Assignment #2

pragma solidity ^0.4.14;

contract Payroll {
    struct Employee {
        address id;
        uint salary;
        uint lastPayday;
    }
    
    uint constant payDuration = 10 seconds;
    
    //added
    uint totalSalary_New = 0 ether; 

    address owner;
    Employee[] employees;

    function Payroll() {
        owner = msg.sender;
    }
    
    function _partialPaid(Employee employee) private {
        uint payment = employee.salary * (now - employee.lastPayday) / payDuration;
        employee.id.transfer(payment);
    }
    
    function _findEmployee(address employeeId) private returns (Employee, uint) {
        for(uint i = 0; i<employees.length;i++){
            if(employees[i].id == employeeId){
                return (employees[i],i);
            }
        }
    }

    function addEmployee(address employeeId, uint salary) {
        require(msg.sender == owner);
        var(employee,index) = _findEmployee(employeeId);
        assert(employee.id == 0x0);
        employees.push(Employee(employeeId,salary * 1 ether,now));
        //added
        totalSalary_New += salary;
    }
    
    function removeEmployee(address employeeId) {
        require(msg.sender == owner);
        var (employee,index) = _findEmployee(employeeId);
        assert(employee.id != 0x0);
        _partialPaid(employee);
        //added
        totalSalary_New -= employees[index].salary;
        delete employees[index];
        employees[index] = employees[employees.length-1];
        employees.length -=1;
    }
    
    function updateEmployee(address employeeId, uint salary) {
        require(msg.sender == owner);
        var(employee,index) = _findEmployee(employeeId);
        assert(employee.id != 0x0);
        
        _partialPaid(employee);
        //added
        totalSalary_New -= employees[index].salary;
        employees[index].salary = salary * 1 ether;
        employees[index].lastPayday = now;
        //added
        totalSalary_New += employees[index].salary;
    }
    
    function addFund() payable returns (uint) {
        return this.balance;
    }
    
    function calculateRunway_Old() returns (uint) {
        uint totalSalary = 0;
       for (uint i = 0; i < employees.length; i++) {
            totalSalary += employees[i].salary;
        }
        return this.balance / totalSalary;
    }
    
    //added
    function calculateRunway_New() returns (uint) {
        return this.balance / totalSalary_New;
    }
    
    function hasEnoughFund() returns (bool) {
        return calculateRunway_New() > 0;
    }
    
    function getPaid() {
        var(employee,index) = _findEmployee(msg.sender);
        assert(employee.id != 0x0);
        
        uint nextPayday = employee.lastPayday + payDuration;
        assert(nextPayday < now);

        employees[index].lastPayday = nextPayday;
        employee.id.transfer(employee.salary);
    }
}

//优化前calculateRunway gas记录
//1:
// "0x14723a09acff6d2a60dcdf7aa4aff308fddc160c",1
// transaction cost 	22966 gas 
// execution cost 	1694 gas 
//2:
// "0x4b0897b0513fdc7c541b6d9d7e929c4e5364d2db",1
// transaction cost 	23747 gas 
// execution cost 	2475 gas 
//3:
// "0x583031d1113ad414f02576bd6afabfb302140225",1
// transaction cost 	24528 gas 
// execution cost 	3256 gas 
//4:
// "0xdd870fa1b7c4700f2bd7f44238821c26f7392148",1
// transaction cost 	25309 gas 
// execution cost 	4037 gas 
//5:
// "0x14723a09acff6d2a60dcdf7aa4aff308fddc160d",1
// transaction cost 	26090 gas 
// execution cost 	4818 gas 
//6:
// "0x4b0897b0513fdc7c541b6d9d7e929c4e5364d2dc",1
// transaction cost 	26871 gas 
// execution cost 	5599 gas 
//7:
// "0x583031d1113ad414f02576bd6afabfb302140226",1
// transaction cost 	27652 gas 
// execution cost 	6380 gas 
//8:
// "0xdd870fa1b7c4700f2bd7f44238821c26f7392149",1
// transaction cost 	28433 gas 
// execution cost 	7161 gas 
//9:
// "0x14723a09acff6d2a60dcdf7aa4aff308fddc160e",1
// transaction cost 	29214 gas 
// execution cost 	7942 gas 
//10:
// "0x4b0897b0513fdc7c541b6d9d7e929c4e5364d2de",1
// transaction cost 	29995 gas 
// execution cost 	8723 gas 

//每次调用calculateRunway，gas都会增加，因为for循环遍历数组，随着员工数的增加，
//for循环执行次数增加

//优化方案：定义totalSalary_New变量，在addEmployee、removeEmployee、updateEmployee
//中更改此变量，可以避免在calculateRunway_New中使用for循环

//优化后，每次运行calculateRunway_New的gas为：
//transaction cost 	22146 gas 
//execution cost 	874 gas