#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

struct tm *proceedMonth1stDay(struct tm *t);
struct tm *getCurrentLocalTimeStructure();
struct tm *proceedInput(struct tm *t);
int proceedCalender(int y, int m, int start);

int main()
{
	struct tm *t = proceedMonth1stDay(NULL);
	printf("\n\n\t\t");
	while(1)
	{
		proceedCalender(t->tm_year + 1900, t->tm_mon + 1, t->tm_wday + 1);
		t = proceedInput(t);
	}
	return 1;
}

struct tm *getCurrentLocalTimeStructure()
{
	static time_t currTimeStamp;
	time(&currTimeStamp);
	return localtime(&currTimeStamp);
}

struct tm *proceedMonth1stDay(struct tm *t)
{
	if(t == NULL) t = getCurrentLocalTimeStructure();
	t->tm_mday = 1;
	mktime(t);
	return t;
}

struct tm *proceedInput(struct tm *t)
{
	char c;
	
	printf("COMMAND : ");
	c = getchar();
	
		if(c == '1' || c == '[') t->tm_mon--;
	else	if(c == '3' || c == ']') t->tm_mon++;
	else	if(c == '7' || c == '-') t->tm_year--;
	else	if(c == '9' || c == '+') t->tm_year++;
	else	if(c == 'Q' || c == 'q') exit(0);
	
	if(t->tm_mon == 12)
	{
		t->tm_year++;
		t->tm_mon = 0;
	}
	if(t->tm_mon == -1)
	{
		t->tm_year--;
		t->tm_mon = 11;
	}
	
	return proceedMonth1stDay(t);
}

int proceedCalender(int y, int m, int start)
{
	static int monthDays[12] = {31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	int i;
	if(((y % 4 == 0) && !(y % 100 == 0)) || (y % 400 == 0)) monthDays[2 - 1] = 29;
	else monthDays[2 - 1] = 28;
	printf("\n\n\t\t\t*** CALENDER ***");
	printf("\n\n\t\t\t\t\t%4d/%02d", y, m);
	printf("\n\n\t\tSUN MON TUE WED THU FRI SAT\n\t\t\n\t\t");
	for(i = 1; i <= start - 1; i++) printf("    ");
	for(i = 1; i <= monthDays[m - 1]; i++, start++)
	{
		printf("%3d ", i);
		if(start % 7 == 0) printf("\n\t\t");
	}
	printf("\n\n\t\t");
	return 1;
}