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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3.  
  4. // ASSUMPTIONS
  5. // 
  6. // - I use the definition of perfect month that's on its wikipedia which states that
  7. //   it's gotta be in Feb, in non leap year, and the 1st feb starts on either sunday or monday
  8. //   depends on each person's first day on their own calendar
  9. // 
  10. // - I made an assumption that the date input from user will always be the first of january of that year
  11. //   which means, if a user put 2026 as the input, I'll assume it's the 1st January of 2026
  12. //   making the next perfect month will be 02-2026 and the prev one will be 2015-02 (if we use sunday)
  13.  
  14. int firstDayOfCalendar = 0; // default = sunday
  15.  
  16. // sakamoto algo's to find the day of the week of a certain date
  17. int getDayOfTheWeek(int y, int m, int d){
  18.  
  19.     int t[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
  20.  
  21.     if(m < 3){
  22.         y -= 1;
  23.     }
  24.  
  25.     // sunday: 0 ; monday: 1 ; and so on ...
  26.     return ((y + y / 4 - y / 100 + y / 400 + t[m - 1] + d) % 7);
  27.  
  28. }
  29.  
  30. bool isLeapYear(int year){
  31.     return(((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0));
  32. }
  33.  
  34. void nearestPerfectMonth(int year){
  35.     int prev = -1;
  36.     int next = -1;
  37.     bool foundPrev = false;
  38.     bool foundNext = false;
  39.     int i = 0;
  40.  
  41.     while(!(foundPrev && foundNext)){
  42.  
  43.         if(!foundPrev && !isLeapYear(year - i -1) && getDayOfTheWeek((year - i -1), 2, 1) == firstDayOfCalendar){
  44.             prev = year - i -1;
  45.             foundPrev =true;
  46.         }
  47.  
  48.         if(!foundNext && !isLeapYear(year + i) && getDayOfTheWeek((year + i), 2, 1) == firstDayOfCalendar){
  49.             next = year + i;
  50.             foundNext = true;
  51.         }
  52.  
  53.         i++;
  54.     }
  55.  
  56.     cout << "> { prev: \"" << prev << "-02\", next: \"" << next << "-02\" }";
  57.  
  58. }
  59.  
  60. int main(){
  61.     int year;
  62.     string firstDay;
  63.     cout << "Enter the first day on your calendar (monday/sunday): ";
  64.     cin >> firstDay;
  65.     transform(firstDay.begin(), firstDay.end(), firstDay.begin(), ::tolower);
  66.     if(firstDay == "monday"){
  67.         firstDayOfCalendar = 1;
  68.     }
  69.     else if(firstDay == "sunday"){
  70.         firstDayOfCalendar = 0;
  71.     }
  72.     else{
  73.         cerr << "invalid first day of calendar, defaulted into sunday\n";
  74.     }
  75.  
  76.     cout << "Enter the year: ";
  77.     cin >> year;
  78.     nearestPerfectMonth(year);
  79.  
  80.     return 0;
  81. }
Success #stdin #stdout 0.01s 5292KB
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stdin
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sunday
2026
stdout
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Enter the first day on your calendar (monday/sunday): Enter the year: > { prev: "2015-02", next: "2026-02" }
https://ideone.com/pZvjqX
language:
C++ (gcc 8.3)
created:
7 days ago
visibility:
secret

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