yt_dlp/webvtt.py

   1 """
   2 A partial parser for WebVTT segments. Interprets enough of the WebVTT stream
   3 to be able to assemble a single stand-alone subtitle file, suitably adjusting
   4 timestamps on the way, while everything else is passed through unmodified.
   5
   6 Regular expressions based on the W3C WebVTT specification
   7 <https://www.w3.org/TR/webvtt1/>. The X-TIMESTAMP-MAP extension is described
   8 in RFC 8216 §3.5 <https://tools.ietf.org/html/rfc8216#section-3.5>.
   9 """
  10
  11 import io
  12 import re
  13
  14 from .utils import int_or_none, timetuple_from_msec
  15
  16
  17 class _MatchParser:
  18     """
  19     An object that maintains the current parsing position and allows
  20     conveniently advancing it as syntax elements are successfully parsed.
  21     """
  22
  23     def __init__(self, string):
  24         self._data = string
  25         self._pos = 0
  26
  27     def match(self, r):
  28         if isinstance(r, re.Pattern):
  29             return r.match(self._data, self._pos)
  30         if isinstance(r, str):
  31             if self._data.startswith(r, self._pos):
  32                 return len(r)
  33             return None
  34         raise ValueError(r)
  35
  36     def advance(self, by):
  37         if by is None:
  38             amt = 0
  39         elif isinstance(by, re.Match):
  40             amt = len(by.group(0))
  41         elif isinstance(by, str):
  42             amt = len(by)
  43         elif isinstance(by, int):
  44             amt = by
  45         else:
  46             raise ValueError(by)
  47         self._pos += amt
  48         return by
  49
  50     def consume(self, r):
  51         return self.advance(self.match(r))
  52
  53     def child(self):
  54         return _MatchChildParser(self)
  55
  56
  57 class _MatchChildParser(_MatchParser):
  58     """
  59     A child parser state, which advances through the same data as
  60     its parent, but has an independent position. This is useful when
  61     advancing through syntax elements we might later want to backtrack
  62     from.
  63     """
  64
  65     def __init__(self, parent):
  66         super().__init__(parent._data)
  67         self.__parent = parent
  68         self._pos = parent._pos
  69
  70     def commit(self):
  71         """
  72         Advance the parent state to the current position of this child state.
  73         """
  74         self.__parent._pos = self._pos
  75         return self.__parent
  76
  77
  78 class ParseError(Exception):
  79     def __init__(self, parser):
  80         super().__init__("Parse error at position %u (near %r)" % (
  81             parser._pos, parser._data[parser._pos:parser._pos + 20]
  82         ))
  83
  84
  85 # While the specification <https://www.w3.org/TR/webvtt1/#webvtt-timestamp>
  86 # prescribes that hours must be *2 or more* digits, timestamps with a single
  87 # digit for the hour part has been seen in the wild.
  88 # See https://github.com/yt-dlp/yt-dlp/issues/921
  89 _REGEX_TS = re.compile(r'''(?x)
  90     (?:([0-9]{1,}):)?
  91     ([0-9]{2}):
  92     ([0-9]{2})\.
  93     ([0-9]{3})?
  94 ''')
  95 _REGEX_EOF = re.compile(r'\Z')
  96 _REGEX_NL = re.compile(r'(?:\r\n|[\r\n])')
  97 _REGEX_BLANK = re.compile(r'(?:\r\n|[\r\n])+')
  98
  99
 100 def _parse_ts(ts):
 101     """
 102     Convert a parsed WebVTT timestamp (a re.Match obtained from _REGEX_TS)
 103     into an MPEG PES timestamp: a tick counter at 90 kHz resolution.
 104     """
 105     return 90 * sum(
 106         int(part or 0) * mult for part, mult in zip(ts.groups(), (3600_000, 60_000, 1000, 1)))
 107
 108
 109 def _format_ts(ts):
 110     """
 111     Convert an MPEG PES timestamp into a WebVTT timestamp.
 112     This will lose sub-millisecond precision.
 113     """
 114     return '%02u:%02u:%02u.%03u' % timetuple_from_msec(int((ts + 45) // 90))
 115
 116
 117 class Block:
 118     """
 119     An abstract WebVTT block.
 120     """
 121
 122     def __init__(self, **kwargs):
 123         for key, val in kwargs.items():
 124             setattr(self, key, val)
 125
 126     @classmethod
 127     def parse(cls, parser):
 128         m = parser.match(cls._REGEX)
 129         if not m:
 130             return None
 131         parser.advance(m)
 132         return cls(raw=m.group(0))
 133
 134     def write_into(self, stream):
 135         stream.write(self.raw)
 136
 137
 138 class HeaderBlock(Block):
 139     """
 140     A WebVTT block that may only appear in the header part of the file,
 141     i.e. before any cue blocks.
 142     """
 143
 144     pass
 145
 146
 147 class Magic(HeaderBlock):
 148     _REGEX = re.compile(r'\ufeff?WEBVTT([ \t][^\r\n]*)?(?:\r\n|[\r\n])')
 149
 150     # XXX: The X-TIMESTAMP-MAP extension is described in RFC 8216 §3.5
 151     # <https://tools.ietf.org/html/rfc8216#section-3.5>, but the RFC
 152     # doesn’t specify the exact grammar nor where in the WebVTT
 153     # syntax it should be placed; the below has been devised based
 154     # on usage in the wild
 155     #
 156     # And strictly speaking, the presence of this extension violates
 157     # the W3C WebVTT spec. Oh well.
 158
 159     _REGEX_TSMAP = re.compile(r'X-TIMESTAMP-MAP=')
 160     _REGEX_TSMAP_LOCAL = re.compile(r'LOCAL:')
 161     _REGEX_TSMAP_MPEGTS = re.compile(r'MPEGTS:([0-9]+)')
 162     _REGEX_TSMAP_SEP = re.compile(r'[ \t]*,[ \t]*')
 163
 164     # This was removed from the spec in the 2017 revision;
 165     # the last spec draft to describe this syntax element is
 166     # <https://www.w3.org/TR/2015/WD-webvtt1-20151208/#webvtt-metadata-header>.
 167     # Nevertheless, YouTube keeps serving those
 168     _REGEX_META = re.compile(r'(?:(?!-->)[^\r\n])+:(?:(?!-->)[^\r\n])+(?:\r\n|[\r\n])')
 169
 170     @classmethod
 171     def __parse_tsmap(cls, parser):
 172         parser = parser.child()
 173
 174         while True:
 175             m = parser.consume(cls._REGEX_TSMAP_LOCAL)
 176             if m:
 177                 m = parser.consume(_REGEX_TS)
 178                 if m is None:
 179                     raise ParseError(parser)
 180                 local = _parse_ts(m)
 181                 if local is None:
 182                     raise ParseError(parser)
 183             else:
 184                 m = parser.consume(cls._REGEX_TSMAP_MPEGTS)
 185                 if m:
 186                     mpegts = int_or_none(m.group(1))
 187                     if mpegts is None:
 188                         raise ParseError(parser)
 189                 else:
 190                     raise ParseError(parser)
 191             if parser.consume(cls._REGEX_TSMAP_SEP):
 192                 continue
 193             if parser.consume(_REGEX_NL):
 194                 break
 195             raise ParseError(parser)
 196
 197         parser.commit()
 198         return local, mpegts
 199
 200     @classmethod
 201     def parse(cls, parser):
 202         parser = parser.child()
 203
 204         m = parser.consume(cls._REGEX)
 205         if not m:
 206             raise ParseError(parser)
 207
 208         extra = m.group(1)
 209         local, mpegts, meta = None, None, ''
 210         while not parser.consume(_REGEX_NL):
 211             if parser.consume(cls._REGEX_TSMAP):
 212                 local, mpegts = cls.__parse_tsmap(parser)
 213                 continue
 214             m = parser.consume(cls._REGEX_META)
 215             if m:
 216                 meta += m.group(0)
 217                 continue
 218             raise ParseError(parser)
 219         parser.commit()
 220         return cls(extra=extra, mpegts=mpegts, local=local, meta=meta)
 221
 222     def write_into(self, stream):
 223         stream.write('WEBVTT')
 224         if self.extra is not None:
 225             stream.write(self.extra)
 226         stream.write('\n')
 227         if self.local or self.mpegts:
 228             stream.write('X-TIMESTAMP-MAP=LOCAL:')
 229             stream.write(_format_ts(self.local if self.local is not None else 0))
 230             stream.write(',MPEGTS:')
 231             stream.write(str(self.mpegts if self.mpegts is not None else 0))
 232             stream.write('\n')
 233         if self.meta:
 234             stream.write(self.meta)
 235         stream.write('\n')
 236
 237
 238 class StyleBlock(HeaderBlock):
 239     _REGEX = re.compile(r'''(?x)
 240         STYLE[\ \t]*(?:\r\n|[\r\n])
 241         ((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
 242         (?:\r\n|[\r\n])
 243     ''')
 244
 245
 246 class RegionBlock(HeaderBlock):
 247     _REGEX = re.compile(r'''(?x)
 248         REGION[\ \t]*
 249         ((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
 250         (?:\r\n|[\r\n])
 251     ''')
 252
 253
 254 class CommentBlock(Block):
 255     _REGEX = re.compile(r'''(?x)
 256         NOTE(?:\r\n|[\ \t\r\n])
 257         ((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
 258         (?:\r\n|[\r\n])
 259     ''')
 260
 261
 262 class CueBlock(Block):
 263     """
 264     A cue block. The payload is not interpreted.
 265     """
 266
 267     _REGEX_ID = re.compile(r'((?:(?!-->)[^\r\n])+)(?:\r\n|[\r\n])')
 268     _REGEX_ARROW = re.compile(r'[ \t]+-->[ \t]+')
 269     _REGEX_SETTINGS = re.compile(r'[ \t]+((?:(?!-->)[^\r\n])+)')
 270     _REGEX_PAYLOAD = re.compile(r'[^\r\n]+(?:\r\n|[\r\n])?')
 271
 272     @classmethod
 273     def parse(cls, parser):
 274         parser = parser.child()
 275
 276         id = None
 277         m = parser.consume(cls._REGEX_ID)
 278         if m:
 279             id = m.group(1)
 280
 281         m0 = parser.consume(_REGEX_TS)
 282         if not m0:
 283             return None
 284         if not parser.consume(cls._REGEX_ARROW):
 285             return None
 286         m1 = parser.consume(_REGEX_TS)
 287         if not m1:
 288             return None
 289         m2 = parser.consume(cls._REGEX_SETTINGS)
 290         if not parser.consume(_REGEX_NL):
 291             return None
 292
 293         start = _parse_ts(m0)
 294         end = _parse_ts(m1)
 295         settings = m2.group(1) if m2 is not None else None
 296
 297         text = io.StringIO()
 298         while True:
 299             m = parser.consume(cls._REGEX_PAYLOAD)
 300             if not m:
 301                 break
 302             text.write(m.group(0))
 303
 304         parser.commit()
 305         return cls(
 306             id=id,
 307             start=start, end=end, settings=settings,
 308             text=text.getvalue()
 309         )
 310
 311     def write_into(self, stream):
 312         if self.id is not None:
 313             stream.write(self.id)
 314             stream.write('\n')
 315         stream.write(_format_ts(self.start))
 316         stream.write(' --> ')
 317         stream.write(_format_ts(self.end))
 318         if self.settings is not None:
 319             stream.write(' ')
 320             stream.write(self.settings)
 321         stream.write('\n')
 322         stream.write(self.text)
 323         stream.write('\n')
 324
 325     @property
 326     def as_json(self):
 327         return {
 328             'id': self.id,
 329             'start': self.start,
 330             'end': self.end,
 331             'text': self.text,
 332             'settings': self.settings,
 333         }
 334
 335     def __eq__(self, other):
 336         return self.as_json == other.as_json
 337
 338     @classmethod
 339     def from_json(cls, json):
 340         return cls(
 341             id=json['id'],
 342             start=json['start'],
 343             end=json['end'],
 344             text=json['text'],
 345             settings=json['settings']
 346         )
 347
 348     def hinges(self, other):
 349         if self.text != other.text:
 350             return False
 351         if self.settings != other.settings:
 352             return False
 353         return self.start <= self.end == other.start <= other.end
 354
 355
 356 def parse_fragment(frag_content):
 357     """
 358     A generator that yields (partially) parsed WebVTT blocks when given
 359     a bytes object containing the raw contents of a WebVTT file.
 360     """
 361
 362     parser = _MatchParser(frag_content.decode())
 363
 364     yield Magic.parse(parser)
 365
 366     while not parser.match(_REGEX_EOF):
 367         if parser.consume(_REGEX_BLANK):
 368             continue
 369
 370         block = RegionBlock.parse(parser)
 371         if block:
 372             yield block
 373             continue
 374         block = StyleBlock.parse(parser)
 375         if block:
 376             yield block
 377             continue
 378         block = CommentBlock.parse(parser)
 379         if block:
 380             yield block  # XXX: or skip
 381             continue
 382
 383         break
 384
 385     while not parser.match(_REGEX_EOF):
 386         if parser.consume(_REGEX_BLANK):
 387             continue
 388
 389         block = CommentBlock.parse(parser)
 390         if block:
 391             yield block  # XXX: or skip
 392             continue
 393         block = CueBlock.parse(parser)
 394         if block:
 395             yield block
 396             continue
 397
 398         raise ParseError(parser)