Troll of Fame - Rust training

This project use Rust with cargo and proptest.

Getting started

• Build the project : cargo build
• Run tests : cargo test
• Run binary : cargo run

Once upon a time ⋯

The King of the Trolls Gnonpom coded the Troll of Fame : a wonderfull application that would help Trolls to learn numbers when they are hunting. Gnonpom was a hard skilled Test Driven Developer king who just released ToF when all tests passed Green.

Sadly he was shooted by a disgusting Elf.

Here come a new King, Hurrah for the great Troll Aklass!

This time it's decided, the elf hunting contest is launched!

At the end of each battle, the trolls want to compare the number and attributes of the slain elves. And with ToF it should be easy ⋯ Should.

Excercices

Work with legacy code

You inherit an application that seems to work fine. Run cargo test (•̀ᴗ•́)و ̑̑

Read elf.rs and troll.rs test modules as a first specification of the software.

Now uncomment the second proptest of elf.rs and run tests again cargo test ⋯ Ooops seems that our unit tests was not so complete. (╥﹏╥)

We will try to improve the quality of Troll of Frame thanks to Property Based Testing

Property testing

Property Based Testing (a.k.a. PBT) is about generating tests instead of manually writing them. Unlike unit tests where you know what goes in and what comes out (a.k.a. oracle tests), you assess properties that should always be true. The PBT library checks for arbitrary inputs that the property is true.

In Rust, we use proptest library to write and run Property Based tests.

Step 1 - Configuration and Invariance

No matter the year, the 31st of December is a New Year's Eve

• For a simpler start, we already configured the build dependencies and created Arbitrary generators for Elf and Troll. It's easy thanks to proptest-derive's high order trait Arbitrary.
• PBT tests are located in a property_tests submodule for each module

...
    #[cfg(test)]
    mod property_tests {
        use super::*;
        use proptest::prelude::*;
        proptest! {
            #[test]
            fn elf_value_should_be_positive(elf : Elf) {
                assert!(elf.value() > 0);
            }
        }
    }

• Did you notice the property test takes a Elf as input? That's where PBT shines! The library will run this test 100 times, and each time will pass a random Elf to it. We no longer care about building input data!

• The first property test we will write aims to assess Invariance property: it means a property should always be true even if the input varies (e.g. the Elf)

  • As an example, no matter the elf, his value is always > 0.

  • Another exemple, an elf value is always the product of his role value and his race value. That's the test which made you discover a bug when you uncommented it while our unit tests were PASS!

• As first exercice, implement an invariant Test for a Troll. No matter the troll, his score is always >= 0 (i.e. is never negative).

• What would the same check with regular unit tests look like?

📌 Most unit tests can actually be converted to Invariance properties

Step 2 - Inverse

Inverse properties check that it's possible to transform some input to an output and back to the original input, no matter the input. This is a useful property because it guarantees some functions don't lose information and/or are consistent.

bar and foo are inverse of each other

• For any Troll and any Elf, if the Troll kills the Elf and then realizes the elf survived, what should be the result?
• Write an inverse property test to check that

Testing it will ensure that i_got_one and oops_he_survived are consistent.

Step 3 - Analogy

Analogous properties check that there are at least 2 different ways from any input to reach an output. This is a useful property because it guarantees some functions are consistent (can also be useful for refactors)

Adding any number to itself is the same as multiplying this number by 2

For any troll, any elf and any positive quantity of killed elves, what should be the difference between:

• killing a single elf and repeating this operation quantity times
• killing in a single strike quantity units of elf?

Write an analogous property test to check that

This ensures that i_got_one and i_got are consistent.

For refactors, copy the function to refactor, do your changes, then write an Analogy property test to check for any input that they return the same output, i.e. the refactor has no regression! Now you can delete the test and the legacy function, and rename the refactored function to the legacy name

Step 4 - Idempotence

Idempotent properties check that running a function once or several times leads to exactly the same result, i.e. an idempotent function brings to a stable state from which this function becomes useless.

Once a list of numbers is sorted, sorting it again doesn't change anything

• For any Troll and any Elf, once all elves have been resurrected, what should happen if these elves are resurrected again?
• Write an idempotent property test to check that

More generally, function is idempotent if applying it to its own result doesn't change anything

This ensures that all_elves_of_a_kind_resurrected brings the Troll killing list to a stable state (i.e. many call should have the same result as once).

[Bonus] Step 5 - Metamorphism

Metamorphic properties check that running a function with variants of the same input should lead to equal or consistent outputs. E.g. if the input is multiplied by 2, is the output also multiplied by 2? Divided by 2? The same?

• For any Troll and any elf, what should the Troll score be compared to the score of the Troll after killing elf?
• Write a metamorphic property test to check that

This ensures that i_got_one correctly increases the kill list (and thus the score) when an elf is killed.

[Bonus] Step 6 - Injection

Injective properties check that different inputs lead to different outputs, i.e. there aren't 2 different inputs that lead to the same output, i.e. each output has at most 1 input.

• For any Troll and any 2 elves elf1 and elf2, assuming elf1 is different from elf2, troll after killing elf1 must be different from Troll after killing elf2
• Write an injective property test to check that

This ensures that iGotOne always updates the provided Troll in a unique way.