RRule.rs

A pure and efficient Rust implementation of recurrence rules as defined in the iCalendar RFC.

## Specification

This crate follows the [iCalendar (RFC-5545) specification][ical_spec] for the "Recurrence Rule".
The Recurrence Rule spec corresponds to the `RRule` object in this crate.
In addition, it allows for adding the ["DTSTART" property][dtstart_property] separated by a newline.

The crate allows for a "BYEASTER" filter. But this is opt-in with the feature flag `"by-easter"`.

### RRuleSet

`RRuleSet` allows for a combination for `RRule`s and some other properties.

- List of [RRules](https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html):
  Allows multiple RRules to be combined. (Union, `A ∪ B`)
- List of [RDates](https://icalendar.org/iCalendar-RFC-5545/3-8-5-2-recurrence-date-times.html):
  A list of datetime combinations to always include. (Union, `A ∪ B`)
- List of [ExRule](https://datatracker.ietf.org/doc/html/rfc2445#section-4.8.5.2) (see note below):
  Allows of RRules that are removed from the results. (Complement `A \ B` or `A - B`)
- List of [ExDate](https://icalendar.org/iCalendar-RFC-5545/3-8-5-1-exception-date-times.html):
  A list of datetime combinations to always exclude. (Complement `A \ B` or `A - B`)

Note: "EXRULE" was originally part of [RFC 2445](https://datatracker.ietf.org/doc/html/rfc2445#section-4.8.5.2),
[RFC 5545][ical_spec] obsoletes this specification.
But "EXRULE" works exactly the same als "RRULE" except that it excludes dates. You can enable "EXRULE" by enabling the "exrule" feature flag which is disabled by default.

If you notice that the implementation differs from the specifications above, please open an issue.

## Library Usage

```rust
use rrule::RRuleSet;

// RRule that starts 2012.02.01 and occurs daily for 3 days.
let rrule: RRuleSet = "DTSTART:20120201T093000Z\nRRULE:FREQ=DAILY;COUNT=3".parse().unwrap();

// Set hard limit in case of infinitely recurring rules.
let limit = 100;
// Get all recurrences of the rrule
let result = rrule.all(limit);
assert_eq!(result.dates.len(), 3);
```

See more examples at [docs.rs](https://docs.rs/rrule)

## Command Line Tool Usage

Install the command line tool with:

```bash
cargo install rrule --features="cli-tool"
```

Then run it with:

```bash
rrule "DTSTART:20120201T093000Z\nRRULE:FREQ=DAILY;COUNT=3"
```

## Security

You should read the [security docs](https://github.com/fmeringdal/rust-rrule/blob/main/SECURITY.md) if you use arbitrary inputs from users for constructing the recurrence rules.

## Limitation and limits

All dates are limited to the range or years +/-262_000[^1] because of [Chrono][chrono] limits.
See [Chrono's limits for more info](https://github.com/chronotope/chrono#limitations).

Supported timezones are limited to by the timezones that [Chrono-Tz][chrono-tz] supports.
This is equivalent to the IANA database.
See [Chrono-Tz's limits for more info](https://github.com/chronotope/chrono-tz/#limiting-the-timezone-table-to-zones-of-interest).

### Validation Limits

Because the specifications do give a lot of flexibilities this can be [abused very easily](#Security).
In order to prevent most of the abuse we've imposed arbitrary limitation when on the `RRuleSet::all`
method. The validation limits aren't enforced for the `RRuleSet::all_unchecked` method or when
using the `Iterator` api directly.

Limitations:
| Description | Arbitrary Limit | Crate Limit |
|----------------------------------|-----------------------|-----------------------------|
| Year range | -10_000..=10_000 | -262_000..=262_000 (Chrono) |
| Max interval with freq Yearly | 10_000 (10000 years) | 65_535 (u16::MAX) |
| Max interval with freq Monthly | 1_000 (~83 years) | 65_535 (u16::MAX) |
| Max interval with freq Weekly | 1_000 (~19 years) | 65_535 (u16::MAX) |
| Max interval with freq Daily | 10_000 (~27 years) | 65_535 (u16::MAX) |
| Max interval with freq Hourly | 10_000 (~416 days) | 65_535 (u16::MAX) |
| Max interval with freq Minutely | 10_000 (~7 days) | 65_535 (u16::MAX) |
| Max interval with freq Secondly | 50_000 (~13 hours) | 65_535 (u16::MAX) |
| Iteration limit | 100_000 | 4_294_967_295 (u32::MAX) |

By default, the "Arbitrary Limit" is used. If you instead want to use the "Crate Limit".
Make sure you [understand the risks that come with this](#safety).

## Inspired by

- [python-dateutil library](http://labix.org/python-dateutil/)
- [rrule.js](https://github.com/jakubroztocil/rrule)

## License

The code in this project is licensed under the [MIT](LICENSE-MIT) or [Apache 2.0](LICENSE-APACHE) license.

All contributions to this project will be similarly licensed.

[chrono]: https://github.com/chronotope/chrono
[chrono-tz]: https://github.com/chronotope/chrono-tz/
[ical_spec]: https://icalendar.org/iCalendar-RFC-5545/3-3-10-recurrence-rule.html
[dtstart_property]: https://icalendar.org/iCalendar-RFC-5545/3-8-2-4-date-time-start.html

[^1]: See [validation limits](#validation_limits) sections more info.
