hdu 2133 What day is it(蔡勒公式)

http://acm.hdu.edu.cn/showproblem.php?pid=2133

　　蔡勒公式求某一天是周几。

/*** 蔡勒公式，求某一天是周几（hdu 2133）***/
#include <cstdio>

const char *ans[7] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

int isLeap(int y) { // 判断是否闰年
    return !(y % 4) && (y % 100) || !(y % 400);
}

int mod(int a, int b) { // 保证取模后是正数
    return (a % b + b) % b;
}
int main() {
    int y, m, d, c, w;

    while (~scanf("%d%d%d", &y, &m, &d)) {
        if (d <= 0 || m <= 0 || d >= 10000 || m > 12 || d > 31) {
            printf("illegal\n");
            continue;
        }
        if (!isLeap(y) && m == 2 && d > 28) {
            printf("illegal\n");
            continue;
        }
        if (isLeap(y) && m == 2 && d > 29) {
            printf("illegal\n");
            continue;
        }
        if ((m == 4 || m == 6 || m == 9 || m == 11) && d > 30) {
            printf("illegal\n");
            continue;
        }
        if (m == 1 || m == 2) m += 12, y--;
        /*** 蔡勒公式 ***/
        c = y / 100;
        y %= 100;
        w = (c >> 2) - (c << 1) + y + (y >> 2) + 13 * (m + 1) / 5 + d - 1;
        printf("%s\n", ans[mod(w, 7)]);
        /****************/
    }
    return 0;
}

——written by Lyon