#include <u.h>
#include <libc.h>
#include <bio.h>

char	dayw[] =
{
	" S  M Tu  W Th  F  S"
};
char	*smon[] =
{
	"January", "February", "March", "April",
	"May", "June", "July", "August",
	"September", "October", "November", "December",
};
char	mon[] =
{
	0,
	31, 29, 31, 30,
	31, 30, 31, 31,
	30, 31, 30, 31,
};
char	string[432];
Biobuf	bout;

void	main(int argc, char *argv[]);
int	number(char *str);
void	pstr(char *str, int n);
void	cal(int m, int y, char *p, int w);
int	jan1(int yr);
int	curmo(void);
int	curyr(void);

void
main(int argc, char *argv[])
{
	int y, i, j, m;

	if(argc > 3) {
		fprint(2, "usage: cal [month] [year]\n");
		exits("usage");
	}
	Binit(&bout, 1, OWRITE);

/*
 * no arg, print current month
 */
	if(argc == 1) {
		m = curmo();
		y = curyr();
		goto xshort;
	}

/*
 * one arg
 *	if looks like a month, print month
 *	else print year
 */
	if(argc == 2) {
		y = number(argv[1]);
		if(y < 0)
			y = -y;
		if(y >= 1 && y <= 12) {
			m = y;
			y = curyr();
			goto xshort;
		}
		goto xlong;
	}

/*
 * two arg, month and year
 */
	m = number(argv[1]);
	if(m < 0)
		m = -m;
	y = number(argv[2]);
	goto xshort;

/*
 *	print out just month
 */
xshort:
	if(m < 1 || m > 12)
		goto badarg;
	if(y < 1 || y > 9999)
		goto badarg;
	Bprint(&bout, "   %s %ud\n", smon[m-1], y);
	Bprint(&bout, "%s\n", dayw);
	cal(m, y, string, 24);
	for(i=0; i<6*24; i+=24)
		pstr(string+i, 24);
	exits(0);

/*
 *	print out complete year
 */
xlong:
	y = number(argv[1]);
	if(y<1 || y>9999)
		goto badarg;
	Bprint(&bout, "\n\n\n");
	Bprint(&bout, "                                %ud\n", y);
	Bprint(&bout, "\n");
	for(i=0; i<12; i+=3) {
		for(j=0; j<6*72; j++)
			string[j] = '\0';
		Bprint(&bout, "         %.3s", smon[i]);
		Bprint(&bout, "                    %.3s", smon[i+1]);
		Bprint(&bout, "                    %.3s\n", smon[i+2]);
		Bprint(&bout, "%s   %s   %s\n", dayw, dayw, dayw);
		cal(i+1, y, string, 72);
		cal(i+2, y, string+23, 72);
		cal(i+3, y, string+46, 72);
		for(j=0; j<6*72; j+=72)
			pstr(string+j, 72);
	}
	Bprint(&bout, "\n\n\n");
	exits(0);

badarg:
	Bprint(&bout, "cal: bad argument\n");
}

struct
{
	char*	word;
	int	val;
} dict[] =
{
	"jan",		1,
	"january",	1,
	"feb",		2,
	"february",	2,
	"mar",		3,
	"march",	3,
	"apr",		4,
	"april",	4,
	"may",		5,
	"jun",		6,
	"june",		6,
	"jul",		7,
	"july",		7,
	"aug",		8,
	"august",	8,
	"sep",		9,
	"sept",		9,
	"september",	9,
	"oct",		10,
	"october",	10,
	"nov",		11,
	"november",	11,
	"dec",		12,
	"december",	12,
	0
};

/*
 * convert to a number.
 * if its a dictionary word,
 * return negative  number
 */
int
number(char *str)
{
	int n, c;
	char *s;

	for(n=0; s=dict[n].word; n++)
		if(strcmp(s, str) == 0)
			return -dict[n].val;
	n = 0;
	s = str;
	while(c = *s++) {
		if(c<'0' || c>'9')
			return 0;
		n = n*10 + c-'0';
	}
	return n;
}

void
pstr(char *str, int n)
{
	int i;
	char *s;

	s = str;
	i = n;
	while(i--)
		if(*s++ == '\0')
			s[-1] = ' ';
	i = n+1;
	while(i--)
		if(*--s != ' ')
			break;
	s[1] = '\0';
	Bprint(&bout, "%s\n", str);
}

void
cal(int m, int y, char *p, int w)
{
	int d, i;
	char *s;

	s = p;
	d = jan1(y);
	mon[2] = 29;
	mon[9] = 30;

	switch((jan1(y+1)+7-d)%7) {

	/*
	 *	non-leap year
	 */
	case 1:
		mon[2] = 28;
		break;

	/*
	 *	1752
	 */
	default:
		mon[9] = 19;
		break;

	/*
	 *	leap year
	 */
	case 2:
		;
	}
	for(i=1; i<m; i++)
		d += mon[i];
	d %= 7;
	s += 3*d;
	for(i=1; i<=mon[m]; i++) {
		if(i==3 && mon[m]==19) {
			i += 11;
			mon[m] += 11;
		}
		if(i > 9)
			*s = i/10+'0';
		s++;
		*s++ = i%10+'0';
		s++;
		if(++d == 7) {
			d = 0;
			s = p+w;
			p = s;
		}
	}
}

/*
 *	return day of the week
 *	of jan 1 of given year
 */
int
jan1(int yr)
{
	int y, d;

/*
 *	normal gregorian calendar
 *	one extra day per four years
 */

	y = yr;
	d = 4+y+(y+3)/4;

/*
 *	julian calendar
 *	regular gregorian
 *	less three days per 400
 */

	if(y > 1800) {
		d -= (y-1701)/100;
		d += (y-1601)/400;
	}

/*
 *	great calendar changeover instant
 */

	if(y > 1752)
		d += 3;

	return d%7;
}

/*
 * system dependent
 * get current month and year
 */
int
curmo(void)
{
	Tm *tm;

	tm = localtime(time(0));
	return tm->mon+1;
}

int
curyr(void)
{
	Tm *tm;

	tm = localtime(time(0));
	return tm->year+1900;
}