.. _library_random:

``random``
==========

This library provides portable random number generators and an
abstraction over the native backend Prolog compiler random number
generator if available. It includes predicates for generating random
floats and random integers in a given interval; predicates for
generating random sequences and sets; predicates for randomly selecting,
enumerating, and swapping elements from a list; predicates that succeed,
fail, or call another predicate with a given probability; and predicates
for sampling common probability distributions.

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

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

Loading
-------

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

::

   | ?- logtalk_load(random(loader)).

Testing
-------

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

::

   | ?- logtalk_load(random(tester)).

Usage
-----

The ``random`` object implements a portable random number generator and
supports multiple random number generators, using different seeds, by
defining derived objects. For example:

::

   :- object(my_random_generator_1,
       extends(random)).

       :- initialization(::reset_seed).

   :- end_object.

The ``fast_random`` object also implements a portable random number
generator but does not support deriving multiple random number
generators, which makes it a bit faster than the ``random`` object.

The ``random`` and ``fast_random`` objects manage the random number
generator seed using internal dynamic state. The predicates that update
the seed are declared as synchronized (when running on Prolog backends
that support threads). Still, care must be taken when using these
objects from multi-threaded applications, as there is no portable
solution to protect seed updates from signals and prevent inconsistent
state when threads are canceled.

The ``random`` and ``fast_random`` objects always initialize the random
generator seed to the same value, thus providing a pseudo random number
generator. The ``randomize/1`` predicate can be used to initialize the
seed with a random value. The argument should be a large positive
integer. In alternative, when using a small integer argument, discard
the first dozen random values.

The ``backend_random`` object abstracts the native backend Prolog
compiler random number generator for the basic ``random/1``,
``get_seed/1``, and ``set_seed/1`` predicates providing a portable
implementation for the remaining predicates. This makes the object
stateless, which allows reliable use from multiple threads. Consult the
backend Prolog compiler documentation for details on its random number
generator properties. Note that several of the supported backend Prolog
systems, notably B-Prolog, CxProlog, ECLiPSe, JIProlog, and Quintus
Prolog, do not provide implementations for both the ``get_seed/1`` and
``set_seed/1`` predicates and calling these predicates simply succeed
without performing any action.

All random objects (``random``, ``fast_random``, and ``backend_random``)
implement the ``sampling_protocol`` protocol. To maximize performance,
the shared implementations of the sampling predicates is defined in the
``sampling.lgt`` file that's included in the random objects. This allows
these predicates to call the basic ``random/1`` and ``random/3``
predicates as locally defined predicates.
