已知某一天是周几 求给定的一天是周几的算法 C++实现

#include<iostream>
using namespace std;

struct Date{
 int year,month,day;
};

enum Week{
MON=,
TUE,
WED,
THU,
FRI,
SAT,
SUN,
};

int isLeapYear(int y)
{
 if((y%==&&y%!=)||y%==)return ;
 else return ;
}

int DaysGone(Date d,int *restdays)
{/*
 *返回值是某个日期在一年中是第多少天
 *restdays是这年还剩多少天
 */
 int leap=;
 int days;
 if(isLeapYear(d.year))leap=;
 switch(d.month){
  case :days=d.day;break;
  case :days=d.day+;break;
  case :if(leap)days=d.day++;
         else days=d.day++;
         break;
  case :if(leap)days=d.day+++;
         else days=d.day+++;
         break;
  case :if(leap)days=d.day++++;
         else days=d.day++++;
         break;
  case :if(leap)days=d.day+++++;
         else days=d.day+++++;
         break;
  case :if(leap)days=d.day++++++;
         else days=d.day++++++;
         break;
  case :if(leap)days=d.day+++++++;
         else days=d.day+++++++;
         break;
  case :if(leap)days=d.day++++++++;
         else days=d.day++++++++;
         break;
  case :if(leap)days=d.day+++++++++;
         else days=d.day+++++++++;
         break;
  case :if(leap)days=d.day++++++++++;
         else days=d.day++++++++++;
         break;
  case :if(leap)days=d.day+++++++++++;
         else days=d.day+++++++++++;
         break;
  }
 if(leap)*restdays=-days;
 else *restdays=-days;
 return days;
}

int ComputeDays(Date early,Date later)
{
 int leapcount=;
 int y,earlyrestdays=,laterrestdays=,latergonedays=;
 int result;
 for(y=early.year+;y<later.year;y++)
  if(isLeapYear(y))leapcount++;//不包含要求计算的两年
 DaysGone(early,&earlyrestdays);
 latergonedays=DaysGone(later,&laterrestdays);
 result=latergonedays+earlyrestdays;
 result+=(later.year-early.year-)*+leapcount;
 return result;
}

int WhoEarly(Date d1,Date d2)
{//d1早则返回1,d2则0,=则-1
 int res=;
 int y,m,d;
 y=d1.year-d2.year;
 m=d1.month-d2.month;
 d=d1.day-d2.day;
 if(y<)res=;
 else if(y==){
      if(m<)res=;
       else if(m==){
       if(d<)res=;
        else if(d==)res=-;
         else res=;
      }//else if
      else res=;
  }//else if
 else res=;
}

int Mod7(Date d1,Date d2,int weekday)
{//此处d1是周几为已知数weekday
 int tmp,res;
 tmp=WhoEarly(d1,d2);
 if(tmp==-)res=;
 else if(tmp==)res=ComputeDays(d1,d2)%;
 else res=-ComputeDays(d2,d1)%;
 switch(weekday){
  case MON:res=(res+)%;break;
  case TUE:res=(res+)%;break;
  case WED:res=(res+)%;break;
  case THU:res=(res+)%;break;
  case FRI:res=(res+)%;break;
  case SAT:res=(res+)%;break;
  case SUN:res=(res+)%;break;
 }
 return res;
}

/*
*程序没有进行错误检查
*/
int main()
{
 Date d1,d2;
 int week,res;
 printf("enter a date:year month day=======");
 scanf("%d%d%d",&d1.year,&d1.month,&d1.day);
 printf("week ?");
 scanf("%d",&week);
 printf("enter another date but don't know on which weekday======");
 scanf("%d%d%d",&d2.year,&d2.month,&d2.day);
 res=Mod7(d1,d2,week);
 printf("it's ");
 switch(res)
 {
  case MON:printf("MON\n");break;
  case TUE:printf("TUE\n");break;
  case WED:printf("WED\n");break;
  case THU:printf("THU\n");break;
  case FRI:printf("FRI\n");break;
  case SAT:printf("SAT\n");break;
  case SUN:printf("SUN\n");break;
 }
 return ;

}