Shift times in a .srt file

shift_srt.rb

#!/usr/bin/env ruby
# frozen_string_literal: false

# Shift times in given .srt file
#
# ./shift_srt.rb [SRT_FILEPATH] [MSECS_TO_SHIFT]
#
# ex: ./shift_srt.rb some_srt_file.srt 420
#
# created by meinside@duck.com
# last update: 2021.07.21.

TIMELINE_REGEX = /(\d\d):(\d\d):(\d\d),(\d\d\d) --> (\d\d):(\d\d):(\d\d),(\d\d\d)/.freeze

def msecs_to_str(msecs)
  s = (msecs / 1000).to_i

  h = (s / 3600).to_i
  m = (s / 60).to_i
  s = s % 60
  ms = msecs % 1000

  format('%<h>02d:%<m>02d:%<s>02d,%<ms>03d', { h: h, m: m, s: s, ms: ms })
end

# shift all occurrences of time with given delta (in msecs)
def shift_all(original, delta)
  original.gsub(TIMELINE_REGEX) do |_|
    from_hours = Regexp.last_match(1).to_i
    from_minutes = Regexp.last_match(2).to_i
    from_seconds = Regexp.last_match(3).to_i
    from_msecs = Regexp.last_match(4).to_i
    from = (from_hours * 60 * 60 + from_minutes * 60 + from_seconds) * 1000 + from_msecs

    to_hours = Regexp.last_match(5).to_i
    to_minutes = Regexp.last_match(6).to_i
    to_seconds = Regexp.last_match(7).to_i
    to_msecs = Regexp.last_match(8).to_i
    to = (to_hours * 60 * 60 + to_minutes * 60 + to_seconds) * 1000 + to_msecs

    # shift
    from += delta
    to += delta

    "#{msecs_to_str(from)} --> #{msecs_to_str(to)}"
  end
end

if __FILE__ == $PROGRAM_NAME
  if ARGV.count < 2
    puts "* usage: #{$PROGRAM_NAME} [SRT_FILEPATH] [MSECS_TO_SHIFT]"
    exit 1
  end

  filepath = File.expand_path(ARGV[0])

  if File.exist? filepath
    msecs = ARGV[1].to_i

    puts "> shifting #{msecs} msecs: #{filepath}"

    original = File.open(filepath, 'r').read
    converted = shift_all(original, msecs)

    converted_filepath = filepath.gsub('.srt', '_shifted.srt')
    File.open(converted_filepath, 'w') do |file|
      file.write converted
    end
    puts "> saved to: #{converted_filepath}"
  else
    puts "* file not found: #{filepath}"
  end

end