@(135- Block Chain| 區塊鏈)

Introduction

• 如何通過數據結構優化降低合約執行成本
• 合約的繼承
• 巧用modifier
• 以太坊函數庫的使用和基本介紹

如何減少gas 消耗？
(本質為節約計算資源，降低時間復雜度的問題)
數組越大，遍歷所需資源越多
遍歷數組 traverse array——> 映射 Mapping

1 Mapping in Solidity

類比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只能作為合約的成員變量，而不能做本地局部變量

  2 Mapping底層實現

• 不使用數組+鏈表，不需要擴容
• hash函數為keccak256hash（keccak 即SHA- 3）
• 在storage 上存儲，理論上為無限大的hash表
  *** 無法naive地遍歷整個mapping**
• 賦值 employees[key] = value
• 取值 value = employees[key]
• value 是引用，在storage上存儲，可以直接修改

• 當key 不存在，value = type‘s default // 不會拋出異常

3 函數返回進階

• 命名參數返回
• 命名返回參數直接賦值

1.命名參數返回

    function checkEmployee(address employeeId) returns(uint salary,uint lastPayday){ //quary the information of employees
    //name the returned parameter 命名參數返回 
    

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

}

2.命名返回參數直接賦值

等效代碼

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

4 可視度

4.1 可視度

• public ：誰都可見
• external：只有“外部調用”可見(特殊可視度)
• internal：外部調用不可見，內部和子類可見（類比c++,protected）
• private：只有當前合約可見

4.2 變量與可見度

• 狀態變量：public,internal,private
  • 默認：internal
  • public：自動定義取值函數
  • private:不代表別人無法看到，只代表別的區塊鏈智能合約無法看到

合約的所有成員變量都是肉眼可見的！！！

• 函數 ：public,external,internal,private
  • 默認public

5 繼承

5.1 繼承-基本語法

private 對繼承類不可見

5.2 繼承-抽象合約

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

5.3 繼承-interface

• 類比 Java 相似
  只是一個告訴之後程序員的一個框架

pragma solidity ^0.4.0;
interface Parent{
//不可繼承其他合約或Interface
//沒有構造函數
//沒有狀態變量
//沒有struct
//沒有enum
//簡單來說，只有function定義，啥都沒有
    function someFunc() returns (uint);
}

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

5.4 多繼承

• 重名函數的override 次序
  ```solidity
  contract Base1{
  function func1(){}
  }

contract Base2{
function func1(){}
}

contract Final is Base1,Base2 {
}

contract test{
Final f = new Final();
f.func1();//Base2 的function1被調用（從後往前，由於先繼承base1，再繼承base2）
}

```

• Super:動態綁定上級函數

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();
}



//調用順序：Base2.func1,Base1.func1,foundation.func1

多繼承 Method Resolution Order使用O3 Linearization

• 與python 相同
• 不能成環路

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;
    //下劃線等效加入一句 modify a=1;
    return a;
    }
}

7 Safe Math 和 Library

7.1 加減乘除在solidity中很危險

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


OUTPUT:整型溢出
a: uint8: 156

手動解決方法_silly

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

庫函數 Zppelin-solidity

• 工程師——學習代碼重用
• 不要重復造輪子
  ```
  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之外的語句
    // }
    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註意整除
            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 較多，不推薦
        return calculateRunway() > 0; //vm jump 操作,使用gas較少,推薦
    }
    
    function checkEmployee(address employeeId) returns(uint salary,uint lastPayday){ //quary the information of employees
    //name the returned parameter 命名參數返回 
    
        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;
         //每一次運算都是真金白銀~
         //原則：不重復運算！——省gas
        assert(nextPayday < now);
        // if( nextPayday > now){
        //     revert();
              //throw or revert
            //throw: 所有的gas 均會被消耗殆盡
            //revert：回滾，return沒有消耗的gas
           
        // }
        
            employees[msg.sender].lastPayday = nextPayday;//原則：先修改內部變量，再給錢——》之後會講，安全問題
            employee.id.transfer(employee.salary);

    }
}

參考閱讀：老董-以太坊智能合約全棧開發

135.003 智能合約後端優化和產品化