SRT parser with validation (!).

srt_parser_with_validation.swift

let timecodeLineParser = timecodeParser
	.skip(StartsWith(" --> ".utf8))
	.take(timecodeParser)
	.filter(<) // `start`- and `endTimeCode`s must strictly increase within a group.

// …

let srtParser = Many(srtGroupParser, separator: Newline().skip(Newline()))
	.skip(End())
	.flatMap { groups in
		groups.sequenceNumbersAndTimecodesAreInIncreasingOrder() ?
			Conditional.first(Always(groups)) :
			.second(Fail())
	}

private extension Collection where Element == SubtitleGroup {
	func sequenceNumbersAndTimecodesAreInIncreasingOrder () -> Bool {
		guard first?.sequenceNumber == 1 else { return false }

		return zip(self, self.dropFirst()) // This trick [Nate Cook](https://twitter.com/nnnnnnnn) taught me
			// never gets old. <3
			.allSatisfy { first, second in
				first.sequenceNumber + 1 == second.sequenceNumber &&
				first.endTimecode <= second.startTimecode // Adjacent groups must have _non-decreasing_ timecodes.
			}
	}
}

let sampleSRTString =
"""
1
00:00:00,540 --> 00:00:00,960
Yo-yo

2
00:00:00,960 --> 00:00:01,490
yo

3
00:00:01,520 --> 00:00:01,830
this

4
00:00:01,830 --> 00:00:02,010
is

5
00:00:02,010 --> 00:00:02,460
an

6
00:00:02,460 --> 00:00:02,760
audio

7
00:00:02,760 --> 00:00:03,240
recording
"""

dump(srtParser.parse(sampleSRTString.utf8)!)

// Outputs:
// ```
// ▿ 7 elements
//   ▿ SubtitleGroup
//     - sequenceNumber: 1
//     - startTimecode: 0.54
//     - endTimecode: 0.96
//     - substring: "Yo-yo"
//   ▿ SubtitleGroup
//     - sequenceNumber: 2
//     - startTimecode: 0.96
//     - endTimecode: 1.49
//     - substring: "yo"
//   ▿ SubtitleGroup
//     - sequenceNumber: 3
//     - startTimecode: 1.52
//     - endTimecode: 1.83
//     - substring: "this"
//   ▿ SubtitleGroup
//     - sequenceNumber: 4
//     - startTimecode: 1.83
//     - endTimecode: 2.01
//     - substring: "is"
//   ▿ SubtitleGroup
//     - sequenceNumber: 5
//     - startTimecode: 2.01
//     - endTimecode: 2.46
//     - substring: "an"
//   ▿ SubtitleGroup
//     - sequenceNumber: 6
//     - startTimecode: 2.46
//     - endTimecode: 2.76
//     - substring: "audio"
//   ▿ SubtitleGroup
//     - sequenceNumber: 7
//     - startTimecode: 2.76
//     - endTimecode: 3.24
//     - substring: "recording"
// ```