.. _library_ulid:

``ulid``
========

This library implements a Universally Unique Lexicographically Sortable
Identifier (ULID) generator.

https://github.com/ulid/spec

Note that most backends provide time stamps with lower granularity than
required (i.e. seconds but not milliseconds). Also note that, per spec,
within the same millisecond, monotonic sort order is not guaranteed.

The generation of ULIDs uses the ``/dev/urandom`` random number
generator when available. This includes macOS, Linux, \*BSD, and other
POSIX operating-systems. On Windows, a pseudo-random generator is used
but randomized using the current wall time.

ULIDs can be generated as atoms, lists of characters, or lists of
character codes.

See also the ``ids`` and ``uuid`` libraries.

API documentation
-----------------

Open the
`../../docs/library_index.html#ulid <../../docs/library_index.html#ulid>`__
link in a web browser.

Loading
-------

To load all entities in this library, load the ``loader.lgt`` file:

::

   | ?- logtalk_load(ulid(loader)).

Testing
-------

To test this library predicates, load the ``tester.lgt`` file:

::

   | ?- logtalk_load(ulid(tester)).

Generating ULIDs
----------------

By default, ULIDs are generated as atoms. For example:

::

   | ?- ulid::generate(ULID).
   ULID = '01H0J31SYQXHJZWPRAKHQ6YVYH'
   yes

To generate a ULID using a list of characters representation, use
instead the ``ulid/1`` parametric object:

::

   | ?- ulid(chars)::generate(ULID).
   ULID = ['0','1','H','0','J','3','2','Y','V','5','V','S','P','K','5','P','4','5','G','G','0','9','8','8','M','2']
   yes

Similar to get a ULID using a list of character codes representation:

::

   | ?- ulid(codes)::generate(ULID).
   ULID = [48,49,72,48,74,51,52,66,54,48,55,57,54,49,67,82,70,65,67,51,67,67,86,82,48,66]
   yes

It's also possible to generate ULIDs from a given timestamp, i.e. the
number of milliseconds since the Unix epoch (00:00:00 UTC on January 1,
1970), using the ``generate/2`` predicate. The timestamp must be an
integer. For example, assuming a backend Prolog system providing a
``time_stamp/1`` predicate returning the Unix epoch in milliseconds:

::

   | ?- time_stamp(Milliseconds), ulid(atom)::generate(Seconds, ULID).
   Seconds = 1684245175344, ULID = '01H0JDBQ1GAWJF35C44Y5S97DX'
   yes

You can also use timestamp discrete components using the ``generate/8``
predicate. For example:

::

   | ?- ulid(atom)::generate(2023, 5, 17, 16, 23, 38, 591, ULID).
   ULID = '01H0N8CDAZK75C5H3BJSGS4VCQ'
   yes

To extract the timestamp from a given ULID, use the ``timestamp/2`` and
``timestamp/8`` predicates. For example:

::

   | ?- ulid(atom)::timestamp('01H0JDBQ1GAWJF35C44Y5S97DX', Milliseconds).
   Milliseconds = 1684245175344
   yes

   | ?- ulid(atom)::timestamp('01H0N8CDAZK75C5H3BJSGS4VCQ', Year, Month, Day, Hours, Minutes, Seconds, Milliseconds).
   Year = 2023, Month = 5, Day = 17, Hours = 16, Minutes = 23, Seconds = 38, Milliseconds = 591
   yes

Type-checking ULIDs
-------------------

This library also defines a ``ulid(Representation)`` type for
type-checking ULIDs. For example, with an atom containing invalid
characters:

::

   | ?- type::check(ulid(atom), '01BX5ZIKBKALTAV9OEVGEMMVRY').
   uncaught exception: domain_error(ulid,'01BX5ZIKBKALTAV9OEVGEMMVRY')
