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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 | _REGEX_OPTIONAL_WHITESPACE = re.compile(r'[ \t]*') | |
99 | ||
100 | ||
101 | def _parse_ts(ts): | |
102 | """ | |
103 | Convert a parsed WebVTT timestamp (a re.Match obtained from _REGEX_TS) | |
104 | into an MPEG PES timestamp: a tick counter at 90 kHz resolution. | |
105 | """ | |
106 | return 90 * sum( | |
107 | int(part or 0) * mult for part, mult in zip(ts.groups(), (3600_000, 60_000, 1000, 1))) | |
108 | ||
109 | ||
110 | def _format_ts(ts): | |
111 | """ | |
112 | Convert an MPEG PES timestamp into a WebVTT timestamp. | |
113 | This will lose sub-millisecond precision. | |
114 | """ | |
115 | return '%02u:%02u:%02u.%03u' % timetuple_from_msec(int((ts + 45) // 90)) | |
116 | ||
117 | ||
118 | class Block: | |
119 | """ | |
120 | An abstract WebVTT block. | |
121 | """ | |
122 | ||
123 | def __init__(self, **kwargs): | |
124 | for key, val in kwargs.items(): | |
125 | setattr(self, key, val) | |
126 | ||
127 | @classmethod | |
128 | def parse(cls, parser): | |
129 | m = parser.match(cls._REGEX) | |
130 | if not m: | |
131 | return None | |
132 | parser.advance(m) | |
133 | return cls(raw=m.group(0)) | |
134 | ||
135 | def write_into(self, stream): | |
136 | stream.write(self.raw) | |
137 | ||
138 | ||
139 | class HeaderBlock(Block): | |
140 | """ | |
141 | A WebVTT block that may only appear in the header part of the file, | |
142 | i.e. before any cue blocks. | |
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 | parser.consume(_REGEX_OPTIONAL_WHITESPACE) | |
291 | if not parser.consume(_REGEX_NL): | |
292 | return None | |
293 | ||
294 | start = _parse_ts(m0) | |
295 | end = _parse_ts(m1) | |
296 | settings = m2.group(1) if m2 is not None else None | |
297 | ||
298 | text = io.StringIO() | |
299 | while True: | |
300 | m = parser.consume(cls._REGEX_PAYLOAD) | |
301 | if not m: | |
302 | break | |
303 | text.write(m.group(0)) | |
304 | ||
305 | parser.commit() | |
306 | return cls( | |
307 | id=id, | |
308 | start=start, end=end, settings=settings, | |
309 | text=text.getvalue() | |
310 | ) | |
311 | ||
312 | def write_into(self, stream): | |
313 | if self.id is not None: | |
314 | stream.write(self.id) | |
315 | stream.write('\n') | |
316 | stream.write(_format_ts(self.start)) | |
317 | stream.write(' --> ') | |
318 | stream.write(_format_ts(self.end)) | |
319 | if self.settings is not None: | |
320 | stream.write(' ') | |
321 | stream.write(self.settings) | |
322 | stream.write('\n') | |
323 | stream.write(self.text) | |
324 | stream.write('\n') | |
325 | ||
326 | @property | |
327 | def as_json(self): | |
328 | return { | |
329 | 'id': self.id, | |
330 | 'start': self.start, | |
331 | 'end': self.end, | |
332 | 'text': self.text, | |
333 | 'settings': self.settings, | |
334 | } | |
335 | ||
336 | def __eq__(self, other): | |
337 | return self.as_json == other.as_json | |
338 | ||
339 | @classmethod | |
340 | def from_json(cls, json): | |
341 | return cls( | |
342 | id=json['id'], | |
343 | start=json['start'], | |
344 | end=json['end'], | |
345 | text=json['text'], | |
346 | settings=json['settings'] | |
347 | ) | |
348 | ||
349 | def hinges(self, other): | |
350 | if self.text != other.text: | |
351 | return False | |
352 | if self.settings != other.settings: | |
353 | return False | |
354 | return self.start <= self.end == other.start <= other.end | |
355 | ||
356 | ||
357 | def parse_fragment(frag_content): | |
358 | """ | |
359 | A generator that yields (partially) parsed WebVTT blocks when given | |
360 | a bytes object containing the raw contents of a WebVTT file. | |
361 | """ | |
362 | ||
363 | parser = _MatchParser(frag_content.decode()) | |
364 | ||
365 | yield Magic.parse(parser) | |
366 | ||
367 | while not parser.match(_REGEX_EOF): | |
368 | if parser.consume(_REGEX_BLANK): | |
369 | continue | |
370 | ||
371 | block = RegionBlock.parse(parser) | |
372 | if block: | |
373 | yield block | |
374 | continue | |
375 | block = StyleBlock.parse(parser) | |
376 | if block: | |
377 | yield block | |
378 | continue | |
379 | block = CommentBlock.parse(parser) | |
380 | if block: | |
381 | yield block # XXX: or skip | |
382 | continue | |
383 | ||
384 | break | |
385 | ||
386 | while not parser.match(_REGEX_EOF): | |
387 | if parser.consume(_REGEX_BLANK): | |
388 | continue | |
389 | ||
390 | block = CommentBlock.parse(parser) | |
391 | if block: | |
392 | yield block # XXX: or skip | |
393 | continue | |
394 | block = CueBlock.parse(parser) | |
395 | if block: | |
396 | yield block | |
397 | continue | |
398 | ||
399 | raise ParseError(parser) |