The Complexity of iCalendar Recurrence Rules

When I first started working on the `rrule` library, I thought parsing iCalendar recurrence rules would be straightforward. After all, how complicated could "every Tuesday" be? As it turns out, the RFC 5545 specification is far more intricate than it initially appears.

Understanding RFC 5545

The iCalendar specification defines recurrence rules (RRULE) as a way to describe repeating events. While simple cases like "FREQ=DAILY" are easy to handle, the specification includes numerous modifiers and edge cases:

• BYDAY, BYMONTH, BYWEEKNO combinations
• Leap year handling
• Timezone considerations
• UNTIL vs COUNT termination

Implementation Challenges

Edge Case Handling

Consider this rule: `FREQ=MONTHLY;BYMONTHDAY=31`. What happens in February? The specification requires that invalid dates be ignored, not shifted to the next valid date.

Performance Considerations

Generating occurrences efficiently requires careful consideration of the expansion algorithm. Naive implementations can be extremely slow for complex rules with large date ranges.

// Efficient occurrence generation
for iter.Step(&occurrence) {
    if occurrence.After(endDate) {
        break
    }
    results = append(results, occurrence)
}

Testing Strategy

The key to a reliable RRULE implementation is comprehensive testing. I created test cases that cover:

• All frequency types (DAILY, WEEKLY, MONTHLY, YEARLY)
• Boundary conditions (leap years, month ends)
• Complex modifier combinations
• Timezone edge cases

The investment in thorough testing pays dividends when other developers depend on your library for their critical calendar systems.