5.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.

Over-constrained Code (2)

It is horribly inefficient, 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…

Over-constrained Code (3)

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!

Over-constrained Code (4)

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!