/* cal - print a calendar		Author: Maritn Minow */

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#define do3months	domonth
#define	IO_SUCCESS	0	/* Unix definitions		 */
#define	IO_ERROR	1
#define	EOS	0

#define	ENTRY_SIZE	3	/* 3 bytes per value		 */
#define DAYS_PER_WEEK	7	/* Sunday, etc.			 */
#define	WEEKS_PER_MONTH	6	/* Max. weeks in a month	 */
#define	MONTHS_PER_LINE	3	/* Three months across		 */
#define	MONTH_SPACE	3	/* Between each month		 */

char *badarg = {"Bad argument\n"};
char *how = {"Usage: cal [month] year\n"};

/* Calendar() stuffs data into layout[],
 * output() copies from layout[] to outline[], (then trims blanks).
 */
char layout[MONTHS_PER_LINE][WEEKS_PER_MONTH][DAYS_PER_WEEK][ENTRY_SIZE];
char outline[(MONTHS_PER_LINE * DAYS_PER_WEEK * ENTRY_SIZE)
       + (MONTHS_PER_LINE * MONTH_SPACE)
       + 1];

char *weekday = " S  M Tu  W Th  F  S";
char *monthname[] = {
	     "???",		/* No month 0	 */
	     "Jan", "Feb", "Mar", "Apr", "May", "Jun",
	     "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
};

_PROTOTYPE(int main, (int argc, char **argv));
_PROTOTYPE(void doyear, (int year));
_PROTOTYPE(void domonth, (int year, int month));
_PROTOTYPE(void output, (int nmonths));
_PROTOTYPE(void calendar, (int year, int month, int indx));
_PROTOTYPE(void usage, (char *s));
_PROTOTYPE(int date, (int year, int month, int week, int wday));
_PROTOTYPE(void setmonth, (int year, int month));
_PROTOTYPE(int getdate, (int week, int wday));
_PROTOTYPE(static int Jan1, (int year));

int main(argc, argv)
int argc;
char *argv[];
{
  register int year;

  register int arg1val;
  int arg1len;
  int arg2val;

  if (argc <= 1) {
	usage(how);
  } else {
	arg1val = atoi(argv[1]);
	arg1len = strlen(argv[1]);
	if (argc == 2) {
		/* Only one argument, if small, it's a month.  If
		 * large, it's a year.  Note: cal		0082	Year
		 * 0082 cal		82	Year 0082 */
		if (arg1len <= 2 && arg1val <= 12)
			do3months(year, arg1val);
		else
			doyear(arg1val);
	} else {
		/* Two arguments, allow 1980 12 or 12 1980 */
		arg2val = atoi(argv[2]);
		if (arg1len > 2)
			do3months(arg1val, arg2val);
		else
			do3months(arg2val, arg1val);
	}
  }
  return(IO_SUCCESS);
}

void doyear(year)
int year;
/* Print the calendar for an entire year. */
{
  register int month;

  if (year < 1 || year > 9999) usage(badarg);
  if (year < 100)
	printf("\n\n\n                                 00%2d\n\n", year);
  else
	printf("\n\n\n%35d\n\n", year);
  for (month = 1; month <= 12; month += MONTHS_PER_LINE) {
	printf("%12s%23s%23s\n",
	       monthname[month],
	       monthname[month + 1],
	       monthname[month + 2]);
	printf("%s   %s   %s\n", weekday, weekday, weekday);
	calendar(year, month + 0, 0);
	calendar(year, month + 1, 1);
	calendar(year, month + 2, 2);
	output(3);
#if MONTHS_PER_LINE != 3
#error  "the above will not work"
#endif
  }
  printf("\n\n\n");
}

void domonth(year, month)
int year;
int month;
/* Do one specific month -- note: no longer used */
{
  if (year < 1 || year > 9999) usage(badarg);
  if (month <= 0 || month > 12) usage(badarg);
  printf("%9s%5d\n\n%s\n", monthname[month], year, weekday);
  calendar(year, month, 0);
  output(1);
  printf("\n\n");
}

void output(nmonths)
int nmonths;			/* Number of months to do	 */
/* Clean up and output the text. */
{
  register int week;
  register int month;
  register char *outp;
  int i;
  char tmpbuf[21], *p;

  for (week = 0; week < WEEKS_PER_MONTH; week++) {
	outp = outline;
	for (month = 0; month < nmonths; month++) {
		/* The -1 in the following removes the unwanted
		 * leading blank from the entry for Sunday. */
		p = &layout[month][week][0][1];
		for (i = 0; i < 20; i++) tmpbuf[i] = *p++;
		tmpbuf[20] = 0;
		sprintf(outp, "%s   ", tmpbuf);
		outp += (DAYS_PER_WEEK * ENTRY_SIZE) + MONTH_SPACE - 1;
	}
	while (outp > outline && outp[-1] == ' ') outp--;
	*outp = EOS;
	printf("%s\n", outline);
  }
}

void calendar(year, month, indx)
int year;
int month;
int indx;			/* Which of the three months		 */
/* Actually build the calendar for this month. */
{
  register char *tp;
  int week;
  register int wday;
  register int today;

  setmonth(year, month);
  for (week = 0; week < WEEKS_PER_MONTH; week++) {
	for (wday = 0; wday < DAYS_PER_WEEK; wday++) {
		tp = &layout[indx][week][wday][0];
		*tp++ = ' ';
		today = getdate(week, wday);
		if (today <= 0) {
			*tp++ = ' ';
			*tp++ = ' ';
		} else if (today < 10) {
			*tp++ = ' ';
			*tp = (today + '0');
		} else {
			*tp++ = (today / 10) + '0';
			*tp = (today % 10) + '0';
		}
	}
  }
}

void usage(s)
char *s;
{
/* Fatal parameter error. */

  fprintf(stderr, "%s", s);
  exit(IO_ERROR);
}

/* Calendar routines, intended for eventual porting to TeX
 *
 * date(year, month, week, wday)
 *	Returns the date on this week (0 is first, 5 last possible)
 *	and day of the week (Sunday == 0)
 *	Note: January is month 1.
 *
 * setmonth(year, month)
 *	Parameters are as above, sets getdate() for this month.
 *
 * int
 * getdate(week, wday)
 *	Parameters are as above, uses the data set by setmonth()
 */

/* This structure is used to pass data between setmonth() and getdate().
 * It needs considerable expansion if the Julian->Gregorian change is
 * to be extended to other countries.
 */

static struct {
  int this_month;		/* month number used in 1752 checking	 */
  int feb;			/* Days in February for this month	 */
  int sept;			/* Days in September for this month	 */
  int days_in_month;		/* Number of days in this month		 */
  int dow_first;		/* Day of week of the 1st day in month	 */
} info;

static int day_month[] = {	/* 30 days hath September...		 */
		  0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};

int date(year, month, week, wday)
int year;			/* Calendar date being computed		 */
int month;			/* January == 1				 */
int week;			/* Week in the month 0..5 inclusive	 */
int wday;			/* Weekday, Sunday == 0			 */
/* Return the date of the month that fell on this week and weekday.
 * Return zero if it's out of range.
 */
{
  setmonth(year, month);
  return(getdate(week, wday));
}

void setmonth(year, month)
int year;			/* Year to compute		 */
int month;			/* Month, January is month 1	 */
/* Setup the parameters needed to compute this month
 * (stored in the info structure).
 */
{
  register int i;

  if (month < 1 || month > 12) {/* Verify caller's parameters	 */
	info.days_in_month = 0;	/* Garbage flag			 */
	return;
  }
  info.this_month = month;	/* used in 1752	checking	 */
  info.dow_first = Jan1(year);	/* Day of January 1st for now	 */
  info.feb = 29;		/* Assume leap year		 */
  info.sept = 30;		/* Assume normal year		 */
  /* Determine whether it's an ordinary year, a leap year or the
   * magical calendar switch year of 1752. */
  switch ((Jan1(year + 1) + 7 - info.dow_first) % 7) {
      case 1:			/* Not a leap year		 */
	info.feb = 28;
      case 2:			/* Ordinary leap year		 */
	break;

      default:			/* The magical moment arrives	 */
	info.sept = 19;		/* 19 days hath September	 */
	break;
  }
  info.days_in_month =
	(month == 2) ? info.feb
	: (month == 9) ? info.sept
	: day_month[month];
  for (i = 1; i < month; i++) {
	switch (i) {		/* Special months?		 */
	    case 2:		/* February			 */
		info.dow_first += info.feb;
		break;

	    case 9:	info.dow_first += info.sept;	break;

	    default:
		info.dow_first += day_month[i];
		break;
	}
  }
  info.dow_first %= 7;		/* Now it's Sunday to Saturday	 */
}

int getdate(week, wday)
int week;
int wday;
{
  register int today;

  /* Get a first guess at today's date and make sure it's in range. */
  today = (week * 7) + wday - info.dow_first + 1;
  if (today <= 0 || today > info.days_in_month)
	return(0);
  else if (info.sept == 19 && info.this_month == 9
	 && today >= 3)		/* The magical month?	 */
	return(today + 11);	/* If so, some dates changed	 */
  else				/* Otherwise,			 */
	return(today);		/* Return the date		 */
}

static int Jan1(year)
int year;
/* Return day of the week for Jan 1 of the specified year. */
{
  register int day;

  day = year + 4 + ((year + 3) / 4);	/* Julian Calendar	 */
  if (year > 1800) {		/* If it's recent, do	 */
	day -= ((year - 1701) / 100);	/* Clavian correction	 */
	day += ((year - 1601) / 400);	/* Gregorian correction	 */
  }
  if (year > 1752)		/* Adjust for Gregorian	 */
	day += 3;		/* calendar		 */
  return(day % 7);
}