第2章 重新组织函数（3）：引入解释性变量、分解临时变量和移除对参数的赋值

//引入解释性变量
//重构前
if((platform.toUpperCase().indexOf("MAC") > -1) &&
   (browser.toUpperCase().indexOf("IE") > -1) &&
    wasInitialized() && resize > 0)
{
    //do something
}

//重构后
const bool isMacOs     = platform.toUpperCase().indexOf("MAC") > -1;
const bool isIEBrowser = browser.toUpperCase().indexOf("IE") > -1;
const bool wasResized = resize > 0; 

if (isMacOs && isIEBrowser && wasInitialized() && wasResized())
{
    //do something
}

//重构前(其中的_quantity和_itemPrice为类的成员变量)
double price()
{
    //价格 = 底价(basePrice) - 批发折扣(quantity discount) + 运费(shipping)
    //底价 = 数量(quantity) * 单价(itemPrice) 
    return  _quantity * _itemPrice -  //底价
            Math.max(0, _quantity - 500) * _itemPrice * 0.05 +
            Math.min(_quantity * _itemPrice * 0.1, 100.0);            
}

//重构中(引入解释性变量)
double price()
{
    const double basePrice = _quantity * _itemPrice;
    const double quantityDiscount = Math.max(0, _quantity - 500) * _itemPrice * 0.05;
    const double shipping = Math.min(basePrice * 0.1, 100.0);
    
    return basePrice - quantityDiscount + shipping;
}

//运用ExtractMethod方法对上述范例进行重构（去除上述的解释性变量，即临时变量）
double price()
{
    return basePrice() - quantityDiscount() + shipping();
}

//底价
double basePrice()
{
    return _quantity * _itemPrice;
}
//批发折扣
double quantityDiscount()
{
    return Math.max(0, _quantity - 500) * _itemPrice * 0.05;
}

//运费
double shipping()
{
    return Math.min(basePrice * 0.1, 100.0);
}

//分解临时变量
//重构前
double temp = 2 * (_height + _width);  //周长
cout << temp << endl;

temp = _height * _width; //面积
cout << temp << endl;

//重构后
const double perimeter = 2 * (_height + _width);  //周长
cout << perimeter << endl;

const double area = _height * _width; //面积
cout << area << endl;

//分解临时变量
//场景：根据牛顿第二定律计算物理从静止开始运动，在
//指定时间内的运动距离（分两个阶段）
//第1阶段：在一个力的作用下。第2个阶段在两个力的共同作用下）
double getDistanceTravelled(int time)
{
    double result = 0.0;
    double acc = _primaryForce / _mass; //F = ma;注意acc被赋值两次。
    int primaryTime = Math.min(time, _delay);
    result = 0.5 * acc * primaryTime * primaryTime;
    
    int secondaryTime = time - _delay;
    if(secondaryTime > 0)
    {
        double primaryVel = acc * _delay; //初始速
        acc = (_primaryForce + _secondaryForce) / _mass;
        
        result += primaryVel * secondaryTime +
                  0.5 * acc * secondaryTime * secondaryTime;
    }
    
    return result;
}

//重构中（对第1次赋值重构）
double getDistanceTravelled(int time)
{
    double result = 0.0;
    const double primaryAcc = _primaryForce / _mass; //F = ma;注意acc被赋值两次。
    int primaryTime = Math.min(time, _delay);
    result = 0.5 * primaryAcc * primaryTime * primaryTime;
    
    int secondaryTime = time - _delay;
    if(secondaryTime > 0)
    {
        double primaryVel = primaryAcc * _delay; //初始速
        double acc = (_primaryForce + _secondaryForce) / _mass;
        
        result += primaryVel * secondaryTime +
                  0.5 * acc * secondaryTime * secondaryTime;
    }
    
    return result;
}
//重构后
double getDistanceTravelled(int time)
{
    double result = 0.0;
    const double primaryAcc = _primaryForce / _mass; //F = ma;注意acc被赋值两次。
    int primaryTime = Math.min(time, _delay);
    result = 0.5 * primaryAcc * primaryTime * primaryTime;
    
    int secondaryTime = time - _delay;
    if(secondaryTime > 0)
    {
        double primaryVel = primaryAcc * _delay; //初始速
        const double secondaryAcc = (_primaryForce + _secondaryForce) / _mass;
        
        result += primaryVel * secondaryTime +
                  0.5 * secondaryAcc * secondaryTime * secondaryTime;
    }
    
    return result;
}

void aMethod(Object* foo)
{
    foo->modifyInSomeWay(); //没问题
    foo = anotherObject;    //对参数赋值！
}

//对参数赋值
//重构前(注意第1个参数为引用类型)
int discount (int& inputVal, int quantity, int yearToDate)
{
    if(inputVal > 50) inputVal = -2;
    if(quantity > 100) inputVal = -1;
    if(yearToDate > 10000) inputVal = -4;
    return inputVal;
}

//重构后
int discount (int& inputVal, int quantity, int yearToDate)
{
    int result = inputVal;
    if(inputVal > 50) result = -2;
    if(quantity > 100) result = -1;
    if(yearToDate > 10000) result = -4;
    return result;
}