What is a Unit Test

• Allows automated testing of independent code units
• No replacement for other test categories
• Write them as early as possible - if possible before the code it tests
• Tests can serve as useful requirements specification
• Create a test for each bug you encounter (refacturing!)

Which problems does unit testing solve?

Unit tests...

• are a first line of defense against bugs
• improve code quality by dictating testable code
• allow releasing higher quality early and often
• give more confidence to programmers
• are a prerequisit to refactoring
• can be used as a contract with regard to desired behavior
• prevent regressions (bugs in features priorly working properly)

Which problems does unit testing cause?

Essentially none, but writing unit tests ...

• seems to cost time
• demands discipline
• is far from trivial
• does not substitute other kinds of testing

What is a Test(case)

• Answers a single question about the code it is testing
• Run completely by itself (automation)
• Results must not require human interpretation
• Should be independent of other tests and their order of execution
• Can serve as documentation of how (not) to use your code

Flaky Tests

• Have different outcomes without changes to the code
• Erode confidence in testing processes
• Fix them (mute / skip them until they are fixed) or delete them

User's view (visible to the user)

• Functionality
• Reliability
• Usability
• Performance/ efficiency

Producer's view

• Portability
• Maintainability
• Testability (vs. legacy code)
• Transparency (is the code readable/ understandable?)

#include <stdint.h>
// Return the sum of all values from 1 to n.
uint64_t add_to(uint32_t n) {
    uint64_t sum = 0;
    for (uint32_t i = 1; i <= n; ++i) {
        sum += i;
    }
    return sum;
}

#include <stdint.h>
// Return the sum of all values from 1 to n.
uint64_t add_to(uint32_t n) {
    return (uint64_t) (n / 2.0) * (n + 1);
}

How can we apply the change without risking to destroy our application?

1. Define good test cases
2. Implement test cases using your project's unit testing library
3. Run the tests before the change to ensure they pass
4. Apply the change, then run the tests again

1. Define Good Test Cases

Using pencil and paper / brainstorming: define relevant test cases

• All code paths should be used
• Add one or more common cases
• Include relevant edge cases

25 is a possible common case. Add 26 as an even input. The edge cases should include 0 and 4294967295.
Expected results for these are 325, 351, 0 and 9223372034707292160.

Let's also add 500000000 (expected result 125000000250000000) as 4294967295 does not terminate with the current implementation.

2. Implement defined test cases

#include <criterion/criterion.h>
#include <criterion/new/assert.h>
unsigned long long add_to(unsigned n);  // usually in header file

Test(sum_to, fast_test) {
  cr_assert(eq(u64, add_to(0), 0), "Sum of 0 is 0.");
  cr_assert(eq(u64, add_to(25), 325), "Sum of 1..25 is 325.");
  cr_assert(eq(u64, add_to(26), 351), "Sum of 1..26 is 351.");
  cr_assert(eq(u64, add_to(500000000), 125000000250000000),
    "This function must work for large unsigned integers.");
}
// does not terminate for the old code version due to an overflow of the
// loop counter (the loop would terminate only at UINT_MAX + 1)
Test(sum_to, slow_test) {
  cr_assert(eq(u64, add_to(4294967295), 9223372034707292160),
    "The function must work for the largest unsigned integer.");
}

3. Run the Unittests Before the Change

clang before_refactoring.c test_sum_to.c -l criterion -o run_tests
./run_tests  # does not terminate; interrupt via [Ctrl]+[C]

What to do now?

./run_tests --help
./run_tests -l  # list all available tests
./run_tests --filter sum_to/fast_test

4. Apply the Change, then Run the Tests Again

For the sake of the presentation, instead of applying the change, we use another source file.

clang after_refactoring.c test_sum_to.c -l criterion -o run_tests
./run_tests

Now the tests are fast, but they are failing. Fix the code, then compile and run the tests again.