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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: iso_week_date_r.c 2394 2013-05-04 10:26:24Z swfltek $ */ | |
30 | ||
31 | /* | |
32 | Compute the ISO 8601 week date corresponding to the given year and day of year. | |
33 | See http://en.wikipedia.org/wiki/ISO_week_date for a full description. To summarize: | |
34 | ||
35 | Weeks are numbered from 1 to 53. | |
36 | Week days are numbered 1 to 7, beginning with Monday as day 1. | |
37 | ||
38 | The first week of the year contains the first Thursday in that year. | |
39 | Dates prior to week 1 belong to the final week of the previous year. | |
40 | ||
41 | The final week of the year contains the last Thursday in that year. | |
42 | Dates after the final week belong to week 1 of the following year. | |
43 | ||
44 | */ | |
45 | ||
46 | #include <time.h> | |
47 | ||
48 | void | |
49 | iso_week_date_r(int y, int yday, struct week_date * iso) | |
50 | { | |
51 | uint16_t years, n, wday; | |
52 | int weeknum; | |
53 | int isLeap; | |
54 | ||
55 | iso->year = y; | |
56 | ||
57 | isLeap = is_leap_year(y); | |
58 | ||
59 | /* compute days elapsed since epoch */ | |
60 | years = y - 2000; | |
61 | n = 365 * years + yday; | |
62 | if (years) { | |
63 | n++; /* epoch was a leap year */ | |
64 | n += years / 4; | |
65 | n -= isLeap; | |
66 | if (years > 100) | |
67 | n--; | |
68 | } | |
69 | ||
70 | /* compute ISO8601 day of week (1 ... 7, Monday = 1) */ | |
71 | wday = n + 6; /* epoch was a Saturday */ | |
72 | wday %= 7; | |
73 | if (wday == 0) | |
74 | wday = 7; | |
75 | ||
76 | iso->day = wday; | |
77 | ||
78 | /* compute tentative week number */ | |
79 | weeknum = yday + 11 - wday; | |
80 | weeknum /= 7; | |
81 | ||
82 | /* if 53, it could be week 1 of the following year */ | |
83 | if (weeknum == 53) { | |
84 | /* | |
85 | The final week must include its Thursday in the year. We determine the yday of this | |
86 | weeks Thursday, and test whether it exceeds this years length. | |
87 | */ | |
88 | ||
89 | /* determine final yday of this year, 364 or 365 */ | |
90 | n = 364 + isLeap; | |
91 | ||
92 | /* compute yday of this weeks Thursday */ | |
93 | wday--; /* convert to zero based week, Monday = 0 */ | |
94 | yday -= wday; /* yday of this weeks Monday */ | |
95 | yday += 3; /* yday of this weeks Thursday */ | |
96 | ||
97 | /* Is this weeks Thursday included in the year? */ | |
98 | if (yday > (int) n) { | |
99 | iso->year++; | |
100 | weeknum = 1; | |
101 | } | |
102 | } | |
103 | iso->week = weeknum; | |
104 | ||
105 | /* | |
106 | If zero, it is the final week of the previous year. | |
107 | We determine that by asking for the week number of Dec 31. | |
108 | */ | |
109 | if (weeknum == 0) { | |
110 | y = y - 1; | |
111 | iso_week_date_r(y, 364 + is_leap_year(y), iso); | |
112 | iso->day = wday; | |
113 | } | |
114 | } |