Invariant.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;
// this need to be older version of solidity from 0.8.0 solidty compiler checks for overflow and underflow

import "forge-std/Test.sol";

/*
Name: Invariant issue

Description:
Assert is used to check invariants. Those are states our contract or variables should never reach, ever. For example,
if we decrease a value then it should never get bigger, only smaller.

In the given code, the Invariant contract contains a receiveMoney function that accepts Ether and 
increments the sender's balance with the amount received. This balance is stored as an uint64.
Unsigned integers can store values from 0 to 2^n - 1, so in this case 2^64 - 1, or roughly 18.4467 Ether.

If the sender sends more Ether than the maximum that can be stored in an uint64, 
an overflow occurs, and the value rolls over to 0 and starts incrementing from there. 
As a result, the balance does not accurately reflect the amount of Ether received by the contract.

Mitigation:
To avoid this problem, it's important to ensure that the types you use for storing values 
are appropriately sized for the values they need to store.

REF:
https://ethereum-blockchain-developer.com/027-exceptions/04-invariants-with-assert/

*/


contract ContractTest is Test {
    Invariant InvariantContract;

    function testInvariant() public {
        InvariantContract = new Invariant();
        InvariantContract.receiveMoney{value: 1 ether}();
        console.log(
            "BalanceReceived:",
            InvariantContract.balanceReceived(address(this))
        );

        InvariantContract.receiveMoney{value: 18 ether}();
        console.log(
            "testInvariant, BalanceReceived:",
            InvariantContract.balanceReceived(address(this))
        );
        /*
That's only 553255926290448384 Wei, or around 0.553 Ether. Where is the rest? What happened?

We are storing the balance in an uint64. Unsigned integers go from 0 to 2^n-1, 
so that's 2^64-1 or 18446744073709551615. So, it can store a max of 18.4467... 
Ether. We sent 19 Ether to the contract. 
It automatically rolls over to 0. So, we end up with 19000000000000000000 - 18446744073709551615 -1 (the 0 value) = 553255926290448384.
*/
    }

    receive() external payable {}
}

contract Invariant {
    mapping(address => uint64) public balanceReceived;

    function receiveMoney() public payable {
        balanceReceived[msg.sender] += uint64(msg.value);
    }

    function withdrawMoney(address payable _to, uint64 _amount) public {
        require(
            _amount <= balanceReceived[msg.sender],
            "Not Enough Funds, aborting"
        );

        balanceReceived[msg.sender] -= _amount;
        _to.transfer(_amount);
    }
}