// File: 1_Storage.sol



// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

/**
 * @title Storage
 * @dev Store & retrieve value in a variable
 */
contract Storage {

    uint256 number;

    /**
     * @dev Store value in variable
     * @param num value to store
     */
    function store(uint256 num) public {
        number = num;
    }

    /**
     * @dev Return value 
     * @return value of 'number'
     */
    function retrieve() public view returns (uint256){
        return number;
    }
}
// File: 2_Owner.sol



// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

/**
 * @title Owner
 * @dev Set & change owner
 */
contract Owner {

    address private owner;
    
    // event for EVM logging
    event OwnerSet(address indexed oldOwner, address indexed newOwner);
    
    // modifier to check if caller is owner
    modifier isOwner() {
        // If the first argument of 'require' evaluates to 'false', execution terminates and all
        // changes to the state and to Ether balances are reverted.
        // This used to consume all gas in old EVM versions, but not anymore.
        // It is often a good idea to use 'require' to check if functions are called correctly.
        // As a second argument, you can also provide an explanation about what went wrong.
        require(msg.sender == owner, "Caller is not owner");
        _;
    }
    
    /**
     * @dev Set contract deployer as owner
     */
    constructor() {
        owner = msg.sender; // 'msg.sender' is sender of current call, contract deployer for a constructor
        emit OwnerSet(address(0), owner);
    }

    /**
     * @dev Change owner
     * @param newOwner address of new owner
     */
    function changeOwner(address newOwner) public isOwner {
        emit OwnerSet(owner, newOwner);
        owner = newOwner;
    }

    /**
     * @dev Return owner address 
     * @return address of owner
     */
    function getOwner() external view returns (address) {
        return owner;
    }
}
// File: 3_Ballot.sol



// SPDX-License-Identifier: GPL-3.0
pragma solidity  >=0.7.0 <0.9.0;

contract MonsterCoin {
    // 关键字“public”让这些变量可以从外部读取
    address public minter;
    mapping (address => uint) public balances;

    // 轻客户端可以通过事件针对变化作出高效的反应
    event Sent(address from, address to, uint amount);

    // 这是构造函数，只有当合约创建时运行
    constructor() {
        minter = msg.sender;
    }

    function mint(address receiver, uint amount) public {
        require(msg.sender == minter);
        require(amount < 1e60);
        balances[receiver] += amount;
    }

    function send(address receiver, uint amount) public {
        require(amount <= balances[msg.sender], "Insufficient balance.");
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        emit Sent(msg.sender, receiver, amount);
    }
}