在Solidity中实现代币买卖及ETH兑换代币功能的开发咨询

你需要在原有ERC20合约基础上新增汇率配置、收款自动兑换逻辑、权限控制和提现功能，以下是适配Solidity 0.4.22版本的完整可运行代码：

pragma solidity 0.4.22;

library SafeMath { 
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
      assert(b <= a);
      return a - b;
    }
    
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
      uint256 c = a + b;
      assert(c >= a);
      return c;
    }
}

contract ERC20Basic {
    string public constant name = "Community Token";
    string public constant symbol = "COMM";
    uint8 public constant decimals = 1;  

    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
    event Transfer(address indexed from, address indexed to, uint tokens);
    // 新增兑换成功事件
    event TokensPurchased(address indexed buyer, uint256 ethAmount, uint256 tokenAmount);

    mapping(address => uint256) balances;
    mapping(address => mapping (address => uint256)) allowed;
    
    uint256 totalSupply_;
    using SafeMath for uint256;

    // 新增：合约所有者地址
    address public owner;
    // 新增：ETH兑换代币汇率，示例为1ETH兑换1000枚COMM，可自行调整
    uint256 public constant ETH_TO_TOKEN_RATE = 1000;

    // 新增：仅所有者可调用的修饰符
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }

    constructor(uint256 total) public {  
        totalSupply_ = total;
        balances[msg.sender] = totalSupply_;
        owner = msg.sender;
    }  

    // 新增：用户直接向合约转ETH时自动触发兑换的fallback函数
    function () payable public {
        buyTokens();
    }

    // 新增：主动调用兑换代币函数
    function buyTokens() public payable {
        // 检查用户转入的ETH大于0
        require(msg.value > 0);
        // 计算可兑换的代币数量
        uint256 tokenAmount = msg.value.mul(ETH_TO_TOKEN_RATE).div(1 ether);
        // 检查合约有足够的代币可以兑付
        require(balances[address(this)] >= tokenAmount);
        
        // 从合约地址向用户转代币
        balances[address(this)] = balances[address(this)].sub(tokenAmount);
        balances[msg.sender] = balances[msg.sender].add(tokenAmount);
        emit Transfer(address(this), msg.sender, tokenAmount);
        emit TokensPurchased(msg.sender, msg.value, tokenAmount);
    }

    // 新增：所有者提取合约内的ETH
    function withdrawETH() public onlyOwner {
        owner.transfer(address(this).balance);
    }

    // 新增：所有者提取合约内剩余的COMM代币
    function withdrawRemainingTokens() public onlyOwner {
        uint256 remaining = balances[address(this)];
        require(remaining > 0);
        balances[address(this)] = balances[address(this)].sub(remaining);
        balances[owner] = balances[owner].add(remaining);
        emit Transfer(address(this), owner, remaining);
    }

    function totalSupply() public view returns (uint256) {
        return totalSupply_;
    }
    
    function balanceOf(address tokenOwner) public view returns (uint) {
        return balances[tokenOwner];
    }

    function transfer(address receiver, uint numTokens) public returns (bool) {
        require(numTokens <= balances[msg.sender]);
        balances[msg.sender] = balances[msg.sender].sub(numTokens);
        balances[receiver] = balances[receiver].add(numTokens);
        emit Transfer(msg.sender, receiver, numTokens);
        return true;
    }

    function approve(address delegate, uint numTokens) public returns (bool) {
        allowed[msg.sender][delegate] = numTokens;
        emit Approval(msg.sender, delegate, numTokens);
        return true;
    }

    function allowance(address owner, address delegate) public view returns (uint) {
        return allowed[owner][delegate];
    }

    function transferFrom(address owner, address buyer, uint numTokens) public returns (bool) {
        require(numTokens <= balances[owner]);    
        require(numTokens <= allowed[owner][msg.sender]);
    
        balances[owner] = balances[owner].sub(numTokens);
        allowed[owner][msg.sender] = allowed[owner][msg.sender].sub(numTokens);
        balances[buyer] = balances[buyer].add(numTokens);
        emit Transfer(owner, buyer, numTokens);
        return true;
    }
}

• 兑换逻辑支持两种使用方式：用户直接向合约地址转账ETH自动兑换，或者主动调用buyTokens方法完成兑换，两种方式都会按照预设汇率自动计算到账的代币数量
• 汇率参数ETH_TO_TOKEN_RATE可自行调整，示例值1000对应1ETH兑换1000枚COMM，如需调整直接修改该常量的数值即可
• 新增了TokensPurchased事件，可用于链上查询兑换记录，方便做数据统计
• 所有兑换逻辑都使用了原有的SafeMath库做运算，避免溢出问题

注意：合约部署完成后，需要先从部署者地址向合约地址转入足够的COMM代币，否则用户兑换时会因为合约代币余额不足触发交易回滚。如果后续需要调整兑换规则，可基于当前逻辑扩展汇率修改、兑换限额等功能。

内容的提问来源于stack exchange，提问作者Nalks_