time/week_of_year.c

   1 /*
   2  * (C)2012 Michael Duane Rice All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
   6  * met:
   7  *
   8  * Redistributions of source code must retain the above copyright notice, this
   9  * list of conditions and the following disclaimer. Redistributions in binary
  10  * form must reproduce the above copyright notice, this list of conditions
  11  * and the following disclaimer in the documentation and/or other materials
  12  * provided with the distribution. Neither the name of the copyright holders
  13  * nor the names of contributors may be used to endorse or promote products
  14  * derived from this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  17  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  19  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  20  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  21  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  22  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  23  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  24  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  25  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  26  * POSSIBILITY OF SUCH DAMAGE.
  27  */
  28
  29 /* $Id: week_of_year.c 2362 2013-04-26 22:22:46Z swfltek $ */
  30
  31 /*
  32         Return the week of year, where 'base' represents the first day of the week.
  33         In the USA, the week is generally considered to start on Sunday (base = 0),
  34         while in Europe it is generally considered to be Monday (base = 1).
  35
  36         Return value ranges from 0 to 52.
  37 */
  38
  39 #include <time.h>
  40
  41 uint8_t
  42 week_of_year(const struct tm * timestruct, uint8_t base)
  43 {
  44         int             first, n;
  45
  46         /* find the first base day of the year (start of week 1) */
  47         first = 7 + timestruct->tm_yday - timestruct->tm_wday + base;
  48     first %= 7;
  49
  50         /* find days since that first base day*/
  51         n = timestruct->tm_yday - first;
  52
  53         /* if negative, we are in week 0 */
  54         if (n < 0)
  55                 return 0;
  56
  57         return n / 7 + 1;
  58 }