cronexpr/README.md

Golang Cron expression parser
=============================
Given a cron expression and a time stamp, you can get the next time stamp which satisfies the cron expression.

In another project, I decided to use cron expression syntax to encode scheduling information. Thus this standalone library to parse and apply time stamps to cron expressions.

The time-matching algorithm in this implementation is efficient, it avoids as much as possible to guess the next matching time stamp, a common technique seen in a number of implementations out there.

There is also a companion command-line utility to evaluate cron time expressions: <https://github.com/gorhill/cronexpr/tree/master/cronexpr> (which of course uses this library).

Implementation
--------------
The reference documentation for this implementation is found at
<https://en.wikipedia.org/wiki/Cron#CRON_expression>, which I copy/pasted here (laziness!) with modifications where this implementation differs:

    Field name     Mandatory?   Allowed values    Allowed special characters
    ----------     ----------   --------------    --------------------------
    Seconds        No           0-59              * / , -
    Minutes        Yes          0-59              * / , -
    Hours          Yes          0-23              * / , -
    Day of month   Yes          1-31              * / , - L W
    Month          Yes          1-12 or JAN-DEC   * / , -
    Day of week    Yes          0-6 or SUN-SAT    * / , - L #
    Year           No           1970–2099         * / , -

#### Asterisk ( * )
The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month.

#### Slash ( / )
Slashes describe increments of ranges. For example `3-59/15` in the minute field indicate the third minute of the hour and every 15 minutes thereafter. The form `*/...` is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field.

#### Comma ( , )
Commas are used to separate items of a list. For example, using `MON,WED,FRI` in the 5th field (day of week) means Mondays, Wednesdays and Fridays.

#### Hyphen ( - )
Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.

#### L
`L` stands for "last". When used in the day-of-week field, it allows you to specify constructs such as "the last Friday" (`5L`) of a given month. In the day-of-month field, it specifies the last day of the month.

#### W
The `W` character is allowed for the day-of-month field. This character is used to specify the business day (Monday-Friday) nearest the given day. As an example, if you were to specify `15W` as the value for the day-of-month field, the meaning is: "the nearest business day to the 15th of the month."

So, if the 15th is a Saturday, the trigger fires on Friday the 14th. If the 15th is a Sunday, the trigger fires on Monday the 16th. If the 15th is a Tuesday, then it fires on Tuesday the 15th. However if you specify `1W` as the value for day-of-month, and the 1st is a Saturday, the trigger fires on Monday the 3rd, as it does not 'jump' over the boundary of a month's days.

The `W` character can be specified only when the day-of-month is a single day, not a range or list of days.

The `W` character can also be combined with `L`, i.e. `LW` to mean "the last business day of the month."

#### Hash ( # )
`#` is allowed for the day-of-week field, and must be followed by a number between one and five. It allows you to specify constructs such as "the second Friday" of a given month.

Predefined cron expressions
---------------------------
(Copied from <https://en.wikipedia.org/wiki/Cron#Predefined_scheduling_definitions>, with text modified according to this implementation)

    Entry       Description                                                             Equivalent to
    @annually   Run once a year at midnight in the morning of January 1                 0 0 0 1 1 * *
    @yearly     Run once a year at midnight in the morning of January 1                 0 0 0 1 1 * *
    @monthly    Run once a month at midnight in the morning of the first of the month   0 0 0 1 * * *
    @weekly     Run once a week at midnight in the morning of Sunday                    0 0 0 * * 0 *
    @daily      Run once a day at midnight                                              0 0 0 * * * *
    @hourly     Run once an hour at the beginning of the hour                           0 0 * * * * *
    @reboot     Not supported

Other details
-------------
* If only six fields are present, a `0` second field is prepended, that is, `* * * * * 2013` internally become `0 * * * * * 2013`.
* If only five fields are present, a `0` second field is prepended and a wildcard year field is appended, that is, `* * * * Mon` internally become `0 * * * * Mon *`.
* Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0). This to comply with http://linux.die.net/man/5/crontab#.
* As of now, the behavior of the code is undetermined if a malformed cron expression is supplied
* By default, on a DST change, it returns the times that would have been skipped when the clock moves forward and returns only once the times that would have been repeated when the clock moves backwards. For customized behavior, see the godoc documentation.

Install
-------
    go get github.com/gorhill/cronexpr

Usage
-----
Import the library:

    import "github.com/gorhill/cronexpr"
    import "time"

Simplest way:

    nextTime := cronexpr.MustParse("0 0 29 2 *").Next(time.Now())

Assuming `time.Now()` is "2013-08-29 09:28:00", then `nextTime` will be "2016-02-29 00:00:00".

You can keep the returned Expression pointer around if you want to reuse it:

    expr := cronexpr.MustParse("0 0 29 2 *")
    nextTime := expr.Next(time.Now())
    ...
    nextTime = expr.Next(nextTime)

Use `time.IsZero()` to find out whether a valid time was returned. For example,

    cronexpr.MustParse("* * * * * 1980").Next(time.Now()).IsZero()

will return `true`, whereas

    cronexpr.MustParse("* * * * * 2050").Next(time.Now()).IsZero()

will return `false` (as of 2013-08-29...)

You may also query for `n` next time stamps:

    cronexpr.MustParse("0 0 29 2 *").NextN(time.Now(), 5)

which returns a slice of time.Time objects, containing the following time stamps (as of 2013-08-30):

    2016-02-29 00:00:00
    2020-02-29 00:00:00
    2024-02-29 00:00:00
    2028-02-29 00:00:00
    2032-02-29 00:00:00

The time zone of time values returned by `Next` and `NextN` is always the
time zone of the time value passed as argument, unless a zero time value is
returned.

API
---
<http://godoc.org/github.com/gorhill/cronexpr>

License
-------

License: pick the one which suits you best:

- GPL v3 see <https://www.gnu.org/licenses/gpl.html>
- APL v2 see <http://www.apache.org/licenses/LICENSE-2.0>