university/S1/GdCP/uebung/day.c

#include<stdio.h>

// Lists to define the calendar
char *day[7] = {"Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag", "Sonntag"};
int months[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int leap = 29;

// Check for leap year
int isleap(int y) {
  return ((y % 4 == 0 && y % 100 != 0) || y % 400 == 0);
}

int main() {
  // Stating date for the 1.1.1600 is a Saturday
  int n = 5;

  // Read in the date
  int d, m, y;
  scanf("%d.%d.%d", &d, &m, &y);
  // Check for errors
  if (d < 1 || m < 1 || m > 12 || d > months[m-1] || y < 1600 || y > 4000) return 1;

  // Get the number of days passed
  for (int i = 1600; i < y; i++) {
    if (isleap(i)) n+= 366;
    else n += 365;
  }
  for (int i = 0; i < m - 1; i++) {
    if (isleap(y)) {
      if (i == 1) n += leap;
      else n += months[i];
    } else n += months[i];
  }
  n += d - 1;

  // Print the day
  printf("%s\n", day[n % 7]);
}