/* cal.cpp - Calendar program.  Matt Mahoney, mmahoney@cs.fit.edu

This program prints a calendar for any given month and year, for
instance:

  cal Feb 2002        Prints a calendar for February 2002
  cal Mar             Prints a calendar for March of the current year
  cal                 Prints a calendar for the current month and year
  cal +10             Prints a calendar for 10 months from now
  cal -20             Prints a calendar for 20 months ago

The name of the month can be any unambiguous prefix, and is not
case sensitive, for example "F" or "febr" for February, or "Ja" (but
not "J") for January.  The year must include all digits.  Assumes
Georgian calendar back to Jan. 1 AD; prior years are not valid.
*/

#include <iostream>
#include <iomanip>
#include <string>
#include <cctype>
#include <ctime>
#include <cstdlib>
using namespace std;

// true if s1 is a prefix of s2, not case sensitive
bool prefix(const string& s1, const char* s2) {
  for (int i=0;; ++i) {
    if (i==int(s1.size()))
      return true;
    if (s2[i]==0 || tolower(s1[i])!=tolower(s2[i]))
      return false;
  }
}

// A Month represents a month and year, which prints as a calendar
class Month {
public:
  Month(const string& m="", int y=0); // Defaults: this month and year
    // m is the month name or prefix, not case sensitive
  class Ambiguous {};  // Thrown if m matches >1 month name, e.g. "JU"
  class Invalid {};  // Thrown if m matches no month name, e.g. "X"
  void print(ostream& out) const;  // Print a one month calendar to out
  void add(int n) {now+=n;}  // Add n months (may be negative)
private:
  int now;  // year * 12 + month (0=Jan, 11=Dec), e.g. 24001 means Feb. 2000
  static const char* const monthNames[12];  // Table of month names
};
const char* const Month::monthNames[12] = {"January", "February", "March",
  "April", "May", "June", "July", "August", "September", "October",
  "November", "December"};

Month::Month(const string& m, int y) {  // m = prefix of month name, y = year
  int mon=12;  // Get month as 0-11 (0=Jan, 11=Dec), 12 = unknown
  if (y<1) {   // Default year: use current year
    time_t t=time(0);
    tm* tp=localtime(&t);
    y=1900+tp->tm_year;
    if (m=="")  // Default month: use current month
      mon=tp->tm_mon;
  }

  // Look up the month name if not ""
  if (mon==12) {
    for (int i=0; i<12; ++i) {  // Find all matches of m to monthNames
      if (prefix(m, monthNames[i])) {
        if (mon==12)  // First matching prefix
          mon=i;
        else  // Second matching prefix
          throw Ambiguous();
      }
    }
    if (mon==12)
      throw Invalid();  // No match found
  }
  now=y*12+mon;
}

// Print the calendar
void Month::print(ostream& out) const {
  if (now<12) throw Invalid();  // Must be at least Jan 1 AD.
  const int year=now/12;
  const int mon=now%12;  // Month, 0 = Jan, ... 11 = Dec

  // Find weekday of first day of month: 0 = Sun, 1 = Mon,... 6 = Sat.
  const int len[] = {31,28,31,30,31,30,31,31,30,31,30,31};  // Days in month
  const bool leapyear = year%400==0 || (year%4==0 && year%100!=0);
  int weekday=year*365+year/4-year/100+year/400; // Days from year 0 to Jan 1
  for (int i=0; i<mon; ++i)  // Add days this year until first of this month
    weekday+=len[i];
  if (leapyear && mon<2)  // Adjust for Jan. or Feb. of leapyear
    --weekday;
  weekday%=7;

  // Print the calendar
  out << "\n" << monthNames[mon] << " " << year << "\n"
      << "Su Mo Tu We Th Fr Sa\n";
  for (int i=0; i<weekday; ++i)
    out << "   ";
  for (int i=1; i <= len[mon]+(mon==1 && leapyear); ++i) {
    out << setw(2) << i << " ";
    if ((i+weekday)%7==0)
      out << "\n";
  }
  out << endl;
}

int main(int argc, char** argv) {
  try {
    Month m;  // Current month and year
    if (argc>1 && (argv[1][0]=='+' || argv[1][0]=='-')) // cal +n or -n
      m.add(atoi(argv[1]));
    else if (argc > 2)
      m = Month(argv[1], atoi(argv[2]));  // cal month year
    else if (argc == 2)
      m = Month(argv[1]);  // cal month
    m.print(cout);
  }
  catch (Month::Ambiguous x) {
    cerr << "Month name is ambiguous\n";
  }
  catch (Month::Invalid x) {
    cerr << "Month name is invalid\n";
  }
  return 0;
}