pragma solidity ^0.4.21;
contract Token{
    uint256 public totalSupply;

    function balanceOf(address _owner) public constant returns (uint256 balance);
    function transfer(address _to, uint256 _value) public returns (bool success);
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
    function approve(address _spender, uint256 _value) public returns (bool success);
    function allowance(address _owner, address _spender) public constant returns (uint256 remaining);

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

contract CYFToken is Token {

    string public name = "乞力马扎罗的雪CYF";                   //名称
    uint8 public decimals = 18;               //返回token使用的小数点后几位。比如如果设置为3，就是支持0.001表示.
    string public symbol = "CYF";               //token简称

    mapping (address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;

    function CYFToken() public {
        totalSupply = 7000000000 * (10 ** (uint256(decimals)));         // 设置初始总量
        balances[msg.sender] = totalSupply; // 初始token数量给予消息发送者，因为是构造函数，所以这里也是合约的创建者
    }

    function transfer(address _to, uint256 _value) public returns (bool success) {
        //默认totalSupply 不会超过最大值 (2^256 - 1).
        //如果随着时间的推移将会有新的token生成，则可以用下面这句避免溢出的异常
        require(balances[msg.sender] >= _value && balances[_to] + _value > balances[_to]);
        require(_to != 0x0);
        balances[msg.sender] -= _value;//从消息发送者账户中减去token数量_value
        balances[_to] += _value;//往接收账户增加token数量_value
        emit Transfer(msg.sender, _to, _value);//触发转币交易事件
        return true;
    }
    function transferFrom(address _from, address _to, uint256 _value) public returns 
    (bool success) {
        require(balances[_from] >= _value && allowed[_from][msg.sender] >= _value);
        balances[_to] += _value;//接收账户增加token数量_value
        balances[_from] -= _value; //支出账户_from减去token数量_value
        allowed[_from][msg.sender] -= _value;//消息发送者可以从账户_from中转出的数量减少_value
        emit Transfer(_from, _to, _value);//触发转币交易事件
        return true;
    }
    function balanceOf(address _owner) public constant returns (uint256 balance) {
        return balances[_owner];
    }
    function approve(address _spender, uint256 _value) public returns (bool success)   
    { 
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }
    function allowance(address _owner, address _spender) public constant returns (uint256 remaining) {
        return allowed[_owner][_spender];//允许_spender从_owner中转出的token数
    }
}