31.VC(custom)=怎么得到执行复杂的算法时需要的时间量

第一种方法:

dwOldTime = GetTickCount();
DoSomething();
dwTimeElapsed = GetTickCount() – dwOldTime;

第二种方法:

class CStopwatch 
{
public:
   CStopwatch() 
   { 
      QueryPerformanceFrequency(&m_liPerfFreq);
      Start();
   }

   void Start() 
   { 
      QueryPerformanceCounter(&m_liPerfStart);
   }

   __int64 Now() const 
   {
      //Returns # of milliseconds since
      //Start was called

      LARGE_INTEGER liPerfNow;
      QueryPerformanceCounter(&liPerfNow);

      return (((liPerfNow.QuadPart - 
         m_liPerfStart.QuadPart) * 1000)/
         m_liPerfFreq.QuadPart);
   }

private:

   //Counts per second
   LARGE_INTEGER m_liPerfFreq;   

   //Starting count
   LARGE_INTEGER m_liPerfStart;  
};

使用方法:

//Create a stopwatch timer
//(which defaults to the current time).
CStopwatch stopwatch;

//Execute the code I want to profile here.

//Get how much time has elapsed up to now.
  __int64 qwElapsedTime = stopwatch.Now();

//qwElapsedTime indicates how long 
//the profiled code executed in milliseconds.

QueryPerformanceFrequency() - 基本介绍

类型：Win32API

原型：BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);

作用：返回硬件支持的高精度计数器的频率。

返回值：非零，硬件支持高精度计数器；零，硬件不支持，读取失败。

QueryPerformanceFrequency() - 技术特点
函数的原形是：

供WIN9X使用的高精度定时器：QueryPerformanceFrequency()和QueryPerformanceCounter()，要求计算机从硬件上支持高精度定时器。

BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);
BOOL QueryPerformanceCounter (LARGE_INTEGER *lpCount);

数据类型LARGEINTEGER既可以是一个作为8字节长的整数，也可以是作为两个4字节长的整数的联合结构，其具体用法根据编译器是否支持64位而定。该类型的定义如下：

typeef union _ LARGE_INTEGER
　　{
　　 struct
　　 {
　　 DWORD LowPart;
　　 LONG HighPart;
　　 };
　　 LONGLONG QuadPart;
　　} LARGE_INTEGER;

在定时前应该先调用QueryPerformanceFrequency()函数获得机器内部计时器的时钟频率。接着在需要严格计时的事件发生前和发生之后分别调用QueryPerformanceCounter()，利用两次获得的计数之差和时钟频率，就可以计算出事件经历的精确时间。测试函数SLEEP(100)的精确持续时间方法：

　　LARGE_INTEGER litmp;
　　LONGLONG qt1,qt2;
　　double dft,dff,dfm;
　　QueryPerformanceFrequency(&litmp);//获得时钟频率
　　dff=(double)litmp.QuadPart;
　　QueryPerformanceCounter(&litmp);//获得初始值
　　qt1=litmp.QuadPart;Sleep(100);
　　QueryPerformanceCounter(&litmp);//获得终止值
　　qt2=litmp.QuadPart;
　　dfm=(double)(qt2-qt1);
　　dft=dfm/dff;//获得对应的时间值

需要注意的是DFT计算的结果单位是秒。
第三种方法:

__int64 FileTimeToQuadWord(PFILETIME pft) 
{
   return (Int64ShllMod32(
      pft->dwHighDateTime, 32) | pft->dwLowDateTime);
}

void PerformLongOperation() 
{
   FILETIME ftKernelTimeStart, ftKernelTimeEnd;
   FILETIME ftUserTimeStart,   ftUserTimeEnd;

   FILETIME ftDummy;
   __int64 qwKernelTimeElapsed, qwUserTimeElapsed,
      qwTotalTimeElapsed;

   //Get starting times.
   GetThreadTimes(GetCurrentThread(), &ftDummy,
      &ftDummy, &ftKernelTimeStart, &ftUserTimeStart);

   //Perform complex algorithm here.

   //Get ending times.
   GetThreadTimes(GetCurrentThread(), &ftDummy,
      &ftDummy, &ftKernelTimeEnd, &ftUserTimeEnd);

   //Get the elapsed kernel and user times by 
   //converting the start and end times 
   //from FILETIMEs to quad words, and then 
   //subtract the start times from the end times.

   qwKernelTimeElapsed = 
      FileTimeToQuadWord(&ftKernelTimeEnd) -
      FileTimeToQuadWord(&ftKernelTimeStart);

   qwUserTimeElapsed = 
      FileTimeToQuadWord(&ftUserTimeEnd) -
      FileTimeToQuadWord(&ftUserTimeStart);

   //Get total time duration by adding the kernel
   //and user times.

   qwTotalTimeElapsed = qwKernelTimeElapsed + 
      qwUserTimeElapsed;

   //The total elapsed time is in 
   //qwTotalTimeElapsed.
}