.. _Minimization:

.. raw:: html

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.. |---| unicode:: U+2014  .. em dash, trimming surrounding whitespace
   :trim:



.. odsalink:: DataStructures/FLA/FLA.css

.. odsalink:: AV/VisFormalLang/FA/Minimization1CON.css

.. odsalink:: AV/VisFormalLang/FA/Minimization2CON.css
.. This file is part of the OpenDSA eTextbook project. See
.. http://algoviz.org/OpenDSA for more details.
.. Copyright (c) 2012-2016 by the OpenDSA Project Contributors, and
.. distributed under an MIT open source license.

.. avmetadata::
   :author: Susan Rodger and Cliff Shaffer
   :requires: Deterministic Finite Automata
   :satisfies: DFA minimization
   :topic: Finite Automata


Minimizing the Number of States in a DFA
========================================

Minimizing the Number of States in a DFA
----------------------------------------

Why do we need to do this?

If you have an NFA with :math:`n` states, what is the maximum number 
of states in the equivalent DFA created? :math:`2^n` 

**Algorithm**

Identify states that are indistinguishable

* These states form a new state in the minimized machine.

**Definition**: Two states :math:`p` and :math:`q` are
indistinquishable if for all :math:`w \in \Sigma^*`

.. math::

   \begin{eqnarray*}
   \delta^*(q, w) \in F &\Rightarrow& \delta^*(p, w) \in F\\
   \delta^*(p, w) \not\in F &\Rightarrow& \delta^*(q, w) \not\in F\\
   \end{eqnarray*}

**Definition**: Two states :math:`p` and :math:`q` are distinquishable
if :math:`\exists\ w \in \Sigma^*` such that

.. math::

   \begin{eqnarray*}
   \delta^*(q, w)\in F &\Rightarrow& \delta^*(p, w) \not\in F\ \mathrm{OR}\\
   \delta^*(q, w) \not\in F &\Rightarrow& \delta^*(p, w) \in F
   \end{eqnarray*}

All that an acceptor cares about is accepting or rejecting strings.

Select any pair of states, :math:`p` and :math:`q`.

* If, in either case, we accept/reject the exact same set set of
  strings, then there is no difference between them.
* So, we can combine them.

Distinguishability is an equivalence relation.
We are trying to break the set of all states into subsets that are
equivances.

**Example**:

.. odsafig:: Images/stmindfa1s.png
   :width: 500
   :align: center
   :capalign: justify
   :figwidth: 90%
   :alt: Minimization 1

   Minimization 1

Look at A on :math:`a`, :math:`ab`:

* :math:`a` goes to a non-final state, :math:`ab` goes to a final
  state
  
Look at F on :math:`a`, :math:`ab`:

* :math:`a` goes to a non-final state, :math:`ab` goes to a non-final
  state
  
Look at D on :math:`a`, :math:`ab`:

* :math:`a` goes to a non-final state, :math:`ab` goes to a final
  state

So, we know that F must be in a different equivalence subset than A
and D.
A and D might or might not end up being equivalent, we don't have
enough evidence yet to decide.

Unfortunately, this is not obviously an algorithm, since we cannot
actually test on all input strings.

* But remember the definition for :math:`\delta^*(p, w)`.
  Look at things this way:
  It is telling us that we don't care about the prior history before
  we got to the current state with whatever remains of the input.
* So, we can look at each transition out of two subsets being
  considered, and verify that they lead to "equivalent" places (which
  is not the same as leading to the same state in the
  non-minimized machine).
* We will start with the maximum possible joining of states as a
  potential equivalence class, and see if we find evidence that forces
  us to break them apart as we consider the various transitions.

We will build a tree, whose root has all states in the original machine.
The first step will always be to split the states into the subset of
non-final vs. the subset of final states, so these are the children of
the root.
We then look at each current leaf node of the tree, and check the
transitions from each of the states in that leaf.
We test a given character against the states in that subset to see if
they all go to the same subset.
Split them up when they do not go to the same place.

The following slideshow presents, step-by-step, the process of
minimizing the DFA.

.. inlineav:: Minimization1CON ss
   :points: 0.0
   :required: False
   :threshold: 1.0
   :id: 163417
   :output: show


**Another Example**:

.. odsafig:: Images/stmindfa2s.png
   :width: 500
   :align: center
   :capalign: justify
   :figwidth: 90%
   :alt: Minimization 2

   Minimization 2

.. inlineav:: Minimization2CON ss
   :points: 0.0
   :required: False
   :threshold: 1.0
   :id: 163418
   :output: show

.. odsascript:: lib/underscore.js
.. odsascript:: DataStructures/FLA/FA.js
.. odsascript:: AV/VisFormalLang/FA/Minimization1CON.js
.. odsascript:: AV/VisFormalLang/FA/Minimization2CON.js