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Calendar Calculation in C++

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Brute-force calculation of any date. Includes some very tricky rules for modern-day Gregorian calendar.

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  1. #include <iostream>
  2. #include <fstream>
  3. #include <string>
  4.  
  5. using namespace std;
  6.  
  7. bool isLeapYear(int year) {
  8.     bool retval = false;
  9.     if (year % 100 == 0) {
  10.         if (year % 400 == 0) {
  11.             retval = true;
  12.         }
  13.     } else if (year % 4 == 0) {
  14.         retval = true;
  15.     }
  16.     return retval;
  17. }
  18.  
  19. int daysInMonth(int month, int year) {
  20.     switch (++month) {
  21.         case 4:
  22.         case 6:
  23.         case 9:
  24.         case 11:
  25.             return 30;
  26.             break;
  27.         case 2:
  28.             if (isLeapYear(year)) {
  29.                 return 29;
  30.             } else return 28;
  31.             break;
  32.         default:
  33.             return 31;
  34.             break;
  35.     }
  36. }
  37.  
  38. int getFirstDayOfMonth(int month, int year, int day) {
  39.     int retval = -1;
  40.     int numdays;
  41.     if (month == 0) {
  42.         numdays = daysInMonth(11, year-1);
  43.     } else {
  44.         numdays = daysInMonth(month-1, year);
  45.     }
  46.     int offset = numdays % 7;
  47.     if (month == 0 && year == 1900) {
  48.         retval = 1;
  49.     } else {
  50.         retval = (day + offset) % 7;
  51.     }
  52.  
  53.     return retval;
  54. }
  55.  
  56. int getThirteenth(int firstday) {
  57.     return ((13 - firstday) % 7);
  58. }
  59.  
  60. int main() {
  61.     ofstream fout ("friday.out");
  62.     ifstream fin ("friday.in");
  63.  
  64.     int numYears;
  65.     fin >> numYears;
  66.     int year = 1900;
  67.     int firstDay[numYears][12];
  68.     int numThirteenths[7];
  69.     int day;
  70.  
  71.     for (int z = 0; z < 7; ++z) numThirteenths[z] = 0;
  72.  
  73.     for (int x = 0; x < numYears; ++x) {
  74.         for (int month = 0; month < 12; month++) {
  75.             if (year == 1900 && month == 0) {
  76.                 firstDay[x][month] = 1;
  77.  
  78.             } else if (month == 0) {
  79.                 firstDay[x][month] = getFirstDayOfMonth(month, year, firstDay[x-1][11]);
  80.             } else {
  81.                 firstDay[x][month] = getFirstDayOfMonth(month, year, firstDay[x][month-1]);
  82.             }
  83.             ++numThirteenths[((firstDay[x][month] + 5) % 7)];
  84.         }
  85.         ++year;
  86.     }
  87.  
  88.     fout << numThirteenths[6] << " ";
  89.     for (int y = 0; y < 5; ++y) fout << numThirteenths[y] << " ";
  90.     fout << numThirteenths[5] << endl;
  91.  
  92.     return 0;
  93. }

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