On Tuesday, 1 September 2015 at 18:25:06 UTC, H. S. Teoh wrote:
> On Tue, Sep 01, 2015 at 06:14:05PM +0000, deadalnix via Digitalmars-d wrote:
>> A ton of code is not unitestable. GUI as a starter. Random number generator.  Very rare events handling outside the control of the program. Concurent code. Etc...
>
> That would explain why GUI apps tend to be buggy and unreliable, and RNGs tend to have unintended biases.
>
> No, actually, GUIs should be written to be testable (e.g., input should be abstracted via dependency injection so that GUI interactions are scriptable, at least inside a unittest, with expected final states).
>
> RNGs can be unittested for uniform distribution within certain bounds. Unittest does not necessarily mean checking for equality (even though that's the most common usage).

Some of this depends on whether you're talking about pseudo-random or 'true' random number generators.  With the former you can e.g. verify that particular seeds produce expected sequences, etc., so you can at least test equivalence of implementations.

Testing (true or pseudo) RNGs for desired statistical properties (not just distribution but also periodicity, etc.) is readily possible, it's just rather more costly than is desirable for unittests.