1/* Copyright (C) 1991-2023 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18#include <errno.h>
19#include <time.h>
20
21#define SECS_PER_HOUR (60 * 60)
22#define SECS_PER_DAY (SECS_PER_HOUR * 24)
23
24/* Compute the `struct tm' representation of T,
25 offset OFFSET seconds east of UTC,
26 and store year, yday, mon, mday, wday, hour, min, sec into *TP.
27 Return nonzero if successful. */
28int
29__offtime (__time64_t t, long int offset, struct tm *tp)
30{
31 __time64_t days, rem, y;
32 const unsigned short int *ip;
33
34 days = t / SECS_PER_DAY;
35 rem = t % SECS_PER_DAY;
36 rem += offset;
37 while (rem < 0)
38 {
39 rem += SECS_PER_DAY;
40 --days;
41 }
42 while (rem >= SECS_PER_DAY)
43 {
44 rem -= SECS_PER_DAY;
45 ++days;
46 }
47 tp->tm_hour = rem / SECS_PER_HOUR;
48 rem %= SECS_PER_HOUR;
49 tp->tm_min = rem / 60;
50 tp->tm_sec = rem % 60;
51 /* January 1, 1970 was a Thursday. */
52 tp->tm_wday = (4 + days) % 7;
53 if (tp->tm_wday < 0)
54 tp->tm_wday += 7;
55 y = 1970;
56
57#define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))
58#define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))
59
60 while (days < 0 || days >= (__isleap (y) ? 366 : 365))
61 {
62 /* Guess a corrected year, assuming 365 days per year. */
63 __time64_t yg = y + days / 365 - (days % 365 < 0);
64
65 /* Adjust DAYS and Y to match the guessed year. */
66 days -= ((yg - y) * 365
67 + LEAPS_THRU_END_OF (yg - 1)
68 - LEAPS_THRU_END_OF (y - 1));
69 y = yg;
70 }
71 tp->tm_year = y - 1900;
72 if (tp->tm_year != y - 1900)
73 {
74 /* The year cannot be represented due to overflow. */
75 __set_errno (EOVERFLOW);
76 return 0;
77 }
78 tp->tm_yday = days;
79 ip = __mon_yday[__isleap(y)];
80 for (y = 11; days < (long int) ip[y]; --y)
81 continue;
82 days -= ip[y];
83 tp->tm_mon = y;
84 tp->tm_mday = days + 1;
85 return 1;
86}
87