dow() by Tomohiko Sakamoto

dow.c

// https://www.zhihu.com/question/30262900/answer/2863211053
// Tomohiko Sakamoto 算法，来确定当前日期是星期几

int dow(int y, int m, int d) { 
  static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4}; 
  y -= m < 3; 
  return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7; 
}

// 同时，Tomohiko Sakamoto 还发布了另外一个更简洁的版本：
dow(m,d,y) { y-=m<3; return(y+y/4-y/100+y/400+"-bed=pen+mad."[m]+d)%7; }