Sketch of an SRT file parser.

srt_parser.swift

import Parsing
import Foundation

// [SubRip file format spec.](https://en.wikipedia.org/wiki/SubRip#File_format)
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
"""

let timecodeHours = Prefix(2).pipe(Int.parser(isSigned: false)).utf8
let timecodeMinutes = Prefix(2).pipe(Int.parser(isSigned: false)).utf8
let timecodeSeconds = Prefix(2).pipe(Int.parser(isSigned: false)).utf8
let timecodeMilliseconds = Prefix(3).pipe(Int.parser(isSigned: false)).utf8

let timecodeParser = timecodeHours
	.skip(StartsWith(":".utf8))
	.take(timecodeMinutes)
	.skip(StartsWith(":".utf8))
	.take(timecodeSeconds)
	.skip(StartsWith(",".utf8))
	.take(timecodeMilliseconds)
	.map { hours, minutes, seconds, milliseconds -> TimeInterval in
		let hoursInSeconds = Double(hours) * 60 * 60
		let minutesInSeconds = Double(minutes) * 60
		let millisecondsInSeconds = Double(milliseconds) * 1 / 1_000

		return hoursInSeconds + minutesInSeconds + Double(seconds) + millisecondsInSeconds
	}

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

struct SubtitleGroup {
	var sequenceNumber: Int
	var startTimecode: TimeInterval
	var endTimecode: TimeInterval
	var substring: String
}

let srtGroupParser = Int.parser(isSigned: false)
	.skip(Newline())
	.take(timecodeLineParser)
	.skip(Newline())
	.take(
		PrefixUpTo("\n\n")
			.orElse(PrefixUpTo("\r\n\r\n"))
			.orElse(Rest())
			.utf8
	)
	.map { sequenceNumber, timecodes, substring in
		SubtitleGroup(
			sequenceNumber: sequenceNumber,
			startTimecode: timecodes.0,
			endTimecode: timecodes.1,
			substring: String(substring)
		)
	}

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())
			.allSatisfy { first, second in
				first.sequenceNumber + 1 == second.sequenceNumber &&
				first.endTimecode <= second.startTimecode // Adjacent groups must have _non-decreasing_ timecodes.
			}
	}
}

dump(srtParser.parse(sampleSRTString.utf8))