// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

/**
 * 合约包含IERC20接口，代表满足标准
 */
interface IERC20 {
    /**
     * 当前合约的 token 总量
     */
    function totalSupply() external view returns (uint);

    /**
     * 某一个账户的当前余额
     */
    function balanceOf(address account) external view returns (uint);

    /**
     * 把账户中的余额，由调用者发送到另一个账户中，并向链外汇报事件
     */
    function transfer(address recipient, uint amount) external returns (bool);

    /**
     * 查询某账户向另一账户批准额度有多少
     */
    function allowance(address owner, address spender) external view returns (uint);

    /**
     * 把账户中的数量批准给另一个账户
     */
    function approve(address spender, uint amount) external returns (bool);

    /**
     * 向另一个合约存款的时候，另一个合约需要调用 transferFrom 把token拿到它的合约中
     */
    function transferFrom(address sender, address recipient, uint amount) external returns (bool);

    event Transfer(address indexed from, address indexed to, uint amount);

    event Approval(address indexed owner, address indexed spender, uint amount);

}


contract ERC20 is IERC20 {
    // 当前合约的token总量
    uint public totalSupply;
    // 账本映射
    mapping(address => uint) public balanceOf;
    // 批准映射
    mapping(address => mapping(address => uint)) public allowance;
    // token name
    string public name = "Test";
    string public symbol = "TEST";
    // token 精度
    uint8 public decimals = 18;

    /**
     * 把账户中的余额，由调用者发送到另一个账户中，并向链外汇报事件
     */
    function transfer(address recipient, uint amount) external returns (bool) {
        // 发送者减掉数量
        balanceOf[msg.sender] -= amount;
        // 接受者增加数量
        balanceOf[recipient] += amount;
        emit Transfer(msg.sender, recipient, amount);
        return true;
    }

    /**
     * 把账户中的数量批准给另一个账户
     */
    function approve(address spender, uint amount) external returns (bool) {
        allowance[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    /**
     * 向另一个合约存款的时候，另一个合约需要调用 transferFrom 把token拿到它的合约中
     */
    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;
    }

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

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