ex5_8.c

#include <stdio.h>

static char daytab[2][13] =  {
			      {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
			      {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
};

int day_of_year(int year, int month, int day)
{
  int i, leap;
	
  leap = (year%4 == 0 && year%100 != 0) || year%400 == 0;
  
  if ((year < 1752) || (month < 1) || (month > 12) || (day < 1) || (day > daytab[leap][month]))
      return -1;
      
  for (i = 1; i < month; i++)
    day += daytab[leap][i];
  return day;
}

int month_day(int year, int yearday, int *pmonth, int *pday)
{
  int i, leap;

  leap = (year%4 == 0 && year%100 != 0) || year%400 == 0;
  
  if ((year < 1752) || (yearday < 1) || (yearday > (leap ? 366 : 365)))
    return -1;
  
  for (i = 1; yearday > daytab[leap][i]; i++)
    yearday -= daytab[leap][i];
  *pmonth = i;
  *pday = yearday;
	
  return 0;
}

/* main: test day_of_year and month_day (taken from clc-wiki) */
int main(void)
{
  int year, month, day, yearday;
	
  for (year = 1970; year <= 2000; ++year) {
    for (yearday = 1; yearday < 368; ++yearday) {
      if (month_day(year, yearday, &month, &day) == -1) {
	printf("month_day failed: %d %d\n",
	       year, yearday);
      } else if (day_of_year(year, month, day) != yearday) {
	printf("bad result: %d %d\n", year, yearday);
	printf("month = %d, day = %d\n", month, day);
      }
    }
  }	
  return 0;
}