Parse full RRULEs to pass on to the rrule gem

rrule_parse.rb

# frozen_string_literal: true
# The rrule gem parses only the recurring rules of the RRULE specification.
# That is, the part that comes after "RRULE:" on the last line of an RRULE. To
# include the DTSTART, and EXDATE as options to the rrule gem, let's add our
# own interface. We remove the "RRULE:" and parse other rules and send them to
# the gem with the desired structure.
#
# Example RRULE which is now parseable as a string:
#
# EXDATE:19960402T010000Z,19960403T010000Z,19960404T010000Z
# DTSTART:19960401T000000Z
# RRULE:FREQ=WEEKLY;BYHOUR=2;BYMINUTE=0;BYDAY=WE
#
# See the RRULE specification:
# https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html
module RRule::Parse
  refine RRule.singleton_class do
    def parse(rrule, **options)
      lines = rrule.split("\n")
      dt_start = nil
      tz_id = nil
      ex_date = nil
      rule = nil
      lines.each do |line|
        case
        when line.match?(/^DTSTART/)
          dt_start = Time.new(line.match(/:(.*)$/)[1]).utc
          tz_id_match = line.match(/TZID=(.*):/)
          tz_id = tz_id_match[1] if tz_id_match
        when line.match?(/^EXDATE/)
          ex_date = line.match(/:(.*)$/)[1].split(',').map { |time| Time.new(time).utc }
        when line.match?(/^RRULE/)
          rule = line.sub(/^RRULE*:/, '')
        when line.include?('FREQ')
          rule = line
        end
      end
      raise RRule::InvalidRRule unless rule

      parsed_options = {
        dtstart: dt_start,
        tzid: tz_id,
        exdate: ex_date
      }.compact
      RRule::Rule.new(rule, **parsed_options.merge(options))
    end
  end
end