guide.txt

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

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

:Author:        Jon Parise
:Contact:       [e-mail]
:Date:          $Date: 2004/01/30 08:55:17 $
:Revision:      $Revision: 1.1 $

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

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

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

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

.. _Finite State Machine: http://en.wikipedia.org/wiki/Finite_state_machine
.. _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 it a transition
from the current state using that symbol is found, the current state will be
updated 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 container.  The payload will be passed to all of
the callback functions, allowing you to provide 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``
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``, ``END``) and
two symbols (``FIRST``, ``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 remain in the ``START`` state.

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