VirtualTime is a time matching class for Crystal. It is used for complex and flexible matching of dates and times, primarily for calendar, scheduling, and reminding purposes.
For example:
vt = VirtualTime.new
vt.year = 2020..2030
vt.day = -8..-1
vt.day_of_week = [6,7]
vt.hour = 12..16
vt.minute = ->( val : Int32) { true }
time = Time.local
vt.matches? time
That VirtualTime
instance will match any Time
that is:
- Between years 2020 and 2030, inclusively
- In the last 7 days of each/any month (day = -8..-1; negative values count from the end)
- Falling on Saturday or Sunday (day_of_week = 6 or 7)
- Between hours noon and 4PM (hour = 12..16)
- And any minute (since example block always returns true)
As a more advanced feature, it is also possible to match VirtualTime
s with other
VirtualTime
s. That is documented further below.
Installation
Add the following to your application's "shard.yml":
dependencies:
virtualtime:
github: crystallabs/virtualtime
version: ~> 1.0
And run shards install
or just shards
.
Introduction
Think of class VirtualTime
as of a very flexible time specification against which
Crystal's Time
instances can be matched.
Crystal's struct Time
has all its fields (year, month, day, hour, minute, second, nanosecond) set
to a specific numeric value. Even if some of its fields aren't required in the constructor,
internally they still get initialized to 0, 1, or other suitable value.
As such, Time
instances always represent specific dates and times ("materialized" dates and times).
On the other hand, VirtualTime
s do not have to represent any specific points in time (although they can
be set or converted so that they do); they are primarily intended for conveniently matching broader sets of
values. VirtualTime instances contain the following properties:
- Year (0..9999)
- Month (1..12)
- Day (1..31)
- Week number of year (0..53)
- Day of week (1..7, Monday == 1)
- Day of year (1..366)
- Hour (0..23)
- Minute (0..59)
- Second (0..59)
- Millisecond (0..999)
- Nanosecond (0..999_999_999)
And each of these properties can have a value of the following types:
- Nil (no setting), to always match as a default value
- Boolean, to always specifically match (
true
) or fail (false
) - Int32, to match a specific value such as 5, 12, 2023, -1, or -5
- Array of Int32s, such as [1,2,10,-1] to match any value in list
- Range of Int32..Int32, such as
10..20
to match any value in range - Range with step, e.g.
day: (10..20).step(2)
, to match all even days between 10th and 20th - Proc, to match a value if the return value from calling a proc is
true
All properties (that are specified, i.e. not nil) must match for the match to succeed.
This VirtualTime
object can then be used for matching arbitrary Time
s against it, to check if
they match.
Matching Times
Once VirtualTime
is created, it can be used for matching Time
objects.
An example showing use of different values was given in the introduction:
vt = VirtualTime.new
vt.year = 2020..2030
vt.day = -8..-1
vt.day_of_week = [6,7]
vt.hour = 12..16
vt.minute = ->( val : Int32) { true }
time = Time.local
vt.matches? time
The syntax allows for specifying simple but functionally intricate rules, of which just some of them are:
day=-1 -- matches last day of month (28th, 29th, 30th, or 31st of particular month)
day_of_week=6, day=24..31 -- matches last Saturday in month
day_of_week=1..5, day=-1 -- matches last day of month if it is a workday
Another example:
vt = VirtualTime.new
vt.month = 3 # Month of March
vt.day = [1,-1] # First and last day of every month
vt.hour = (10..20)
vt.minute = (0..59).step(2) # Every other (even) minute in an hour
vt.second = true # Unconditional match
vt.millisecond = ->( val : Int32) { true } # Will match any value as block returns true
Matching other VirtualTimes
In addition to matching Time
structs, VirtualTimes can match other VirtualTimes.
For example, if you had a VirtualTime
that matches every March 15:
vt = VirtualTime.new month: 3, day: 15
And you wanted to check whether this was scheduled on any day in the first 6 months of the year, you could do:
vt = VirtualTime.new month: 3, day: 15
vt2 = VirtualTime.new month: 1..6
vt.matches?(vt2) # ==> true
When matching VirtualTime
s to VirtualTime
s, comparisons between fields' values
which are both a Proc
is not supported and will throw ArgumentError
in runtime.
Field Values
As can be seen above, fields can have some interesting values, such as negative numbers.
Here is a list of all non-obvious values that are supported:
- Negative integer values
Negative integer values count from the end of the range, if the max / wrap-around value is
specified. Typical end values are 7, 12, 30/31, 365/366, 23, 59, and 999, and virtualtime
implicitly knows which one to apply in every case.
For example, a day of -1
would always match the last day of the month, be that 28th, 29th,
30th, or 31st in a particular case.
If the wrap-around value is not specified, negative values are not converted to positive and they enter matching as-is.
- Week numbers
Another interesting case is week number, which is calculated as number of Mondays in the year. The first Monday in a year starts week number 1, but not every year starts on Monday so up to the first 3 days of new year can still technically belong to the last week of the previous year.
That means it is possible for this field to have values between 0 and 53. Value 53 indicates a week that has started in one year (53rd Monday seen in a year), but up to 3 of its days will overflow into the new year.
Similarly, a value 0 matches up to the first 3 days (which inevitably must be Friday, Saturday, and/or Sunday) of the new year that belong to the week started in the previous year.
- Range values
Crystal allows one to define Range
s that have end
value smaller than begin
.
Such objects will simply not contain any elements.
Because creating such ranges is allowed, VirtualTime detects such cases and creates copies of objects with values converted to positive and in the correct order.
In other words, if you specify a range of say, day: (10..-7).step(2)
, this will properly
match every other day from 10th to a day 7 days before the end of the month.
- Days in month and year
When matching VirtualTime
s to other VirtualTime
s, helper functions days_in_month
and
days_in_year
return nil
. As a consequence, matching is performed without converting
negative values to positive ones.
This choice was made because it is only possible to know the exact values if/when year
and month
happen to be integers. If they are a value of any other type (e.g. a range,
2023..2030
), it is ambiguous or indeterminable what the value should be.
Materialization
VirtualTimes sometimes need to be "materialized" for
the purpose of display, calculation, comparison, or conversion. An obvious such case
which happens implicitly is when to_time()
is invoked on a VT, because a Time object
must have all of its fields set.
Because VirtualTimes can be very broadly defined, often times there are many equal choices to which VTs can be materialized. To avoid the problem of too many choices, materialization takes as argument a time hint, and the materialized time will be as close as possible to that time.
For example:
vt= VirtualTime.new
# These fields will be used as-is since they have a value:
vt.year= 2018
vt.day= 15
vt.hour= 0
# While others (which are nil) will have their value inserted from the "hint" object:
hint= Time.local # 2023-12-09 12:56:26.837441132 +01:00 Local
vt.materialize(hint).to_tuple # ==> {2018, 12, 15, nil, nil, nil, 0, 56, 26, nil, 837441132, nil}
Time Zones
VirtualTime
is timezone-agnostic. Values are compared against VirtualTime
values directly.
However, VirtualTime
has property #location
which, if set and different than the other
object's #location
, will cause the object to be duplicated and have its time converted to
VirtualTime
's location before matching.
vt = VirtualTime.new
vt.hour = 16..20
t = Time.local 2023, 10, 10, hour: 0, location: Time::Location.load("Europe/Berlin")
vt.matches?(t) # ==> nil, because 00 hours is not between 16 and 20
vt.location = Time::Location.load("America/New_York")
vt.matches?(t) # ==> true, because time instant 0 hours converted to NY time (-6) is 18 hours
When comparing VirtualTime
s to VirtualTime
s, comparisons between objects with different
location
values are not supported and will throw ArgumentError
in runtime.
Considerations
Alias Virtual
is defined as:
alias Virtual = Nil | Bool | Int32 |
Array(Int32) | Range(Int32, Int32) | Steppable::StepIterator(Int32, Int32, Int32) |
VirtualProc
Array
, Range
, and Steppable::StepIterator
are mentioned explicitly instead of just
being replaced with Enumerable(Int32)
due to a bug in Crystal
(https://github.com/crystal-lang/crystal/issues/14047).
Another, related consideration is related to matching fields that contain these enumerable types.
Some enumerables change internal state when they are used, so in the matching function accepting
Enumerable
data types they are #dup
-ed before use, to make sure the original objects
remain intact.
An alternative approach, to avoid duplicating objects in every case, would be to define more
specific function overloads for matching Array
s, Range
s, and StepIterator
s, and only have
the Enumerable
function overload as a fallback, unless a more specific match is found.
Currently the first option for doing all matching via Enumerable
s is used because it
results is a smaller amount of active code to maintain. But the code for other types exists;
it is just disabled.
Please open an issue on the project to discuss if you would advise differently.
Tests
Run crystal spec
or just crystal s
.
API Documentation
Run crystal docs
or crystal do
and firefox ./docs/index.html
.
Other Projects
List of interesting or similar projects in no particular order:
- https://dianne.skoll.ca/projects/remind/ - a sophisticated calendar and alarm program