[stonemetal12]

As long as we can agree that we are testing the application logic and not the compiler or hardware, then if (a > 4) {...} else {...} can be tested with just 3, 4, 5 no need to test -430 or 5036.

Known as boundary value testing, you partition all input into equivalence classes, then make sure your tests contain a sample from each class.

[SkiFire13]

Making sure the test contains a sample from each class is the hard part. For example in your `if` example above it may happen that the code computing `a` is such that `a >= 5` is impossible, so that equivalence class is never going to happen. As such you can't have a test for it, and instead you'll have to prove that it can never happen, but this reduces to the halting problem and is not computable.

And even ignoring that problem, there may be an infinite amount of equivalence classes when you introduce loops/recursion, as the loops can run a different amount of times and thus lead to different executions.

Even just considering `if` statements, the amount of equivalence classes can be exponential in the amount of `if` (for example consider a series of `if` where each check a different bit of the input; ultimately you'll need any combination of bits to check every combination of `if`, and the number is 2^number of ifs).