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Diffstat (limited to 'time/iso_week_date_r.c')
| -rw-r--r-- | time/iso_week_date_r.c | 114 |
1 files changed, 114 insertions, 0 deletions
diff --git a/time/iso_week_date_r.c b/time/iso_week_date_r.c new file mode 100644 index 0000000..ac1feb4 --- /dev/null +++ b/time/iso_week_date_r.c @@ -0,0 +1,114 @@ +/* + * (c)2012 Michael Duane Rice All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions are + * met: + * + * Redistributions of source code must retain the above copyright notice, this + * list of conditions and the following disclaimer. Redistributions in binary + * form must reproduce the above copyright notice, this list of conditions + * and the following disclaimer in the documentation and/or other materials + * provided with the distribution. Neither the name of the copyright holders + * nor the names of contributors may be used to endorse or promote products + * derived from this software without specific prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE + * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE + * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF + * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS + * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN + * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) + * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE + * POSSIBILITY OF SUCH DAMAGE. + */ + +/* $Id: iso_week_date_r.c 2394 2013-05-04 10:26:24Z swfltek $ */ + +/* + Compute the ISO 8601 week date corresponding to the given year and day of year. + See http://en.wikipedia.org/wiki/ISO_week_date for a full description. To summarize: + + Weeks are numbered from 1 to 53. + Week days are numbered 1 to 7, beginning with Monday as day 1. + + The first week of the year contains the first Thursday in that year. + Dates prior to week 1 belong to the final week of the previous year. + + The final week of the year contains the last Thursday in that year. + Dates after the final week belong to week 1 of the following year. + +*/ + +#include <time.h> + +void +iso_week_date_r(int y, int yday, struct week_date * iso) +{ + uint16_t years, n, wday; + int weeknum; + int isLeap; + + iso->year = y; + + isLeap = is_leap_year(y); + + /* compute days elapsed since epoch */ + years = y - 2000; + n = 365 * years + yday; + if (years) { + n++; /* epoch was a leap year */ + n += years / 4; + n -= isLeap; + if (years > 100) + n--; + } + + /* compute ISO8601 day of week (1 ... 7, Monday = 1) */ + wday = n + 6; /* epoch was a Saturday */ + wday %= 7; + if (wday == 0) + wday = 7; + + iso->day = wday; + + /* compute tentative week number */ + weeknum = yday + 11 - wday; + weeknum /= 7; + + /* if 53, it could be week 1 of the following year */ + if (weeknum == 53) { + /* + The final week must include its Thursday in the year. We determine the yday of this + weeks Thursday, and test whether it exceeds this years length. + */ + + /* determine final yday of this year, 364 or 365 */ + n = 364 + isLeap; + + /* compute yday of this weeks Thursday */ + wday--; /* convert to zero based week, Monday = 0 */ + yday -= wday; /* yday of this weeks Monday */ + yday += 3; /* yday of this weeks Thursday */ + + /* Is this weeks Thursday included in the year? */ + if (yday > (int) n) { + iso->year++; + weeknum = 1; + } + } + iso->week = weeknum; + + /* + If zero, it is the final week of the previous year. + We determine that by asking for the week number of Dec 31. + */ + if (weeknum == 0) { + y = y - 1; + iso_week_date_r(y, 364 + is_leap_year(y), iso); + iso->day = wday; + } +} |