position is set by indent PAC (see below) in 4-column
      // increments, with an additional offset code of 1-3 to reach any
      // of the 32 columns specified by CEA-608. So all we need to do
      // here is increment the column cursor by the given offset.
      this.column_ += (char1 & 0x03);

      // Detect PACs (Preamble Address Codes)
    } else if (this.isPAC(char0, char1)) {

      // There's no logic for PAC -> row mapping, so we have to just
      // find the row code in an array and use its index :(
      var row = ROWS.indexOf(data & 0x1f20);

      // Configure the caption window if we're in roll-up mode
      if (this.mode_ === 'rollUp') {
        // This implies that the base row is incorrectly set.
        // As per the recommendation in CEA-608(Base Row Implementation), defer to the number
        // of roll-up rows set.
        if (row - this.rollUpRows_ + 1 < 0) {
          row = this.rollUpRows_ - 1;
        }

        this.setRollUp(packet.pts, row);
      }

      if (row !== this.row_) {
        // formatting is only persistent for current row
        this.clearFormatting(packet.pts);
        this.row_ = row;
      }
      // All PACs can apply underline, so detect and apply
      // (All odd-numbered second bytes set underline)
      if ((char1 & 0x1) && (this.formatting_.indexOf('u') === -1)) {
        this.addFormatting(packet.pts, ['u']);
      }

      if ((data & 0x10) === 0x10) {
        // We've got an indent level code. Each successive even number
        // increments the column cursor by 4, so we can get the desired
        // column position by bit-shifting to the right (to get n/2)
        // and multiplying by 4.
        this.column_ = ((data & 0xe) >> 1) * 4;
      }

      if (this.isColorPAC(char1)) {
        // it's a color code, though we only support white, which
        // can be either normal or italicized. white italics can be
        // either 0x4e or 0x6e depending on the row, so we just
        // bitwise-and with 0xe to see if italics should be turned on
        if ((char1 & 0xe) === 0xe) {
          this.addFormatting(packet.pts, ['i']);
        }
      }

      // We have a normal character in char0, and possibly one in char1
    } else if (this.isNormalChar(char0)) {
      if (char1 === 0x00) {
        char1 = null;
      }
      text = getCharFromCode(char0);
      text += getCharFromCode(char1);
      this[this.mode_](packet.pts, text);
      this.column_ += text.length;

    } // finish data processing

  }

  // Trigger a cue point that captures the current state of the
  // display buffer
  flushDisplayed(pts) {
    var content = this.displayed_
      // remove spaces from the start and end of the string
      .map(function(row) {
        try {
          return row.trim();
        } catch (e) {
          // Ordinarily, this shouldn't happen. However, caption
          // parsing errors should not throw exceptions and
          // break playback.
          // eslint-disable-next-line no-console
          console.error('Skipping malformed caption.');
          return '';
        }
      })
      // combine all text rows to display in one cue
      .join('\n')
      // and remove blank rows from the start and end, but not the middle
      .replace(/^\n+|\n+$/g, '');

    if (content.length) {
      this.trigger('data', {
        startPts: this.startPts_,
        endPts: pts,
        text: content,
        stream: this.name_
      });
    }
  }
  /**
   * Zero out the data, used for startup and on seek
   */
  reset() {
    this.mode_ = 'popOn';
    // When in roll-up mode, the index of the last row that will
    // actually display captions. If a caption is shifted to a row
    // with a lower index than this, it is cleared from the display
    // buffer
    this.topRow_ = 0;
    this.startPts_ = 0;
    this.displayed_ = createDisplayBuffer();
    this.nonDisplayed_ = createDisplayBuffer();
    this.lastControlCode_ = null;

    // Track row and column for proper line-breaking and spacing
    this.column_ = 0;
    this.row_ = BOTTOM_ROW;
    this.rollUpRows_ = 2;

    // This variable holds currently-applied formatting
    this.formatting_ = [];
  }
  /**
   * Sets up control code and related constants for this instance
   */
  setConstants() {
    // The following attributes have these uses:
    // ext_ :    char0 for mid-row codes, and the base for extended
    //           chars (ext_+0, ext_+1, and ext_+2 are char0s for
    //           extended codes)
    // control_: char0 for control codes, except byte-shifted to the
    //           left so that we can do this.control_ | CONTROL_CODE
    // offset_:  char0 for tab offset codes
    //
    // It's also worth noting that control codes, and _only_ control codes,
    // differ between field 1 and field2. Field 2 control codes are always
    // their field 1 value plus 1. That's why there's the "| field" on the
    // control value.
    if (this.dataChannel_ === 0) {
      this.BASE_ = 0x10;
      this.EXT_ = 0x11;
      this.CONTROL_ = (0x14 | this.field_) << 8;
      this.OFFSET_ = 0x17;
    } else if (this.dataChannel_ === 1) {
      this.BASE_ = 0x18;
      this.EXT_ = 0x19;
      this.CONTROL_ = (0x1c | this.field_) << 8;
      this.OFFSET_ = 0x1f;
    }

    // Constants for the LSByte command codes recognized by Cea608Stream. This
    // list is not exhaustive. For a more comprehensive listing and semantics see
    // http://www.gpo.gov/fdsys/pkg/CFR-2010-title47-vol1/pdf/CFR-2010-title47-vol1-sec15-119.pdf
    // Padding
    this.PADDING_ = 0x0000;
    // Pop-on Mode
    this.RESUME_CAPTION_LOADING_ = this.CONTROL_ | 0x20;
    this.END_OF_CAPTION_ = this.CONTROL_ | 0x2f;
    // Roll-up Mode
    this.ROLL_UP_2_ROWS_ = this.CONTROL_ | 0x25;
    this.ROLL_UP_3_ROWS_ = this.CONTROL_ | 0x26;
    this.ROLL_UP_4_ROWS_ = this.CONTROL_ | 0x27;
    this.CARRIAGE_RETURN_ = this.CONTROL_ | 0x2d;
    // paint-on mode
    this.RESUME_DIRECT_CAPTIONING_ = this.CONTROL_ | 0x29;
    // Erasure
    this.BACKSPACE_ = this.CONTROL_ | 0x21;
    this.ERASE_DISPLAYED_MEMORY_ = this.CONTROL_ | 0x2c;
    this.ERASE_NON_DISPLAYED_MEMORY_ = this.CONTROL_ | 0x2e;
  }
  /**
   * Detects if the 2-byte packet data is a special character
   *
   * Special characters have a second byte in the range 0x30 to 0x3f,
   * with the first byte being 0x11 (for data channel 1) or 0x19 (for
   * data channel 2).
   *
   * @param  {Integer} char0 The first byte
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the 2 bytes are an special character
   */
  isSpecialCharacter(char0, char1) {
    return (char0 === this.EXT_ && char1 >= 0x30 && char1 <= 0x3f);
  }
  /**
   * Detects if the 2-byte packet data is an extended character
   *
   * Extended characters have a second byte in the range 0x20 to 0x3f,
   * with the first byte being 0x12 or 0x13 (for data channel 1) or
   * 0x1a or 0x1b (for data channel 2).
   *
   * @param  {Integer} char0 The first byte
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the 2 bytes are an extended character
   */
  isExtCharacter(char0, char1) {
    return ((char0 === (this.EXT_ + 1) || char0 === (this.EXT_ + 2)) &&
      (char1 >= 0x20 && char1 <= 0x3f));
  }
  /**
   * Detects if the 2-byte packet is a mid-row code
   *
   * Mid-row codes have a second byte in the range 0x20 to 0x2f, with
   * the first byte being 0x11 (for data channel 1) or 0x19 (for data
   * channel 2).
   *
   * @param  {Integer} char0 The first byte
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the 2 bytes are a mid-row code
   */
  isMidRowCode(char0, char1) {
    return (char0 === this.EXT_ && (char1 >= 0x20 && char1 <= 0x2f));
  }
  /**
   * Detects if the 2-byte packet is an offset control code
   *
   * Offset control codes have a second byte in the range 0x21 to 0x23,
   * with the first byte being 0x17 (for data channel 1) or 0x1f (for
   * data channel 2).
   *
   * @param  {Integer} char0 The first byte
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the 2 bytes are an offset control code
   */
  isOffsetControlCode(char0, char1) {
    return (char0 === this.OFFSET_ && (char1 >= 0x21 && char1 <= 0x23));
  }
  /**
   * Detects if the 2-byte packet is a Preamble Address Code
   *
   * PACs have a first byte in the range 0x10 to 0x17 (for data channel 1)
   * or 0x18 to 0x1f (for data channel 2), with the second byte in the
   * range 0x40 to 0x7f.
   *
   * @param  {Integer} char0 The first byte
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the 2 bytes are a PAC
   */
  isPAC(char0, char1) {
    return (char0 >= this.BASE_ && char0 < (this.BASE_ + 8) &&
      (char1 >= 0x40 && char1 <= 0x7f));
  }
  /**
   * Detects if a packet's second byte is in the range of a PAC color code
   *
   * PAC color codes have the second byte be in the range 0x40 to 0x4f, or
   * 0x60 to 0x6f.
   *
   * @param  {Integer} char1 The second byte
   * @return {Boolean}       Whether the byte is a color PAC
   */
  isColorPAC(char1) {
    return ((char1 >= 0x40 && char1 <= 0x4f) || (char1 >= 0x60 && char1 <= 0x7f));
  }
  /**
   * Detects if a single byte is in the range of a normal character
   *
   * Normal text bytes are in the range 0x20 to 0x7f.
   *
   * @param  {Integer} char  The byte
   * @return {Boolean}       Whether the byte is a normal character
   */
  isNormalChar(char) {
    return (char >= 0x20 && char <= 0x7f);
  }
  /**
   * Configures roll-up
   *
   * @param  {Integer} pts         Current PTS
   * @param  {Integer} newBaseRow  Used by PACs to slide the current window to
   *                               a new position
   */
  setRollUp(pts, newBaseRow) {
    // Reset the base row to the bottom row when switching modes
    if (this.mode_ !== 'rollUp') {
      this.row_ = BOTTOM_ROW;
      this.mode_ = 'rollUp';
      // Spec says to wipe memories when switching to roll-up
      this.flushDisplayed(pts);
      this.nonDisplayed_ = createDisplayBuffer();
      this.displayed_ = createDisplayBuffer();
    }

    if (newBaseRow !== undefined && newBaseRow !== this.row_) {
      // move currently displayed captions (up or down) to the new base row
      for (var i = 0; i < this.rollUpRows_; i++) {
        this.displayed_[newBaseRow - i] = this.displayed_[this.row_ - i];
        this.displayed_[this.row_ - i] = '';
      }
    }

    if (newBaseRow === undefined) {
      newBaseRow = this.row_;
    }

    this.topRow_ = newBaseRow - this.rollUpRows_ + 1;
  }
  // Adds the opening HTML tag for the passed character to the caption text,
  // and keeps track of it for later closing
  addFormatting(pts, format) {
    this.formatting_ = this.formatting_.concat(format);
    var text = format.reduce(function(text, format) {
      return text + '<' + format + '>';
    }, '');
    this[this.mode_](pts, text);
  }
  // Adds HTML closing tags for current formatting to caption text and
  // clears remembered formatting
  clearFormatting(pts) {
    if (!this.formatting_.length) {
      return;
    }
    var text = this.formatting_.reverse().reduce(function(text, format) {
      return text + '</' + format + '>';
    }, '');
    this.formatting_ = [];
    this[this.mode_](pts, text);
  }
  // Mode Implementations
  popOn(pts, text) {
    var baseRow = this.nonDisplayed_[this.row_];

    // buffer characters
    baseRow += text;
    this.nonDisplayed_[this.row_] = baseRow;
  }
  rollUp(pts, text) {
    var baseRow = this.displayed_[this.row_];

    baseRow += text;
    this.displayed_[this.row_] = baseRow;

  }
  shiftRowsUp_() {
    var i;
    // clear out inactive rows
    for (i = 0; i < this.topRow_; i++) {
      this.displayed_[i] = '';
    }
    for (i = this.row_ + 1; i < BOTTOM_ROW + 1; i++) {
      this.displayed_[i] = '';
    }
    // shift displayed rows up
    for (i = this.topRow_; i < this.row_; i++) {
      this.displayed_[i] = this.displayed_[i + 1];
    }
    // clear out the bottom row
    this.displayed_[this.row_] = '';
  }
  paintOn(pts, text) {
    var baseRow = this.displayed_[this.row_];

    baseRow += text;
    this.displayed_[this.row_] = baseRow;
  }
}

export class CaptionStream extends Stream {
  constructor() {
    super();

    this.captionPackets_ = [];

    this.ccStreams_ = [
      new Cea608Stream(0, 0),
      new Cea608Stream(0, 1),
      new Cea608Stream(1, 0),
      new Cea608Stream(1, 1) // eslint-disable-line no-use-before-define
    ];

    this.reset();

    // forward data and done events from CCs to this CaptionStream
    this.ccStreams_.forEach(function(cc) {
      cc.on('data', this.trigger.bind(this, 'data'));
      cc.on('partialdone', this.trigger.bind(this, 'partialdone'));
      cc.on('done', this.trigger.bind(this, 'done'));
    }, this);

  }

  push(event) {
    var sei, userData, newCaptionPackets;

    // only examine SEI NALs
    if (event.nalUnitType !== 'sei_rbsp') {
      return;
    }

    // parse the sei
    sei = parseSei(event.escapedRBSP);

    // no payload data, skip
    if (!sei.payload) {
      return;
    }

    // ignore everything but user_data_registered_itu_t_t35
    if (sei.payloadType !== USER_DATA_REGISTERED_ITU_T_T35) {
      return;
    }

    // parse out the user data payload
    userData = parseUserData(sei);

    // ignore unrecognized userData
    if (!userData) {
      return;
    }

    // Sometimes, the same segment # will be downloaded twice. To stop the
    // caption data from being processed twice, we track the latest dts we've
    // received and ignore everything with a dts before that. However, since
    // data for a specific dts can be split across packets on either side of
    // a segment boundary, we need to make sure we *don't* ignore the packets
    // from the *next* segment that have dts === this.latestDts_. By constantly
    // tracking the number of packets received with dts === this.latestDts_, we
    // know how many should be ignored once we start receiving duplicates.
    if (event.dts < this.latestDts_) {
      // We've started getting older data, so set the flag.
      this.ignoreNextEqualDts_ = true;
      return;
    } else if ((event.dts === this.latestDts_) && (this.ignoreNextEqualDts_)) {
      this.numSameDts_--;
      if (!this.numSameDts_) {
        // We've received the last duplicate packet, time to start processing again
        this.ignoreNextEqualDts_ = false;
      }
      return;
    }

    // parse out CC data packets and save them for later
    newCaptionPackets = parseCaptionPackets(event.pts, userData);
    this.captionPackets_ = this.captionPackets_.concat(newCaptionPackets);
    if (this.latestDts_ !== event.dts) {
      this.numSameDts_ = 0;
    }
    this.numSameDts_++;
    this.latestDts_ = event.dts;
  }
  flushCCStreams(flushType) {
    this.ccStreams_.forEach(function(cc) {
      return flushType === 'flush' ? cc.flush() : cc.partialFlush();
    }, this);
  }
  flushStream(flushType) {
    // make sure we actually parsed captions before proceeding
    if (!this.captionPackets_.length) {
      this.flushCCStreams(flushType);
      return;
    }

    // In Chrome, the Array#sort function is not stable so add a
    // presortIndex that we can use to ensure we get a stable-sort
    this.captionPackets_.forEach(function(elem, idx) {
      elem.presortIndex = idx;
    });

    // sort caption byte-pairs based on their PTS values
    this.captionPackets_.sort(function(a, b) {
      if (a.pts === b.pts) {
        return a.presortIndex - b.presortIndex;
      }
      return a.pts - b.pts;
    });

    this.captionPackets_.forEach(function(packet) {
      if (packet.type < 2) {
        // Dispatch packet to the right Cea608Stream
        this.dispatchCea608Packet(packet);
      }
      // this is where an 'else' would go for a dispatching packets
      // to a theoretical Cea708Stream that handles SERVICEn data
    }, this);

    this.captionPackets_.length = 0;
    this.flushCCStreams(flushType);
  }
  flush() {
    return this.flushStream('flush');
  }
  // Only called if handling partial data
  partialFlush() {
    return this.flushStream('partialFlush');
  }
  reset() {
    this.latestDts_ = null;
    this.ignoreNextEqualDts_ = false;
    this.numSameDts_ = 0;
    this.activeCea608Channel_ = [null, null];
    this.ccStreams_.forEach(function(ccStream) {
      ccStream.reset();
    });
  }
  // From the CEA-608 spec:
  /*
   * When XDS sub-packets are interleaved with other services, the end of each sub-packet shall be followed
   * by a control pair to change to a different service. When any of the control codes from 0x10 to 0x1F is
   * used to begin a control code pair, it indicates the return to captioning or Text data. The control code pair
   * and subsequent data should then be processed according to the FCC rules. It may be necessary for the
   * line 21 data encoder to automatically insert a control code pair (i.e. RCL, RU2, RU3, RU4, RDC, or RTD)
   * to switch to captioning or Text.
  */
  // With that in mind, we ignore any data between an XDS control code and a
  // subsequent closed-captioning control code.
  dispatchCea608Packet(packet) {
    // NOTE: packet.type is the CEA608 field
    if (this.setsTextOrXDSActive(packet)) {
      this.activeCea608Channel_[packet.type] = null;
    } else if (this.setsChannel1Active(packet)) {
      this.activeCea608Channel_[packet.type] = 0;
    } else if (this.setsChannel2Active(packet)) {
      this.activeCea608Channel_[packet.type] = 1;
    }
    if (this.activeCea608Channel_[packet.type] === null) {
      // If we haven't received anything to set the active channel, or the
      // packets are Text/XDS data, discard the data; we don't want jumbled
      // captions
      return;
    }
    this.ccStreams_[(packet.type << 1) + this.activeCea608Channel_[packet.type]].push(packet);
  }
  setsChannel1Active(packet) {
    return ((packet.ccData & 0x7800) === 0x1000);
  }
  setsChannel2Active(packet) {
    return ((packet.ccData & 0x7800) === 0x1800);
  }
  setsTextOrXDSActive(packet) {
    return ((packet.ccData & 0x7100) === 0x0100) ||
      ((packet.ccData & 0x78fe) === 0x102a) ||
      ((packet.ccData & 0x78fe) === 0x182a);
  }
}