399 lines
11 KiB
Python
399 lines
11 KiB
Python
"""
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A partial parser for WebVTT segments. Interprets enough of the WebVTT stream
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to be able to assemble a single stand-alone subtitle file, suitably adjusting
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timestamps on the way, while everything else is passed through unmodified.
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Regular expressions based on the W3C WebVTT specification
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<https://www.w3.org/TR/webvtt1/>. The X-TIMESTAMP-MAP extension is described
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in RFC 8216 §3.5 <https://tools.ietf.org/html/rfc8216#section-3.5>.
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"""
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import io
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import re
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from .utils import int_or_none, timetuple_from_msec
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class _MatchParser:
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"""
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An object that maintains the current parsing position and allows
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conveniently advancing it as syntax elements are successfully parsed.
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"""
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def __init__(self, string):
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self._data = string
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self._pos = 0
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def match(self, r):
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if isinstance(r, re.Pattern):
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return r.match(self._data, self._pos)
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if isinstance(r, str):
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if self._data.startswith(r, self._pos):
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return len(r)
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return None
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raise ValueError(r)
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def advance(self, by):
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if by is None:
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amt = 0
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elif isinstance(by, re.Match):
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amt = len(by.group(0))
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elif isinstance(by, str):
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amt = len(by)
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elif isinstance(by, int):
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amt = by
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else:
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raise ValueError(by)
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self._pos += amt
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return by
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def consume(self, r):
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return self.advance(self.match(r))
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def child(self):
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return _MatchChildParser(self)
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class _MatchChildParser(_MatchParser):
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"""
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A child parser state, which advances through the same data as
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its parent, but has an independent position. This is useful when
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advancing through syntax elements we might later want to backtrack
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from.
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"""
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def __init__(self, parent):
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super().__init__(parent._data)
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self.__parent = parent
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self._pos = parent._pos
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def commit(self):
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"""
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Advance the parent state to the current position of this child state.
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"""
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self.__parent._pos = self._pos
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return self.__parent
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class ParseError(Exception):
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def __init__(self, parser):
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super().__init__("Parse error at position %u (near %r)" % (
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parser._pos, parser._data[parser._pos:parser._pos + 20]
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))
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# While the specification <https://www.w3.org/TR/webvtt1/#webvtt-timestamp>
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# prescribes that hours must be *2 or more* digits, timestamps with a single
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# digit for the hour part has been seen in the wild.
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# See https://github.com/yt-dlp/yt-dlp/issues/921
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_REGEX_TS = re.compile(r'''(?x)
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(?:([0-9]{1,}):)?
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([0-9]{2}):
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([0-9]{2})\.
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([0-9]{3})?
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''')
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_REGEX_EOF = re.compile(r'\Z')
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_REGEX_NL = re.compile(r'(?:\r\n|[\r\n]|$)')
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_REGEX_BLANK = re.compile(r'(?:\r\n|[\r\n])+')
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_REGEX_OPTIONAL_WHITESPACE = re.compile(r'[ \t]*')
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def _parse_ts(ts):
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"""
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Convert a parsed WebVTT timestamp (a re.Match obtained from _REGEX_TS)
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into an MPEG PES timestamp: a tick counter at 90 kHz resolution.
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"""
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return 90 * sum(
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int(part or 0) * mult for part, mult in zip(ts.groups(), (3600_000, 60_000, 1000, 1)))
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def _format_ts(ts):
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"""
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Convert an MPEG PES timestamp into a WebVTT timestamp.
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This will lose sub-millisecond precision.
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"""
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return '%02u:%02u:%02u.%03u' % timetuple_from_msec(int((ts + 45) // 90))
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class Block:
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"""
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An abstract WebVTT block.
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"""
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def __init__(self, **kwargs):
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for key, val in kwargs.items():
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setattr(self, key, val)
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@classmethod
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def parse(cls, parser):
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m = parser.match(cls._REGEX)
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if not m:
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return None
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parser.advance(m)
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return cls(raw=m.group(0))
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def write_into(self, stream):
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stream.write(self.raw)
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class HeaderBlock(Block):
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"""
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A WebVTT block that may only appear in the header part of the file,
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i.e. before any cue blocks.
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"""
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pass
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class Magic(HeaderBlock):
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_REGEX = re.compile(r'\ufeff?WEBVTT([ \t][^\r\n]*)?(?:\r\n|[\r\n])')
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# XXX: The X-TIMESTAMP-MAP extension is described in RFC 8216 §3.5
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# <https://tools.ietf.org/html/rfc8216#section-3.5>, but the RFC
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# doesn’t specify the exact grammar nor where in the WebVTT
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# syntax it should be placed; the below has been devised based
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# on usage in the wild
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#
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# And strictly speaking, the presence of this extension violates
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# the W3C WebVTT spec. Oh well.
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_REGEX_TSMAP = re.compile(r'X-TIMESTAMP-MAP=')
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_REGEX_TSMAP_LOCAL = re.compile(r'LOCAL:')
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_REGEX_TSMAP_MPEGTS = re.compile(r'MPEGTS:([0-9]+)')
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_REGEX_TSMAP_SEP = re.compile(r'[ \t]*,[ \t]*')
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# This was removed from the spec in the 2017 revision;
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# the last spec draft to describe this syntax element is
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# <https://www.w3.org/TR/2015/WD-webvtt1-20151208/#webvtt-metadata-header>.
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# Nevertheless, YouTube keeps serving those
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_REGEX_META = re.compile(r'(?:(?!-->)[^\r\n])+:(?:(?!-->)[^\r\n])+(?:\r\n|[\r\n])')
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@classmethod
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def __parse_tsmap(cls, parser):
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parser = parser.child()
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while True:
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m = parser.consume(cls._REGEX_TSMAP_LOCAL)
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if m:
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m = parser.consume(_REGEX_TS)
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if m is None:
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raise ParseError(parser)
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local = _parse_ts(m)
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if local is None:
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raise ParseError(parser)
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else:
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m = parser.consume(cls._REGEX_TSMAP_MPEGTS)
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if m:
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mpegts = int_or_none(m.group(1))
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if mpegts is None:
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raise ParseError(parser)
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else:
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raise ParseError(parser)
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if parser.consume(cls._REGEX_TSMAP_SEP):
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continue
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if parser.consume(_REGEX_NL):
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break
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raise ParseError(parser)
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parser.commit()
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return local, mpegts
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@classmethod
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def parse(cls, parser):
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parser = parser.child()
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m = parser.consume(cls._REGEX)
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if not m:
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raise ParseError(parser)
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extra = m.group(1)
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local, mpegts, meta = None, None, ''
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while not parser.consume(_REGEX_NL):
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if parser.consume(cls._REGEX_TSMAP):
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local, mpegts = cls.__parse_tsmap(parser)
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continue
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m = parser.consume(cls._REGEX_META)
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if m:
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meta += m.group(0)
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continue
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raise ParseError(parser)
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parser.commit()
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return cls(extra=extra, mpegts=mpegts, local=local, meta=meta)
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def write_into(self, stream):
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stream.write('WEBVTT')
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if self.extra is not None:
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stream.write(self.extra)
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stream.write('\n')
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if self.local or self.mpegts:
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stream.write('X-TIMESTAMP-MAP=LOCAL:')
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stream.write(_format_ts(self.local if self.local is not None else 0))
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stream.write(',MPEGTS:')
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stream.write(str(self.mpegts if self.mpegts is not None else 0))
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stream.write('\n')
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if self.meta:
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stream.write(self.meta)
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stream.write('\n')
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class StyleBlock(HeaderBlock):
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_REGEX = re.compile(r'''(?x)
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STYLE[\ \t]*(?:\r\n|[\r\n])
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((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
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(?:\r\n|[\r\n])
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''')
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class RegionBlock(HeaderBlock):
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_REGEX = re.compile(r'''(?x)
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REGION[\ \t]*
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((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
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(?:\r\n|[\r\n])
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''')
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class CommentBlock(Block):
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_REGEX = re.compile(r'''(?x)
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NOTE(?:\r\n|[\ \t\r\n])
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((?:(?!-->)[^\r\n])+(?:\r\n|[\r\n]))*
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(?:\r\n|[\r\n])
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''')
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class CueBlock(Block):
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"""
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A cue block. The payload is not interpreted.
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"""
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_REGEX_ID = re.compile(r'((?:(?!-->)[^\r\n])+)(?:\r\n|[\r\n])')
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_REGEX_ARROW = re.compile(r'[ \t]+-->[ \t]+')
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_REGEX_SETTINGS = re.compile(r'[ \t]+((?:(?!-->)[^\r\n])+)')
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_REGEX_PAYLOAD = re.compile(r'[^\r\n]+(?:\r\n|[\r\n])?')
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@classmethod
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def parse(cls, parser):
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parser = parser.child()
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id = None
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m = parser.consume(cls._REGEX_ID)
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if m:
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id = m.group(1)
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m0 = parser.consume(_REGEX_TS)
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if not m0:
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return None
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if not parser.consume(cls._REGEX_ARROW):
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return None
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m1 = parser.consume(_REGEX_TS)
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if not m1:
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return None
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m2 = parser.consume(cls._REGEX_SETTINGS)
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parser.consume(_REGEX_OPTIONAL_WHITESPACE)
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if not parser.consume(_REGEX_NL):
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return None
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start = _parse_ts(m0)
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end = _parse_ts(m1)
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settings = m2.group(1) if m2 is not None else None
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text = io.StringIO()
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while True:
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m = parser.consume(cls._REGEX_PAYLOAD)
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if not m:
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break
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text.write(m.group(0))
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parser.commit()
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return cls(
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id=id,
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start=start, end=end, settings=settings,
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text=text.getvalue()
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)
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def write_into(self, stream):
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if self.id is not None:
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stream.write(self.id)
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stream.write('\n')
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stream.write(_format_ts(self.start))
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stream.write(' --> ')
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stream.write(_format_ts(self.end))
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if self.settings is not None:
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stream.write(' ')
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stream.write(self.settings)
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stream.write('\n')
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stream.write(self.text)
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stream.write('\n')
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@property
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def as_json(self):
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return {
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'id': self.id,
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'start': self.start,
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'end': self.end,
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'text': self.text,
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'settings': self.settings,
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}
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def __eq__(self, other):
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return self.as_json == other.as_json
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@classmethod
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def from_json(cls, json):
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return cls(
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id=json['id'],
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start=json['start'],
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end=json['end'],
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text=json['text'],
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settings=json['settings']
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)
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def hinges(self, other):
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if self.text != other.text:
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return False
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if self.settings != other.settings:
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return False
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return self.start <= self.end == other.start <= other.end
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def parse_fragment(frag_content):
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"""
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A generator that yields (partially) parsed WebVTT blocks when given
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a bytes object containing the raw contents of a WebVTT file.
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"""
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parser = _MatchParser(frag_content.decode())
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yield Magic.parse(parser)
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while not parser.match(_REGEX_EOF):
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if parser.consume(_REGEX_BLANK):
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continue
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block = RegionBlock.parse(parser)
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if block:
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yield block
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continue
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block = StyleBlock.parse(parser)
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if block:
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yield block
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continue
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block = CommentBlock.parse(parser)
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if block:
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yield block # XXX: or skip
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continue
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break
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while not parser.match(_REGEX_EOF):
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if parser.consume(_REGEX_BLANK):
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continue
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block = CommentBlock.parse(parser)
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if block:
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yield block # XXX: or skip
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continue
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block = CueBlock.parse(parser)
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if block:
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yield block
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continue
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raise ParseError(parser)
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