class TZInfo::DataSources::PosixTimeZoneParser

def parse(tz_string)
  raise InvalidPosixTimeZone unless tz_string.kind_of?(String)
  return nil if tz_string.empty?
  s = StringScanner.new(tz_string)
  check_scan(s, /([^-+,\d<][^-+,\d]*) | <([^>]+)>/x)
  std_abbrev = @string_deduper.dedupe(RubyCoreSupport.untaint(s[1] || s[2]))
  check_scan(s, /([-+]?\d+)(?::(\d+)(?::(\d+))?)?/)
  std_offset = get_offset_from_hms(s[1], s[2], s[3])
  if s.scan(/([^-+,\d<][^-+,\d]*) | <([^>]+)>/x)
    dst_abbrev = @string_deduper.dedupe(RubyCoreSupport.untaint(s[1] || s[2]))
    if s.scan(/([-+]?\d+)(?::(\d+)(?::(\d+))?)?/)
      dst_offset = get_offset_from_hms(s[1], s[2], s[3])
    else
      # POSIX is negative for ahead of UTC.
      dst_offset = std_offset - 3600
    end
    dst_difference = std_offset - dst_offset
    start_rule = parse_rule(s, 'start')
    end_rule = parse_rule(s, 'end')
    raise InvalidPosixTimeZone, "Expected the end of a POSIX-style time zone string but found '#{s.rest}'." if s.rest?
    if start_rule.is_always_first_day_of_year? && start_rule.transition_at == 0 &&
         end_rule.is_always_last_day_of_year? && end_rule.transition_at == 86400 + dst_difference
      # Constant daylight savings time.
      # POSIX is negative for ahead of UTC.
      TimezoneOffset.new(-std_offset, dst_difference, dst_abbrev)
    else
      AnnualRules.new(
        TimezoneOffset.new(-std_offset, 0, std_abbrev),
        TimezoneOffset.new(-std_offset, dst_difference, dst_abbrev),
        start_rule,
        end_rule)
    end
  elsif !s.rest?
    # Constant standard time.
    # POSIX is negative for ahead of UTC.
    TimezoneOffset.new(-std_offset, 0, std_abbrev)
  else
    raise InvalidPosixTimeZone, "Expected the end of a POSIX-style time zone string but found '#{s.rest}'."
  end
end