You need to treat it as a black box ...
Agreed. But ...
PIN length is almost an invariant because it almost doesn't depend on any input data
I couldn't disagree more.
As pin() has no inputs, your assertion is based upon knowledge of what you know is inside the box, which is wrong. For a black box that takes no inputs and produces variable outputs, then only way to test is compare a representative sample of outputs against the specifications for those outputs.
Assuming pin() is defined to produce a random 4-digit integer; or better pin() is defined to produce a random value of type PIN, which is (currently) defined as a 4-digit integer; then the tests need to be:
- Does it always produce a 4-digit integer.
- Can the full range of integers expected be produced.
For large ranges, you would rely upon statistics to sample the distribution, but since this is such a small range, and the generation takes so little time, you might as well saturation test it.
If you rely upon knowledge of the implementation to not bother checking for length 4, then a modification by someone not understanding the significance of "%04d" that omitted the zero would not be detected.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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