きったんの頭_ん中☆

/*
 * pe19.c
 * https://mind.kittttttan.info/c/pe19
 */
#include <stdio.h>
#include <stdlib.h>

enum {
  SUN,
  MON,
  TUE,
  WED,
  THU,
  FRI,
  SAT
};

int isLeapYear(int year) {
  return (year & 3) == 0 && (year % 100 || (year % 400) == 0);
}

int daysOfMonth(int year, int month) {
  switch (month) {
    case 2:
      return isLeapYear(year) ? 29 : 28;
    case 4:
    case 6:
    case 9:
    case 11:
      return 30;
  }
  
  return 31;
}

void pe19(int n) {
  int sum = 0;
  int dow = TUE;  // 1901-01-01 is Tuesday
  int year;
  int month;

  for (year = 1901; year <= n; ++year) {
    for (month = 1; month <= 12; ++month) {
      dow += daysOfMonth(year, month);
      dow %= 7;
      if (dow == SUN) ++sum;
    }
  }
  
  printf("%d (1901 to %d)\n", sum, n);
}

int main(int argc, char* argv[]) {
  puts("Probrem 19:");
  pe19(argc > 1 ? atoi(argv[1]) : 2000);

  return 0;
}