INTRODUCTION

Debugging and testing Solidity smart contracts are crucial steps in the development process to ensure that the contracts function as intended and do not contain any security vulnerabilities. This article will provide an overview of the tools and best practices for debugging and testing Solidity contracts.

CONCEPTS COMPREHENSION

Before diving into debugging and testing, it's important to understand the basics of the Solidity programming language. Solidity is a contract-oriented, high-level programming language for writing smart contracts. It is used for implementing smart contracts on various blockchain platforms, such as Ethereum. Smart contracts are self-executing contracts with the terms of the agreement written directly into code.

Debugging

Debugging is the process of identifying and resolving errors or bugs in a program. There are several tools and methods available for debugging Solidity contracts.
One of the most popular tools for debugging Solidity contracts is Truffle. Truffle is a development environment, testing framework, and asset pipeline for Ethereum. It includes a suite of tools for compiling, deploying, and testing contracts. It also provides an interactive console for debugging and interacting with contracts on the blockchain.

Another useful tool for debugging Solidity contracts is Remix, an online solidity compiler and IDE. It allows developers to write, test, and debug their contracts in the browser. It also includes a debugger that allows developers to step through the execution of their contracts, set breakpoints, and examine the state of variables at different points in the execution.
There are also various plugins and extensions for development environments like** Visual Studio Code** that include debugging capabilities and are useful for debugging solidity contract

Testing

Testing is the process of verifying that a program or system meets its specified requirements and behaves as expected. It is an essential step in the development process to ensure that a contract is functioning correctly and does not contain any bugs or security vulnerabilities.

The most common approach to testing Solidity contracts is to use a testing framework. Truffle, as mentioned earlier, includes a built-in testing framework for Solidity contracts. It allows developers to write tests in JavaScript and then run them against their contracts to check that they function as expected.
Another popular testing framework for Solidity is OpenZeppelin's "Test Environment", which allows for the testing of smart contract in a simulated blockchain.

It's important to thoroughly test a smart contract before deploying it to a production environment. This includes testing for common security vulnerabilities such as the reentrancy vulnerability and the integer overflow/underflow vulnerability.

In addition to automated testing, it's also recommended to perform manual testing, reviewing the codebase, and conducting security audit by a third-party or specialized auditor.

A practical example

A practical example that demonstrates the concepts discussed in this article could be a simple smart contract for a token transfer system. Here is an example of what such a contract might look like:

`pragma solidity ^0.8.0;

contract TokenTransfer {
    mapping (address => uint) public balances;

    function transfer(address _to, uint _value) public {
        require(balances[msg.sender] >= _value && _value > 0, "Not enough balance or invalid value");
        balances[msg.sender] -= _value;
        balances[_to] += _value;
    }
}`

** Code comprehension**
This contract defines a mapping called "balances" that keeps track of the number of tokens held by each address.
It also includes a function called "transfer" that is used to transfer tokens from one address to another.
The function takes two arguments: an address to which the tokens will be transferred, and the number of tokens to be transferred.
The transfer function uses the Solidity require statement to check that the sender has enough tokens and the _value is more than 0 before making the transfer.

Debugging
To debug this contract, a developer could use Truffle or Remix to deploy the contract to a local test blockchain, and then use the interactive console or debugger to step through the execution of the transfer function, and examine the state of the contract's variables at different points in the execution. They could also set breakpoints to pause the execution and inspect variable values to detect errors or bugs in the contract's logic.

Testing
For testing, the developer could use Truffle or OpenZeppelin's "Test Environment" to write test cases for the contract, checking that it correctly transfers tokens between addresses, that it correctly updates the balance mapping, and that it has the expected behavior when passed invalid input or called by unauthorized addresses.

Exemple of testing code : 

`const TokenTransfer = artifacts.require("TokenTransfer");

contract("TokenTransfer", (accounts) => {

  let contract;
  beforeEach(async () => {
    contract = await TokenTransfer.new();
  });

  it("should transfer tokens correctly", async () => {
    await contract.transfer(accounts[1], 100);
    let senderBalance = await contract.balances(accounts[0]);
    let receiverBalance = await contract.balances(accounts[1]);
    assert.equal(senderBalance.toNumber(), 0, "Sender balance not updated correctly");
    assert.equal(receiverBalance.toNumber(), 100, "Receiver balance not updated correctly");
  });

  it("should fail when trying to transfer more tokens than the sender has", async () => {
    try {
      await contract.transfer(accounts[1], 100, {from: accounts[0]});
      await contract.transfer(accounts[1], 100, {from: accounts[0]});
    } catch (error) {
      assert.equal(error.reason, "Not enough balance or invalid value");
    }
  });

  it("should fail when trying to transfer negative tokens", async () => {
    try {
        await contract.transfer(accounts[1], -1, {from: accounts[0]});
    } catch (error) {
        assert.equal(error.reason, "Not enough balance or invalid value");
    }
  });

  it("should fail when trying to transfer 0 tokens", async () => {
    try {
        await contract.transfer(accounts[1], 0, {from: accounts[0]});
    } catch (error) {
        assert.equal(error.reason, "Not enough balance or invalid value");
    }
  });
});`

Test cases comprehension

These test cases test the following scenarios:

1. The correct transfer of tokens between accounts

2. Attempts to transfer more tokens than the sender has

3. Attempts to transfer negative or zero tokens

In this example :

• the beforeEach function is used to create a new instance of the contract before each test case is run. This ensures that the tests are isolated from one another and do not affect the state of the contract in unexpected ways.

• The it functions are used to define individual test cases. The first test case, "should transfer tokens correctly," calls the transfer function and then checks that the balance of the sender and receiver have been updated correctly.

• The next three test cases checks for the edge cases where it should fail when the sender doesn't have enough balance or trying to transfer negative or zero tokens and they check if the reason of the failure is the expected one.

They also test for common security vulnerabilities such as the reentrancy vulnerability and the integer overflow/underflow vulnerability by adding edge cases and malicious input to their test suite.

CONCLUSION

In summary, debugging and testing are crucial steps in the development of Solidity smart contracts. Using the tools and techniques described in this article can help ensure that contracts are functioning correctly and do not contain any security vulnerabilities.