.. This file is part of the OpenDSA eTextbook project. See
.. http://opendsa.org for more details.
.. Copyright (c) 2012-2020 by the OpenDSA Project Contributors, and
.. distributed under an MIT open source license.

.. avmetadata::
   :author: Cliff Shaffer

.. slideconf::
   :autoslides: False

=====================
Over-Constrained Code
=====================

Over-Constrained Code
---------------------

.. slide:: Over-constrained Code (1)

   * Consider the situation where we have two points. We want to know which
     quadrant the second point (b) is in w.r.t. the first point (a)::

       if ((b.x < a.x) && (b.y < a.y))
         doNW();
       else if ((b.x < a.x) && (b.y >= a.y))
         doSW();
       else if ((b.x >= a.x) && (b.y < a.y))
         doNE();
       else if ((b.x >= a.x) && (b.y >= a.y))
         doSE();

   * This has the virtue of being quite logical and clear. However, it has
     some problems.


.. slide:: Over-constrained Code (2)

   * It is horribly inefficient (up to 8 comparisons),
     compared to alternatives.
   * But our real concern has to do with testing and code coverage.
   * Fact: No series of tests will cover all branches in this code.
   * Q: Why?
   * A: Consider every possible branch and see what can get
     triggered. Consider that there have to be at least 8+ branches, and
     only 4 possible inputs!!
   * Try to hit every branch by brute force, one at a time...


.. slide:: Over-constrained Code (3)
     
   * Reconsider this code::
       
       if ((b.x < a.x) && (b.y < a.y))
         doNW();
       else if ((b.x < a.x) && (b.y >= a.y))
         doSW();

   * Why would we be in the SW branch?
      * Because (b.x < a.x) and (b.y >= a.y).
      * But if (b.x < a.x), then WE ALREAY KNOW that (b.y >= a.y),
        or we WOULD HAVE gone NW!!
      * So MT setting this comparison to TRUE can't break any test,
        that HAS to happen anyway.


.. slide:: Over-constrained Code (4)

   * Q: If we want complete code coverage when there are only four
     logically distinct inputs, then we had better do what?
   * A: Come up with code that has only four branches!


.. slide:: Over-constrained Code (5)

   * Refactored code::

       if (b.x < a.x)
         if (b.y < a.y)
           doNW();
         else
           doSW();
       else
         if (b.y < a.y)
           doNE();
         else
           doSE();

   * Not only can you test every branch, but this is a lot more efficient!
     Every branch requires 2 tests!