# Fuzz Testing

Smart contracts on Stacks are immutable. Bugs are forever. Test early. Test often. Fuzzing finds edge cases that unit tests often miss.

## What is Fuzz Testing?

Fuzzing hits your code with random inputs. It helps uncover unexpected behavior and subtle bugs. Unlike unit tests, it explores paths you didn't think of.

## What is Rendezvous?

Rendezvous (`rv`) is a Clarity fuzzer. It supports:

### Property-Based Testing

You extract properties about your smart contract using Clarity. Rendezvous checks them multiple times with random inputs, in a stateful manner (the smart contract's state is not refreshed during the run).

**What is a property?**

A property is a universal truth about your smart contract's state, functions, etc.

**How to extract a property?**

Say that your smart contract has a function that reverses a list of `uint`s. In this case, one property can be that "reversing a list twice returns the original list". The property will look like this:

```clarity
(define-public (test-reverse-list (seq (list 127 uint)))
  (begin
    (asserts!
      (is-eq seq
        (reverse-uint
          (reverse-uint seq)
        )
      )
      (err u999)
    )
    (ok true)
  )
)
```

**Making your property valid for Rendezvous**

> For a property to be cosidered valid by Rendezvous, it has to comply with the following rules:
>
> * Function name starts with `test-`
> * Function is declared as `public`
> * Test passes when it returns `(ok true)`
> * Test would be discarded if it returned `(ok false)`
> * Test fails if it returns an error or throws an exception

***

### Invariant Testing

You define read-only conditions in Clarity that must always hold true. Rendezvous attempts to create state transitions in your smart contract and continuously checks the conditions you defined to hold.

**What is an invariant?**

An invariant is a general truth regarding your smart contract's internal state. It will not be able to mutate the state, its role being solely to check the integrity of the state.

**How to extract an invariant?**

Say that you have a counter contract, having functions to `increment` and `decrement`. In this case, you could use the Rendezvous [`context`](https://stacks-network.github.io/rendezvous/chapter_6.html?#the-rendezvous-context) to extract an invariant regarding your smart contract's internal state:

```clarity
(define-read-only (invariant-counter-gt-zero)
  (let
    (
      (increment-num-calls
        (default-to u0 (get called (map-get? context "increment")))
      )
      (decrement-num-calls
        (default-to u0 (get called (map-get? context "decrement")))
      )
    )
    (if
      (<= increment-num-calls decrement-num-calls)
      true
      (> (var-get counter) u0)
    )
  )
)
```

**Making your invariant valid for Rendezvous**

> For an invariant to be cosidered valid by Rendezvous, it has to complain to the following ruleset:
>
> * Function name starts with invariant-
> * Function is declared as read-only (not public)
> * Function returns a boolean value (true if the invariant holds, false if violated)
> * The test can use the special context map to access execution history

## Why Test in Clarity?

Rendezvous tests run in Clarity, just like your contracts.

1. Tests operate under the exact same constraints as production code.
2. Better understanding of Clarity.
3. No need to expose internals as public functions.
4. Fewer tools to manage.

## Getting Started

Put tests next to contracts. Rendezvous will find them.

```
my-project/
├── Clarinet.toml
├── contracts/
│   ├── my-contract.clar       # Contract
│   ├── my-contract.tests.clar # Tests
└── settings/
    └── Devnet.toml
```

### Installation

To install Rendezvous as a dependency in your project, use `npm`:

```
npm install @stacks/rendezvous
```

This will add Rendezvous to your project's `node_modules` and update your `package.json`.

## Rendezvous Docs

See full docs at: [https://stacks-network.github.io/rendezvous](https://stacks-network.github.io/rendezvous/)
