=================
 The FSM Package
=================

--------------------
 User Documentation
--------------------

:Author:        Jon Parise
:Contact:       jon@php.net
:Date:          $Date: 2008/02/10 22:42:16 $
:Revision:      $Revision: 1.3 $

.. contents:: Contents
.. section-numbering::

About the FSM Package
=====================

The `FSM Package`_ implements a `Finite State Machine`_.  In addition to
maintaining state, this FSM also manages a user-defined payload, therefore
effectively making the machine a `Pushdown Automaton`_ (a finite state machine
with memory).

This code is based largely on Noah Spurrier's excellent `FSM Python class`_.

.. _FSM Package: http://pear.php.net/package/FSM
.. _Finite State Machine: http://wikipedia.org/wiki/Finite_state_machine
.. _Pushdown Automaton: http://wikipedia.org/wiki/Push-down_automata
.. _FSM Python class: http://www.noah.org/python/FSM/


Building a Finite State Machine
===============================

The first step in building a Finite State Machine involves listing the finite
set of states.  Then, all of the permissible transitions between these states
must be defined.  A symbol and an optional callback function are associated
with each transition.  The input processing routine will attempt to match its
current symbol against the list of registered transitions.  If a transition
from the current state using that symbol is found, the machine will move to
the new state specified by the transition and, if one has been specified, the
associated callback function will be invoked.

Creating a New FSM Object
-------------------------
Start by including the FSM package in your script::

    require 'FSM.php';

When constructing a new FSM object, you must specify the machine's initial
state and provide a payload variable.  The payload will be passed to all of
the callback functions, supplying them with state information without
(ab)using global variables.

In this example, we pass an array representing a stack as the payload.  The
machine's initial state is set to ``START``.

::

    $stack = array();
    $fsm = new FSM('START', $stack);

Defining Transitions
--------------------
We'll need to define some transitions in order to make our machine useful.
Let's assume our machine has two additional states: ``MIDDLE`` and ``END``.
Here's how we would define transitions to move us from ``START`` to ``MIDDLE``
and from ``MIDDLE`` to ``END``::

    function FirstCallback($symbol, $payload)
    {
        echo "First Transition\n";
    }

    function SecondCallback($symbol, $payload)
    {
        echo "Second Transition\n";
    }

    $fsm->addTransition('FIRST', 'START', 'MIDDLE', 'FirstCallback');
    $fsm->addTransition('SECOND', 'MIDDLE', 'END', 'SecondCallback');

Our machine is now aware of three states (``START``, ``MIDDLE``, and ``END``)
and two symbols (``FIRST`` and ``SECOND``).  Two transitions (``START`` to
``MIDDLE`` and ``MIDDLE`` to ``END``) have been defined and associated with
callbacks.  The following code will process the symbols ``FIRST`` and
``SECOND`` and move us from our initial state (``START``) through the
``MIDDLE`` state to the ``END`` state.

::

    $fsm->process('FIRST');
    $fsm->process('SECOND');

The processing routine will invoke our two callbacks along the way, as well,
resulting in the following being printed::

    First Transition
    Second Transition

Setting Default Transitions
---------------------------
Now we'll set up a default transition.  This transition will be used whenever
the processing routine cannot find a better match for the current state and
symbol.  For our example, we'll consider this an error and print a warning for
the user.

::

    function ErrorCallback($symbol, $payload)
    {
        echo "This symbol does not compute: $symbol\n";
    }

    $fsm->setDefaultTransition('START', 'ErrorCallback');

Now let's process our symbols in an unexcepted order::

    $fsm->process('SECOND');
    $fsm->process('FIRST');

Because the ``SECOND`` transition doesn't specify ``START`` as its initial
state, the default transition will be used and the error callback will be
invoked.  The ``FIRST`` transition will work as expected, however, because the
machine will still be in the ``START`` state.

Plotting a State Machine
========================

The FSM package optionally supports the ability to plot a machine's states
with the help of the `Image_GraphViz`_ package.  Doing so is as simple as
creating a new ``FSM_GraphViz`` object using an existing state machine
instance and then exporting the graph.

::

    require_once 'FSM/GraphViz.php';
    $converter = new FSM_GraphViz($fsm);
    $graph = $converter->export();

The resulting graph object is an ``Image_GraphViz`` instance.  To export the
graph as an image, use the ``image()`` method::

    $graph->image('png');

This will produce an image similar to the following:

.. figure:: graphviz.png
   :alt: Example State Machine Plot

Consult the `Image_GraphViz documentation`_ for additional usage information.

.. _Image_GraphViz: http://pear.php.net/package/Image_GraphViz
.. _Image_GraphViz documentation: http://pear.php.net/package/Image_GraphViz/docs

Development and Support
=======================

Reporting Problems and Suggestions
----------------------------------
If you run into a problem or would like to make a suggestion, please use the
`PEAR Bug Tracker`_.  Feel free to contact me directly for other issues, but
please try to use the bug tracker whenever possible so that others in the
community will benefit from your feedback and my responses.

- `Open Bugs`_
- `Report a New Bug`_

.. _PEAR Bug Tracker: http://pear.php.net/bugs/
.. _Open Bugs: http://pear.php.net/package/FSM/bugs
.. _Report a New Bug: http://pear.php.net/bugs/report.php?package=FSM

Coming Soon
-----------
This section contains a list of "todo" items that will hopefully be addressed
in future releases.

- *No items at this time.*

If you have feature suggestions, please submit them using the `PEAR Bug
Tracker`_.

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