1    /******************************************************************************
     2     *
     3     * ISOWeekNum.c
     4     * return a date's week number, according to ISO 8601
     5     * Version: 1.0
     6     *
     7     * Copyright (c) 2002 Robert Watkins
     8     *
     9     * This program is free software; you can redistribute it and/or modify
    10     * it under the terms of the GNU General Public License as published by
    11     * the Free Software Foundation; either version 2 of the License, or
    12     * (at your option) any later version.
    13     * 
    14     * This program is distributed in the hope that it will be useful,
    15     * but WITHOUT ANY WARRANTY; without even the implied warranty of
    16     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    17     * GNU General Public License for more details.
    18     * 
    19     * You should have received a copy of the GNU General Public License
    20     * along with this program; if not, write to the Free Software
    21     * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    22     *
    23     *****************************************************************************/
    24    
    25    #include <PalmOS.h>
    26    #include "ISOWeekNum.h"  // this is no more than the function declaration
    27    
    28    Int32 ISOWeekNum(DateType *dP)
    29    {
    30        Int32    weekNum;
    31        DateType firstMonday = { firstYear,  1,  1 };
    32        DateType date0101    = { firstYear,  1,  1 };
    33        DateType date3112    = { firstYear, 12, 31 };
    34        // Note: "dow" stands for "day of the week"
    35        Int16   dowDP   = DayOfWeek(dP->month, dP->day, dP->year + firstYear);
    36        Int16   dow0101 = DayOfWeek(1,  1,  dP->year + firstYear);
    37        Int16   dow3112 = DayOfWeek(12, 31, dP->year + firstYear);
    38        Int16   dow0[7] = { 6, 0, 1, 2, 3, 4, 5 };  // 0-based dow array (PalmOS thinks Sunday is day 0!)
    39    
    40        firstMonday.year = dP->year;
    41        date0101.year    = dP->year;
    42        date3112.year    = dP->year;
    43    
    44        if ((dP->month == 12) && (dP->day > (31 - dow3112)) && (dow3112 < thursday)) {
    45            // day in question belongs in first week of following year
    46            weekNum = 1;
    47        }
    48        else {
    49            if (dow0[dow0101] > dow0[thursday]) {  // jan1 is Fri, Sat or Sun
    50                // see if day in question belongs in last week of previous year
    51                if ((dP->month == 1) && (dP->day < (8 - dow0[dow0101]))) {
    52                    Int16 dowPrev0101;
    53                    if (firstMonday.year == 0) { return weekNum = 53; }  // special case due to PalmOS limitations
    54                    firstMonday.year--;
    55                    firstMonday.month = 1;
    56                    firstMonday.day   = 1;
    57                    dowPrev0101 = DayOfWeek(1, 1, firstMonday.year + firstYear);
    58                    if (dow0[dowPrev0101] > dow0[thursday]) {  // jan1 of previous year is Fri, Sat or Sun
    59                        // set firstMonday to monday after jan1 of previous year
    60                        DateAdjust(&firstMonday, (7 - dow0[dowPrev0101]));
    61                    }
    62                    else {
    63                        // set firstMonday to monday before jan1 of previous year
    64                        DateAdjust(&firstMonday, -(dow0[dowPrev0101]));
    65                    }
    66                }
    67                else {
    68                    // set firstMonday to monday after jan1
    69                    DateAdjust(&firstMonday, (7 - dow0[dow0101]));
    70                }
    71            }
    72            else {
    73                // set firstMonday to monday before jan1
    74                DateAdjust(&firstMonday, -(dow0[dow0101]));
    75            }
    76            // count weeks from firstMonday (zero based) and add 1 to get week number
    77            weekNum = ((DateToDays(*dP) - DateToDays(firstMonday)) / daysInWeek) + 1;
    78        }
    79    
    80        return weekNum;
    81    }
    82