Goal: learn about the various types of test doubles

There are bugs in the code (marked with a Bug! comment), but all the tests are passing.

For each test, you will find a TODO comment. Your job is to finish the implementation of each test and make them fail for the right reason, i.e, triggering the bugs. You must do so by adding only test code - you are not allowed to change the production code.

Here's the order in which you should fix the test - they go from easiest to trickiest :)

1. SafeCalculator
2. DiscountApplier
3. MailSender

For this part, you are not allowed to add any external mock library (using the test framework is ok)

Start by writing the tests naively - and then make it run (or compile) by adding clases to the test code (this is what we call a test double).

Use a library like mockito for Java, unittest.mock for Python or sinon for JavaScript to rewrite all the tests.

Which version do you prefer? Why?