Published: May 11 2019

It’s been a few months that I wrote about stateful properties. Since then I’ve given a couple of workshops about property-based testing and jqwik has reached version 1.1.3.

However, while I’m getting more and more committed to PBT in my side-projects when developing software for clients my main approach is still Test-Driven Development with mostly example-based test cases. Wouldn’t it be nice if both techniques could be merged into some sort of grand unified development approach? It turns out that they do go together quite well, at least sometimes.

Prime Factorization - Property-Test-Driven

To get the discussion going I’ll demonstrate my ideas by tackling a heavily used exercise: prime factorization. The goal of this code kata is to compute the prime factors of any given natural number from 2 upwards. I usually start all my TDD work with collecting test ideas in an “inbox”. I’ll then grab and implement the ideas from this inbox one by one until there is either none left or none of the remaining test ideas seems to provide any further value.

In a conventional example-driven TDD session the test ideas often come in the form of concrete examples. With properties as an additional means of expression I tend to mix concrete examples with properties in my initial inbox. As for the kata at hand I came up with the following list:

factorize(2) -> [2]
factorize(prime) -> [prime]
factorize(prime^2) -> [prime, prime]
factorize(prime^n) -> [prime, ..., prime]
factorize(prime1 * prime2) -> [prime1, prime2]
factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

The items did not come to my mind in this order but I tried to sort them by assumed implementation complexity. At the end of the list I appended a few generic properties.

Starting with an Example

More often than not I start a TDD session with an easy example test. This helps me to figure out a function’s interface. Luckily, the first item in our inbox is a perfect fit for that purpose:

class PrimeFactorizationTests {
	@Example
	void factorizing_2_returns_list_with_just_2() {
		List<Integer> factors = Primes.factorize(2);
		Assertions.assertThat(factors).containsExactly(2);
	}
}

Getting this test to pass is also trivial:

public class Primes {
	public static List<Integer> factorize(int number) {
		return Collections.singletonList(2);
	}
}

Et voilà, the first inbox item can be ticked off:

✓ factorize(2) -> [2]
factorize(prime) -> [prime]
factorize(prime^2) -> [prime, prime]
factorize(prime^n) -> [prime, ..., prime]
factorize(prime1 * prime2) -> [prime1, prime2]
factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

A first Property

At this point test-driven developers will often ask themselves: Is the current test enough to justify a more generic implementation of factorize() or do we need additional examples from which to triangulate. With property-based testing in mind we have another option. Converting a concrete example into a generic property will often enforce generic implementation more or less by itself. Let’s see how that works out by taking translating the second inbox item into a property test:

@Property
void factorizing_a_prime_returns_list_with_just_the_prime(
        @ForAll("primes") int prime
) {
    List<Integer> factors = Primes.factorize(prime);
    Assertions.assertThat(factors).containsExactly(prime);
}

Running this property will yield an exception:

net.jqwik.api.CannotFindArbitraryException:
    Cannot find an Arbitrary [primes] for Parameter of type
        [@net.jqwik.api.ForAll(value=primes) int]

The error message tells us that we have to implement a provider method for “primes”. We could now search in our brains for long forgotten algorithms to generate prime numbers. We could also import a maths library. Or we can be pragmatic and just enumerate a few primes and let jqwik choose among them:

@Provide
Arbitrary<Integer> primes() {
    return Arbitraries.of(2, 3, 5, 7, 23, 101);
}

Trying just 6 different prime numbers might look like too much of a simplification, especially since one of the promises of PBT is to provide a wide coverage of the problem space. However, the six examples are enough to drive generality into our implementation. We can always add more thorough prime number generation later - and we will.

Rerunning the property will still fail, but now with a real assertion failure:

jaa.lang.AssertionError:
    Expecting:
      <[2]>
    to contain exactly (and in same order):
      <[3]>

This can be fixed by a simple change:

public static List<Integer> factorize(int number) {
    return Collections.singletonList(number);
}

Knowing that in the end we’ll sometimes have to add more than a single factor, the following refactoring towards this capability seems natural to me:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    factors.add(number);
    return factors;
}

All tests and properties still succeed and the first property on our list is done:

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
factorize(prime^2) -> [prime, prime]
factorize(prime^n) -> [prime, ..., prime]
factorize(prime1 * prime2) -> [prime1, prime2]
factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

If we wanted we could now get rid of the initial example test since it’s fully covered by the property. I prefer to keep a few concrete examples around because they are often easier to understand.

From Example to Property - Again

Tackling the next property - squared primes - gives us the opportunity to revisit a tactic that we’ve already seen in the section above. Let’s start with the property test method:

@Property
void factorizing_squared_prime_returns_prime_twice(@ForAll("primes") int prime) {
    List<Integer> factors = Primes.factorize(prime * prime);
    Assertions.assertThat(factors).containsExactly(prime, prime);
}

Run and see it fail as expected! When I tried to think of a simple way to make it work for all primes I couldn’t. I bit off more than I could chew in one bit. My standard trick to tackle this kind of situation is to scale down from property to single example. One way to do that without having to write yet another (example) test is to temporarily fix the incoming value of a property test:

@Property
void factorizing_squared_prime_returns_prime_twice(@ForAll("primes") int prime) {
	prime = 2;
    List<Integer> factors = Primes.factorize(prime * prime);
    Assertions.assertThat(factors).containsExactly(prime, prime);
}

This will result in the same test failure but it is easier to repair:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    if (number == 4) {
        factors.add(2);
        number /= 2;
    }
    factors.add(number);
    return factors;
}

Extracting 2 into a variable seems a good refactoring because 2 is just the current representative of another concept: a prime candidate.

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    if (number == candidate * candidate) {
        factors.add(candidate);
        number /= candidate;
    }
    factors.add(number);
    return factors;
}

From here the general property seems achievable:

@Property
void factorizing_squared_prime_returns_prime_twice(@ForAll("primes") int prime) {
    List<Integer> factors = Primes.factorize(prime * prime);
    Assertions.assertThat(factors).containsExactly(prime, prime);
}

fails with message:

java.lang.AssertionError:
    Expecting:
      <[9]>
    to contain exactly (and in same order):
      <[3, 3]>

Fixing this requires only a small change to increase the candidate until it fits:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number % candidate != 0) {
        candidate++;
    }
    if (number == candidate * candidate) {
        factors.add(candidate);
        number /= candidate;
    }
    factors.add(number);
    return factors;
}

This code can further be simplified without breaking existing tests:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number % candidate != 0) {
        candidate++;
    }
    if (number > candidate) {
        factors.add(candidate);
    }
    factors.add(candidate);
    return factors;
}

The code’s not pretty (yet); in TDD you sometimes have to be patient with your design. In fact, being more patient with not so perfect code was probably my personal breakthrough as for dealing with larger code bases. As long as I have trust in my tests I can be confident that I can improve the design as soon as I have the necessary insight; I no longer feel obliged to fix everything at once.

There’s one more item to check off now:

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
✓ factorize(prime^2) -> [prime, prime]
factorize(prime^n) -> [prime, ..., prime]
factorize(prime1 * prime2) -> [prime1, prime2]
factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

Generalizing an Existing Property

The next point on our list - dealing with prime^n is a generalized version of prime^2. Thus, we proceed by generalizing the existing property: introduce the new parameter, rename the method appropriately, but stick with already working examples.

@Property
void factorizing_prime_raised_to_n_returns_n_times_prime(
        @ForAll("primes") int prime,
        @ForAll @IntRange(min = 1, max = 2) int n
) {
    List<Integer> factors = Primes.factorize((int) Math.pow(prime, n));
    Assertions.assertThat(factors).containsOnly(prime);
    Assertions.assertThat(factors).hasSize(n);
}

This works out of the box and show us that we didn’t break the test during generalization. Now we can increase the upper limit of n:

@Property
void factorizing_prime_raised_to_n_returns_n_times_prime(
        @ForAll("primes") int prime,
        @ForAll @IntRange(min = 1, max = 5) int n
) { ... }

This will fail with a sample of [2, 3]. But again, it’s a rather smallish change that can make all of our tests succeed:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number % candidate != 0) {
        candidate++;
    }
    while (number >= candidate) {
        factors.add(candidate);
        number /= candidate;
    }
    return factors;
}

Well, at least I thought it would work. In reality the following error message was showing up:

sample = [101, 5]

java.lang.AssertionError:
    Expecting:
      <[2147483647]>
    to contain only:
      <[101]>

What’s happening here is that 101^5 is bigger than the maximum representable value of type int. Java’s policy to just overflow and be quiet makes that not so obvious. What we have to do is make sure that prime^n will not overflow. To reach that goal we have several choices:

  • Generate only primes that are below Integer.MAX_VALUE when raised to the power of 5
  • Reduce the max value of n to 4
  • Filter out those combinations where prime^n exceeds Integer.MAX_VALUE

I chose the latter option here because it results in more and more different tests. Exploring the possible range of values to its limits is one of the promised virtues of PBT. Since the filter criteria covers more than one parameter we have to use assumptions within the property method itself:

@Property
void factorizing_prime_raised_to_n_returns_n_times_prime(
        @ForAll("primes") int prime,
        @ForAll @IntRange(min = 1, max = 5) int n
) {
    BigInteger numberToFactorize = BigInteger.valueOf(prime).pow(n);
    Assume.that(
        numberToFactorize.compareTo(BigInteger.valueOf(Integer.MAX_VALUE)) <= 0);

    List<Integer> factors = Primes.factorize(numberToFactorize.intValueExact());
    Assertions.assertThat(factors).containsOnly(prime);
    Assertions.assertThat(factors).hasSize(n);
}

Now the property will succeed and it’s interesting to look at the report:

PrimeFactorizationTests:factorizing prime raised to n returns n times prime =
                              |-----------------------jqwik-----------------------
tries = 30                    | # of calls to property
checks = 29                   | # of not rejected calls
generation-mode = EXHAUSTIVE  | parameters are exhaustively generated
after-failure = SAMPLE_FIRST  | try previously failed sample, then previous seed
seed = 1584749916605677180    | random seed to reproduce generated values

The values of tries and checks reveals two things: Only a single combination was filtered out. And only 30 sets of input values were created at all! Why not 1000, which is the default number of tries? Well, in cases where jqwik can figure out that the number of all possible combinations of values is lower than the number of tries, it will just go ahead and generate all possible combinations. We currently only provide 6 different primes to choose from and the exponent is between 1 and 5: 6 * 5 is 30. And sure enough, one more item can be marked as “done”.

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
✓ factorize(prime^2) -> [prime, prime]
✓ factorize(prime^n) -> [prime, ..., prime]
factorize(prime1 * prime2) -> [prime1, prime2]
factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

Adding the Remaining Functional Requirements

From a functional perspective all that’s missing is factorization of numbers with mixed prime factors. The next item in the inbox - factorize(prime1 * prime2) -> [prime1, prime2] - would be another stepping stone. Since I can almost visualize the general implementation I feel bold enough to skip this step and go the full mile:

factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]

Knowing from experience that integer overflow might strike here, I put in the guarding assumption from the start:

@Property
void factorizing_product_of_list_of_primes_will_return_original_list(
        @ForAll("listOfPrimes") List<Integer> primes
) {
    BigInteger product =
            primes.stream()
                  .map(BigInteger::valueOf)
                  .reduce(BigInteger.ONE, BigInteger::multiply);
    Assume.that(product.compareTo(BigInteger.valueOf(Integer.MAX_VALUE)) <= 0);

    List<Integer> factors = Primes.factorize(product.intValueExact());
    Assertions.assertThat(factors).isEqualTo(primes);
}

@Provide
Arbitrary<List<Integer>> listOfPrimes() {
    return primes().list().ofMinSize(1).ofMaxSize(5);
}

As expected, running the property reveals the simplest example that fails:

org.opentest4j.AssertionFailedError:
    Expecting:
     <[2, 2]>
    to be equal to:
     <[2, 3]>
    but was not.

Sure enough, all I have to do is pulling in the first loop into the second:

public static List<Integer> factorize(int number) {
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number >= candidate) {
        while (number % candidate != 0) {
            candidate++;
        }
        factors.add(candidate);
        number /= candidate;
    }
    return factors;
}

I was too optimistic, though:

org.opentest4j.AssertionFailedError:
    Expecting:
     <[2, 7]>
    to be equal to:
     <[7, 2]>
    but was not.

The problem lies in the order of factors. Since they should be - and are indeed - returned in ascending order the original list of primes must also be in ascending order to match the result. We could do the sorting in the provider method, but the property itself seems to be as good a place:

@Property
void factorizing_product_of_list_of_primes_will_return_original_list(
        @ForAll("listOfPrimes") List<Integer> primes
) {
    primes.sort(Integer::compareTo);
    BigInteger product = ... ;
    Assume.that(product.compareTo(BigInteger.valueOf(Integer.MAX_VALUE)) <= 0);

    List<Integer> factors = Primes.factorize(product.intValueExact());
    Assertions.assertThat(factors).isEqualTo(primes);
}

And indeed, all tests succeed now and we can happily revisit our inbox:

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
✓ factorize(prime^2) -> [prime, prime]
✓ factorize(prime^n) -> [prime, ..., prime]
✓ factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

Error Cases

Testing illegal input and rejecting it is straightforward in our case. That’s why I’ll cut it short and just show the property and the implementation that fulfills it:

@Property
void numbers_below_2_are_illegal(
        @ForAll @IntRange(min = Integer.MIN_VALUE, max = 1) int number
) {
    Assertions.assertThatThrownBy(() -> {
        Primes.factorize(number);
    }).isInstanceOf(IllegalArgumentException.class);
}
public static List<Integer> factorize(int number) {
    if (number < 2) {
        throw new IllegalArgumentException();
    }
    ...
}

Here’s the remaining inbox:

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
✓ factorize(prime^2) -> [prime, prime]
✓ factorize(prime^n) -> [prime, ..., prime]
✓ factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
✓ factorize(n < 2) -> IllegalArgumentException
factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
product of all returned numbers must be equal to input number
all numbers in produced list must be primes

Putting the Implementation under Pressure

Up to now we’ve been quite careful with our selection of primes and the number of primes the product of which we factorize in the tests. Let’s first enhance the list of primes to consider the first 26:

@Provide
Arbitrary<Integer> primes() {
    return Arbitraries.of(
            2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47,
            53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101
    );
}

No problems with that. As a next step, we can raise the maximum list size to 20:

@Provide
Arbitrary<List<Integer>> listOfPrimes() {
    return primes().list().ofMinSize(1).ofMaxSize(20);
}

Still no problem. Checking the jqwik report, however, reveals that now more than two thirds of generated examples will be thrown away due to the filtering assumption in the property:

PrimeFactorizationTests:factorizing product of list of primes will return original list =
                              |-----------------------jqwik-----------------------
tries = 1000                  | # of calls to property
checks = 301                  | # of not rejected calls

Let’s take the next two items from our inbox, combine them into one property method and use it to explore how big numbers are handled by the factorization algorithm:

@Property
void all_numbers_above_1_can_be_factorized(
        @ForAll @IntRange(min = 2, max = 10000) int number
) {
    List<Integer> factors = Primes.factorize(number);
    Integer product = factors.stream().reduce(1, (a, b) -> a * b);
    Assertions.assertThat(product).isEqualTo(number);
}

On my machine max = 100_000_000 is still finishing within a few seconds. Turning it up to 1_000_000_000 got the runtime above my personal threshold of patience for microtests. Maybe we can optimize the algorithm a little bit? Next try:

public static List<Integer> factorize(int number) {
    if (number < 2) {
        throw new IllegalArgumentException();
    }
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number >= candidate) {
        while (number % candidate != 0) {
            if (candidate * candidate > number) {
                candidate = number;
            } else {
                candidate++;
            }
        }
        factors.add(candidate);
        number /= candidate;
    }
    return factors;
}

This will get us to max = Integer.MAX_VALUE - 1 but max = Integer.MAX_VALUE does not finish. Again, we’re running into an overflow; this time within a condition of the the factorize method:

if (candidate * candidate > number) { ... }

Using the square root instead of the square gets rid of this problem.

So here’s the final version of our prime factorization algorithm, which works for all int numbers above 1. At least jqwik has not found a counter example yet:

public static List<Integer> factorize(int number) {
    if (number < 2) {
        throw new IllegalArgumentException();
    }
    List<Integer> factors = new ArrayList<>();
    int candidate = 2;
    while (number >= candidate) {
        while (number % candidate != 0) {
            if (Math.sqrt(number) < candidate) {
                candidate = number;
            } else {
                candidate++;
            }
        }
        factors.add(candidate);
        number /= candidate;
    }
    return factors;
}

If you really really want to know if it works for all valid numbers you can configure the property method like that:

@Property(generation = GenerationMode.EXHAUSTIVE)
void all_numbers_above_1_can_be_factorized(
        @ForAll @IntRange(min = 2) int number
) { ... }

I am pretty sure this will run for a few hours on my machine though…

The last Property

We are now left with the following inbox:

✓ factorize(2) -> [2]
✓ factorize(prime) -> [prime]
✓ factorize(prime^2) -> [prime, prime]
✓ factorize(prime^n) -> [prime, ..., prime]
✓ factorize(prime1 * .... * primeN) -> [prime1, ..., primeN]
✓ factorize(n < 2) -> IllegalArgumentException
✓ factorize(2 <= number <= Integer.MAX_VALUE) -> no exception  
✓ product of all returned numbers must be equal to input number
all numbers in produced list must be primes

There is one property left that we haven’t implemented yet: “All numbers in produced list must be primes.” Can you think of an implementation that would succeed in all tested properties but fail this last one? Frankly, I can’t. For the sceptics among you: Go ahead and write the missing property. For all the others: Be happy with what we have!

Conclusion

As I could hopefully demonstrate in this example, Property-Test-Driven Development can work - at least sometimes. A pattern that you could watch here twice is to start with an example first and to promote it to a property later. In cases where this approach works I end up with both more concise tests and more trust in my code. That’s why I always look for those opportunities in my existing example tests.

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