// SPDX-License-Identifier: MIT

pragma solidity ^0.8;

/* runner.json
{
    "differential": true,
    "actions": [
        {
            "Upload": {
                "code": {
                    "Solidity": {
                        "contract": "ERC20"
                    }
                }
            }
        },
        {
            "Instantiate": {
                "code": {
                    "Solidity": {
                        "contract": "ERC20Tester"
                    }
                }
            }
        }
    ]
}
*/

// https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.0.0/contracts/token/ERC20/IERC20.sol
interface IERC20 {
    function totalSupply() external view returns (uint);

    function balanceOf(address account) external view returns (uint);

    function transfer(address recipient, uint amount) external returns (bool);

    function allowance(
        address owner,
        address spender
    ) external view returns (uint);

    function approve(address spender, uint amount) external returns (bool);

    function transferFrom(
        address sender,
        address recipient,
        uint amount
    ) external returns (bool);

    event Transfer(address indexed from, address indexed to, uint value);
    event Approval(address indexed owner, address indexed spender, uint value);
}

contract ERC20 is IERC20 {
    uint public totalSupply;
    mapping(address => uint) public balanceOf;
    mapping(address => mapping(address => uint)) public allowance;
    string public name = "Solidity by Example";
    string public symbol = "SOLBYEX";
    uint8 public decimals = 18;

    function transfer(address recipient, uint amount) external returns (bool) {
        balanceOf[msg.sender] -= amount;
        balanceOf[recipient] += amount;
        emit Transfer(msg.sender, recipient, amount);
        return true;
    }

    function approve(address spender, uint amount) external returns (bool) {
        allowance[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(
        address sender,
        address recipient,
        uint amount
    ) external returns (bool) {
        allowance[sender][msg.sender] -= amount;
        balanceOf[sender] -= amount;
        balanceOf[recipient] += amount;
        emit Transfer(sender, recipient, amount);
        return true;
    }

    function mint(uint amount) external {
        balanceOf[msg.sender] += amount;
        totalSupply += amount;
        emit Transfer(address(0), msg.sender, amount);
    }

    function burn(uint amount) external {
        balanceOf[msg.sender] -= amount;
        totalSupply -= amount;
        emit Transfer(msg.sender, address(0), amount);
    }
}

contract ERC20Tester {
    constructor() {
        address BOB = address(0xffffffffffffffffffffffffffffffffffffff);
        ERC20 token = new ERC20();
        assert(token.decimals() == 18);

        token.mint(300);
        assert(token.balanceOf(address(this)) == 300);
        token.transfer(BOB, 100);
        assert(token.balanceOf(address(this)) == 200);
        assert(token.balanceOf(BOB) == 100);

        token.approve(address(this), 100);

        token.transferFrom(address(this), BOB, 100);
        assert(token.balanceOf(BOB) == 200);
        assert(token.balanceOf(address(this)) == 100);

        token.burn(100);
        assert(token.balanceOf(address(this)) == 0);
    }
}