findweekday.c

#include <stdio.h>

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

int dateDiff(struct Date, struct Date);
int yearDays(int);
int monthDays(int, int);
int whichWeekday(struct Date);
int isLeapYear(int);
int print(int);



int main(){
    int n, i, results[1000];
    struct Date date;

    scanf("%d", &n);
    for(i=0; i<n; i++){
        scanf("%d %d %d", &date.year, &date.month, &date.day);
        results[i] = whichWeekday(date);
    }
    
    for(i=0; i<n; i++){
        print(results[i]);
    }
}

int print(int wday){
    char *t[] = {
        "Sun.", "Mon.", "Tue.", "Wed.", "Thu.", "Fri.", "Sat."
    };
    printf("%s\n", t[wday]);
}

int whichWeekday(struct Date d){
    struct Date compare = {
        .year = 2016,
        .month = 12,
        .day = 18
    };
    int cweekday = 0; // sunday
    int diff = dateDiff(d, compare);
    int offset, ret;
    offset = diff % 7;
    ret = cweekday + offset;
    if(ret > 6){
        ret -= 7;
    }
    if(ret < 0){
        ret += 7;
    }
    return ret;
}

int dateDiff(struct Date d1, struct Date d2){
    int i;
    long dd1 = 0, dd2 = 0;
    for(i=0; i<d1.year; i++) dd1 += yearDays(i);
    for(i=1; i<d1.month; i++) dd1 += monthDays(d1.year, i);
    dd1 += d1.day;
    
    for(i=0; i<d2.year; i++) dd2 += yearDays(i);
    for(i=1; i<d2.month; i++) dd2 += monthDays(d2.year, i);
    dd2 += d2.day;
    return dd1 - dd2;;
}


int yearDays(int year){
    if(isLeapYear(year)) return 366;
    return 365;
}

int monthDays(int year, int month){
    if(
        1 == month || 3 == month || 5 == month || 7 == month ||
        8 == month || 10 == month || 12 == month
    ){
        return 31;
    } else if(2 == month){
        return isLeapYear(year) ? 29 : 28;
    } else {
        return 30;
    }
}

int isLeapYear(int year){
    if(0 == year % 400){
        return 1;
    } else if (0 == year % 100){
        return 0;
    } else {
        return (0 == year % 4);
    }
}