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120 lines
3.2 KiB
C
120 lines
3.2 KiB
C
#include <stdio.h>
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/**
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* returns number of days in a month.
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* Month is identified by an integer -
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* 0 = Jan and 11 = December
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* For February, adjust for leap year outside the function.
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**/
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char get_month_days(short month)
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{
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if (month == 1) /* February has 28 days. Adjust leap year in the loop */
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return 28;
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else if (month <= 6) /* odd months till July have 30 days - Jan = 0 (even)*/
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{
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if (month & 0x01)
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return 30;
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else
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return 31;
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} else if (month >= 7) /* odd months after July have 31 days*/
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{
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if (month & 0x01)
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return 31;
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else
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return 30;
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}
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/* should never reach here! */
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perror("Should never have reached this point!\n");
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return -1;
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}
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/**
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* return 1 if input year is a leap year
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* otherwise, return 0
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**/
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char is_leap_year(short year)
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{
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if ((year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0)))
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return 1;
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return 0;
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}
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#ifdef DEBUG
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const char* day_string(int day)
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{
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switch(day)
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{
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case 0:
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return "Sunday";
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case 1:
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return "Monday";
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case 2:
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return "Tuesday";
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case 3:
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return "Wednesday";
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case 4:
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return "Thursday";
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case 5:
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return "Friday";
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case 6:
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return "Saturday";
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default:
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return "Shouldnt see this!";
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}
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}
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#endif
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int main(int argc, char **argv)
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{
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int count_sundays = 0;
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const short start_year = 1901;
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const short end_year = 2000;
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/**
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* Let us identify days i.e., Sunday thru Saturday with integers - 0 thru 6 respectively
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* Jan 1 1901 was a Tuesday
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**/
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char start_day = 2;
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for (int year = start_year; year <= end_year; year++)
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{
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char is_leap = is_leap_year(year);
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for (char month = 0; month < 12; month ++)
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{
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/**
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* These two for-loops count the start of day for the next month. Hence,
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* we have to skip the last December count */
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if (year == end_year && month == 11)
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continue;
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int days = get_month_days(month);
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if (is_leap && month == 1) /* for a leap year february, add a day */
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days ++;
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#ifdef DEBUG
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if (year == end_year)
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{
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printf("Year: %d\t Month: %d\t Days: %d\t First of day: %s\n", year, month, days, day_string(start_day));
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}
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#endif
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/** Main Algorithm:
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* every week has 7 days hence, the start of next day would be modulo 7
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* add to this, the current start date and ensure the result is still
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* modulo 7!
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**/
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start_day = ((days % 7) + start_day) % 7;
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/* If start-day is a Sunday, increment counter */
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if (start_day == 0)
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count_sundays ++;
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}
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}
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printf("Total number of Sundays that happened on the 1st of a month in the last century: %d\n", count_sundays);
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return 0;
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} |