euler19

You are given the following information, but you may prefer to do some research for yourself.

• 1 Jan 1900 was a Monday.
• Thirty days has September,
      April, June and November.
      All the rest have thirty-one,
      Saving February alone,
      Which has twenty-eight, rain or shine.
      And on leap years, twenty-nine.
• A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
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#include <stdio.h>

int dayofmouth(int year, int mouth)
{
    int ret;

    switch(mouth) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            ret =  31;
            break;
        case 4:
        case 6:
        case 9:
        case 11:
            ret =  30;
            break;
        case 2:
            ret = (!(year%4) && (year%100) || !(year%400))?29:28;
            break;
        default:
            ret = -1;
    }

    return ret;
}

int main(int argc, const char *argv[])
{
    int remain;
    int i, j;
    int result = 0;

    remain = 365 % 7;    /* 1900 */

    for (i=1901; i<2001; i++) {
        for (j=1; j<=12; j++) {
            remain = (remain + dayofmouth(i, j)) % 7;
            if(remain == 6)
                result++;
        }
    }

    printf("result: %d\n", result);

    return 0;
}