Trait fuzzcheck::Mutator

pub trait Mutator: Sized {
    type Value: Clone;
    type Cache: Clone;
    type MutationStep;
    type UnmutateToken;
    fn cache_from_value(&self, value: &Self::Value) -> Self::Cache;
    fn mutation_step_from_value(
        &self, 
        value: &Self::Value
    ) -> Self::MutationStep;
    fn max_complexity(&self) -> f64;
    fn min_complexity(&self) -> f64;
    fn complexity(&self, value: &Self::Value, cache: &Self::Cache) -> f64;
    fn arbitrary(
        &self, 
        seed: usize, 
        max_cplx: f64
    ) -> (Self::Value, Self::Cache);
    fn mutate(
        &self, 
        value: &mut Self::Value, 
        cache: &mut Self::Cache, 
        step: &mut Self::MutationStep, 
        max_cplx: f64
    ) -> Self::UnmutateToken;
    fn unmutate(
        &self, 
        value: &mut Self::Value, 
        cache: &mut Self::Cache, 
        t: Self::UnmutateToken
    );
}

A Mutator is an object capable of mutating a value for the purpose of fuzz-testing.

For example, a mutator could change the value v1 = [1, 4, 2, 1] to v1' = [1, 5, 2, 1]. The idea is that if v1 is an “interesting” value to test, then v1' also has a high chance of being “interesting” to test.

Complexity

A mutator is also responsible for keeping track of the complexity of a value. The complexity is, roughly speaking, how large the value is.

For example, the complexity of a vector is the complexity of its length, plus the sum of the complexities of its elements. So vec![] would have a complexity of 0.0 and vec![76] would have a complexity of 9.0: 1.0 for its short length and 8.0 for the 8-bit integer “76”. But there is no fixed rule for how to compute the complexity of a value, and it is up to you to judge how “large” something is.

Cache

In order to mutate values efficiently, the mutator is able to make use of a per-value cache. The Cache contains information associated with the value that will make it faster to compute its complexity or apply a mutation to it. For a vector, its cache is its total complexity, along with a vector of the cache of each of its element.

MutationStep

The same values will be passed to the mutator many times, so that it is mutated in many different ways. There are different strategies to choose what mutation to apply to a value. The first one is to create a list of mutation operations, and choose one to apply randomly from this list.

However, one may want to have better control over which mutation operation is used. For example, if the value to be mutated is of type Option<T>, then you may want to first mutate it to None, and then always mutate it to another Some(t). This is where MutationStep comes in. The mutation step is a type you define to allow you to keep track of which mutation operation has already been tried. This allows you to deterministically apply mutations to a value such that better mutations are tried first, and duplicate mutations are avoided.

Unmutate

Finally, it is important to note that values and caches are mutated in-place. The fuzzer does not clone them before handing them to the mutator. Therefore, the mutator also needs to know how to reverse each mutation it performed. To do so, each mutation needs to return a token describing how to reverse it. The unmutate method will later be called with that token to get the original value and cache back.

For example, if the value is [[1, 3], [5], [9, 8]], the mutator may mutate it to [[1, 3], [5], [9, 1, 8]] and return the token: Element(2, Remove(1)), which means that in order to reverse the mutation, the element at index 2 has to be unmutated by removing its element at index 1. In pseudocode:

value = [[1, 3], [5], [9, 8]];
cache: c1 (ommitted from example)
step: s1 (ommitted from example)

let unmutate_token = self.mutate(&mut value, &mut cache, &mut step, max_cplx);

// value = [[1, 3], [5], [9, 1, 8]]
// token = Element(2, Remove(1))
// cache = c2
// step = s2

test(&value);

self.unmutate(&mut value, &mut cache, unmutate_token);

// value = [[1, 3], [5], [9, 8]]
// cache = c1 (back to original cache)
// step = s2 (step has not been reversed)

Associated Types

type Value: Clone

type Cache: Clone

type MutationStep

type UnmutateToken

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Required methods

fn cache_from_value(&self, value: &Self::Value) -> Self::Cache

Compute the cache for the given value

fn mutation_step_from_value(&self, value: &Self::Value) -> Self::MutationStep

Compute the initial mutation step for the given value

fn max_complexity(&self) -> f64

The maximum complexity of an input of this type

fn min_complexity(&self) -> f64

The minimum complexity of an input of this type

fn complexity(&self, value: &Self::Value, cache: &Self::Cache) -> f64

The complexity of the current input

fn arbitrary(&self, seed: usize, max_cplx: f64) -> (Self::Value, Self::Cache)

Create an arbitrary value

fn mutate(
    &self,
    value: &mut Self::Value,
    cache: &mut Self::Cache,
    step: &mut Self::MutationStep,
    max_cplx: f64
) -> Self::UnmutateToken

fn unmutate(
    &self,
    value: &mut Self::Value,
    cache: &mut Self::Cache,
    t: Self::UnmutateToken
)

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Implementors

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