135.003 智能合约后端优化和产品化

@(135- Block Chain| 区块链)

Introduction

• 如何通过数据结构优化降低合约执行成本
• 合约的继承
• 巧用modifier
• 以太坊函数库的使用和基本介绍

如何减少gas 消耗？
(本质为节约计算资源，降低时间复杂度的问题)
数组越大，遍历所需资源越多
遍历数组 traverse array——> 映射 Mapping

1 Mapping in Solidity

类比map(c++),dict(python)
Hash table
Key - Value

• types of Key in solidity
  （bool,int ,address,string）

• types of Value in solidity
  （any type）

• usage
  mapping (address=>Employee)employees

• mapping只能作为合约的成员变量，而不能做本地局部变量

2 Mapping底层实现

• 不使用数组+链表，不需要扩容
• hash函数为keccak256hash（keccak 即SHA- 3）
• 在storage 上存储，理论上为无限大的hash表
  *** 无法naive地遍历整个mapping**
• 赋值 employees[key] = value
• 取值 value = employees[key]
• value 是引用，在storage上存储，可以直接修改
• 当key 不存在，value = type's default // 不会抛出异常

3 函数返回进阶

• 命名参数返回
• 命名返回参数直接赋值

1.命名参数返回

    function checkEmployee(address employeeId) returns(uint salary,uint lastPayday){ //quary the information of employees
    //name the returned parameter 命名参数返回 
    

OUTPUT
｛
"0": "uint256: salary 1000000000000000000",
"1": "uint256: lastPayday 1530411588"
}

2.命名返回参数直接赋值

等效代码

        // return (employee.salary,employee.lastPayday);
        salary = employee.salary;
        lastPayday = employee.lastPayday;

4 可视度

4.1 可视度

• public ：谁都可见
• external：只有“外部调用”可见(特殊可视度)
• internal：外部调用不可见，内部和子类可见（类比c++,protected）
• private：只有当前合约可见

4.2 变量与可见度

• 状态变量：public,internal,private
  • 默认：internal
  • public：自动定义取值函数
  • private:不代表别人无法看到，只代表别的区块链智能合约无法看到

合约的所有成员变量都是肉眼可见的！！！

• 函数 ：public,external,internal,private
  • 默认public

5 继承

5.1 继承-基本语法

private 对继承类不可见

5.2 继承-抽象合约

contract Parent{
	function someFunc() returns (uint);
}
contract Child is Parent{
	function someFunc() returns (uint){
		return 1;
	}
}

5.3 继承-interface

• 类比 Java 相似
  只是一个告诉之后程序员的一个框架

pragma solidity ^0.4.0;
interface Parent{
//不可继承其他合约或Interface
//没有构造函数
//没有状态变量
//没有struct
//没有enum
//简单来说，只有function定义，啥都没有
	function someFunc() returns (uint);
}

contract Child is Parent{
	function someFunc() returns (uint){
		return 1;
	}
}

5.4 多继承

• 重名函数的override 次序

contract Base1{
	function func1(){}
}

contract Base2{
	function func1(){}
}

contract Final is Base1,Base2 {
}

contract test{
	Final f = new Final();
	f.func1();//Base2 的function1被调用（从后往前，由于先继承base1，再继承base2）
}

• Super:动态绑定上级函数

contract foundation{
	function func1{
	//do stuff
	}
}


contract Base1 is foundation{
	function func1(){
		super.func1();
	}
}

contract Base2 is foundation{
	function func1(){
	super.func1();
	}
}

contract Final is Base1,Base2{
	Final f = new Final();
	f.func1();
}



//调用顺序：Base2.func1,Base1.func1,foundation.func1

多继承 Method Resolution Order使用O3 Linearization

• 与python 相同
• 不能成环路

6 Modifier

pragma solidity ^0.4.0
contract Parent {
	uint public a=2;
	modifier someModifier(){
		_;
		a = 1;
	}
	function parentFunc2(uint value) someModifer public returns(uint){
	a = value;
	//下划线等效加入一句 modify a=1;
	return a;
	}
}

7 Safe Math 和 Library

7.1 加减乘除在solidity中很危险

contract Test {
	uint8 public a = 0;
	function set(){
		a -= 100;
	}
}


OUTPUT:整型溢出
a: uint8: 156

手动解决方法_silly

contract Test {
	uint8 public a = 0;
	function set(){
		uint c = a - 100;
		assert(c<a);
		a = c;
	}
}

库函数 Zppelin-solidity

• 工程师——学习代码重用
• 不要重复造轮子

solidity ^0.4.0;

import './SafeMath.sol'; //local file,download from github

contract Test {
	using SafeMath for uint8;
	uint8 public a = 0;
	function set(){
		a = a.sub(100);
		//a = SafeMath.sub(a,100);
	}
}

8 Code in Remix (Solidity IDE)

pragma solidity ^0.4.14;

import './SafeMath.sol';
import './Ownable.sol'; //Github - Zppelin-solidity


contract Payroll{
    using SafeMath for uint;
    
    struct Employee{
        address id;
        uint salary;
        uint lastPayday;
    }
    
    
  // uint constant payDuration = 30 days;
   uint constant payDuration = 10 seconds;//for test:10 seconds
   
   uint totalSalary;
   address owner;
   mapping(address => Employee)public employees;
   
   
    // function Payroll(){//construction function
    // owner = msg.sender; // in ownable.sol
    // }
    
    // modifier onlyOwner{ //in Ownable.sol
    //     require(msg.sender == owner);
    //     _; //represent the function been modified， 除了return之外的语句
    // }
    modifier employeeExist(address employeeId){
        var employee = employees[employeeId];
        assert(employee.id != 0x0);//confirm that exist
        _;
    }
    function _partialPaid(Employee employee) private{
            uint payment = employee.salary * (now - employee.lastPayday) /payDuration;//if exist , calculate payment注意整除
            employee.id.transfer(payment);
    }
    

    function addEmployee(address employeeId,uint salary)onlyOwner{
       // require(msg.sender == owner);//whether is onwner
        var employee = employees[employeeId];//var - any type
        assert(employee.id == 0x0);//confirm that exist
        totalSalary += salary *1 ether;
        employees[employeeId] = Employee(employeeId,salary* 1 ether,now);
    }
    
    function removeEmployee(address employeeId)onlyOwner employeeExist(employeeId){
        // require(msg.sender == owner);//whether is onwner
        //  var (employee,index) = _findEmloyee(employeeId); 
        var employee = employees[employeeId];
        _partialPaid(employee);
        totalSalary -=employees[employeeId].salary;
        delete employees[employeeId];//left with a blank in the array,wasteful
}
    
    function updateEmployee(address employeeId,uint salary)onlyOwner employeeExist(employeeId){
        //require(msg.sender == owner);
        //Equivalently 等效 
        // if (msg.sender != owner){//avoid employee cheating
        //     revert();
        // }
         var employee = employees[employeeId];
        _partialPaid(employee);
        totalSalary -= employees[employeeId].salary;
        employees[employeeId].salary = salary *1 ether;
        totalSalary += employees[employeeId].salary;
        employees[employeeId].lastPayday = now;     
         

    }
    
    function addFund() payable returns(uint){
        return this.balance;//address.balance
    }
    
    
    function calculateRunway()returns(uint)
    { //how many times left to pay
        return this.balance / totalSalary; 
    }
    function hasEnoughFund() returns(bool){
        // return this.balance >=salary;
        //return this.calculateRunway() > 0; //this方法 使用的gas 较多，不推荐
        return calculateRunway() > 0; //vm jump 操作,使用gas较少,推荐
    }
    
    function checkEmployee(address employeeId) returns(uint salary,uint lastPayday){ //quary the information of employees
    //name the returned parameter 命名参数返回 
    
        var employee = employees[employeeId];
        // return (employee.salary,employee.lastPayday);
        salary = employee.salary;
        lastPayday = employee.lastPayday;
    }
    
    function getPaid() employeeExist(msg.sender){

         var employee = employees[msg.sender];
         //assert(employee.id != 0x0);//confirm that exist
        uint nextPayday = employee.lastPayday + payDuration;
         //每一次运算都是真金白银~
         //原则：不重复运算！——省gas
        assert(nextPayday < now);
        // if( nextPayday > now){
        //     revert();
              //throw or revert
            //throw: 所有的gas 均会被消耗殆尽
            //revert：回滚，return没有消耗的gas
           
        // }
        
            employees[msg.sender].lastPayday = nextPayday;//原则：先修改内部变量，再给钱——》之后会讲，安全问题
            employee.id.transfer(employee.salary);

    }
}

参考阅读：老董-以太坊智能合约全栈开发