source: lmdz_wrf/WRFV3/external/io_grib1/WGRIB/PDS_date.c @ 1

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#include "pds4.h"
#include "grib.h"

/*
 * PDS_date.c  v1.2 wesley ebisuzaki
 *
 * prints a string with a date code
 *
 * PDS_date(pds,option, v_time)
 *   options=0  .. 2 digit year
 *   options=1  .. 4 digit year
 *
 *   v_time=0   .. initial time
 *   v_time=1   .. verification time
 *
 * assumption: P1 and P2 are unsigned integers (not clear from doc)
 *
 * v1.2 years that are multiple of 400 are leap years, not 500
 * v1.2.1  make the change to the source code for v1.2
 * v1.2.2  add 3/6/12 hour forecast time units
 */

static int msg_count = 0;
extern int minute;

int PDS_date(unsigned char *pds, int option, int v_time) {

    int year, month, day, hour, min;

    if (v_time == 0) {
        year = PDS_Year4(pds);
        month = PDS_Month(pds);
        day  = PDS_Day(pds);
        hour = PDS_Hour(pds);
    }
    else {
        if (verf_time(pds, &year, &month, &day, &hour) != 0) {
            if (msg_count++ < 5) fprintf(stderr, "PDS_date: problem\n");
        }
    }
    min =  PDS_Minute(pds);

    switch(option) {
        case 0:
            printf("%2.2d%2.2d%2.2d%2.2d", year % 100, month, day, hour);
            if (minute) printf("-%2.2d", min);
            break;
        case 1:
            printf("%4.4d%2.2d%2.2d%2.2d", year, month, day, hour);
            if (minute) printf("-%2.2d", min);
            break;
        default:
            fprintf(stderr,"missing code\n");
            exit(8);
    }
    return 0;
}

#define  FEB29   (31+29)
static int monthjday[12] = {
        0,31,59,90,120,151,181,212,243,273,304,334};

static int leap(int year) {
        if (year % 4 != 0) return 0;
        if (year % 100 != 0) return 1;
        return (year % 400 == 0);
}


int add_time(int *year, int *month, int *day, int *hour, int dtime, int unit) {
    int y, m, d, h, jday, i;

    y = *year;
    m = *month;
    d = *day;
    h = *hour;

    if (unit == YEAR) {
        *year = y + dtime;
        return 0;
    }
    if (unit == DECADE) {
        *year =  y + (10 * dtime);
        return 0;
    }
    if (unit == CENTURY) {
        *year = y + (100 * dtime);
        return 0;
    }
    if (unit == NORMAL) {
        *year = y + (30 * dtime);
        return 0;
    }
    if (unit == MONTH) {
        dtime += (m - 1);
        *year = y + (dtime / 12);
        *month = 1 + (dtime % 12);
        return 0;
    }

    if (unit == SECOND) {
        dtime /= 60;
        unit = MINUTE;
    }
    if (unit == MINUTE) {
        dtime /= 60;
        unit = HOUR;
    }

    if (unit == HOURS3) {
        dtime *= 3;
        unit = HOUR;
    }
    else if (unit == HOURS6) {
        dtime *= 6;
        unit = HOUR;
    }
    else if (unit == HOURS12) {
        dtime *= 12;
        unit = HOUR;
    }

    if (unit == HOUR) {
        dtime += h;
        *hour = dtime % 24;
        dtime = dtime / 24;
        unit = DAY;
    }

    /* this is the hard part */

    if (unit == DAY) {
        /* set m and day to Jan 0, and readjust dtime */
        jday = d + monthjday[m-1];
        if (leap(y) && m > 2) jday++;
        dtime += jday;

        /* 4 year chuncks */
        i = dtime / (4 * 365 + 1);
        if (i) {
            /* assume century years are leap */
            y = y + i*4;
            dtime -= i*(4 * 365 + 1);
            /* see if we have gone past feb 28, 1900, 2000, etc */
            if ((y - 1) / 100 != (*year-1) / 100) {
                /* crossed the feb 28, xx00 */
                /* correct for only one century mark */
                if ((y / 100) % 4 != 0) dtime++;
            }
        }

        /* one year chunks */
        while (dtime > 365 + leap(y)) {
            dtime -= (365 + leap(y));
            y++;
        }

        /* calculate the month and day */

        if (leap(y) && dtime == FEB29) {
            m = 2;
            d = 29;
        }
        else {
            if (leap(y) && dtime > FEB29) dtime--;
            for (i = 11; monthjday[i] >= dtime; --i);
            m = i + 1;
            d = dtime - monthjday[i];
        }
        *year = y;
        *month = m;
        *day = d;
        return 0;
   }
   fprintf(stderr,"add_time: undefined time unit %d\n", unit);
   return 1;
}


/*
 * verf_time:
 *
 * this routine returns the "verification" time
 * should have behavior similar to gribmap
 *
 */

int verf_time(unsigned char *pds, int *year, int *month, int *day, int *hour) {
    int tr, dtime, unit;

    *year = PDS_Year4(pds);
    *month = PDS_Month(pds);
    *day  = PDS_Day(pds);
    *hour = PDS_Hour(pds);

    /* find time increment */

    dtime = PDS_P1(pds);
    tr = PDS_TimeRange(pds);
    unit = PDS_ForecastTimeUnit(pds);

    if (tr == 10) dtime = PDS_P1(pds) * 256 + PDS_P2(pds);
    if (tr > 1 && tr < 6 ) dtime = PDS_P2(pds);

    if (dtime == 0) return 0;

    return add_time(year, month, day, hour, dtime, unit);
}