date_conversion2.c

#include "date_conversion.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}
};

/* day_of_year：将月和日转换为指定年份的第几天，如果参数值非法则返回-1 */
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 < 0 || month < 1 || month > 12 || day < 1 || day > daytab[leap][month])
        return -1;
    for (i = 1; i < month; ++i)
        day += daytab[leap][i];
    return day;
}

/* month_day：将指定年份的第几天转换为月和日，如果参数值非法则返回-1, -1 */
void 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 < 0 || yearday < 1 || yearday > (leap ? 366 : 365)) {
        *pmonth = *pday = -1;
        return;
    }
    for (i = 1; yearday > daytab[leap][i]; ++i)
        yearday -= daytab[leap][i];
    *pmonth = i;
    *pday = yearday;
}