求星期公式

今天写嵌入式程序，不能开太多变量，而且最好不能对原来的变量修改，所以求星期就一句话搞定

#define week   (int )((month<=2?year-1:year)%100+ (month<=2?year-1:year)%100 / 4 + ((month<=2?year-1:year)/100) / 4 - 2*( (month<=2?year-1:year)/100)+ 26 * ( (month<=2?month+12:month)+1) / 10 + day - 1)%7;