1 //===-- Implementation of mktime function ---------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #include "src/time/time_utils.h"
10 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
11 #include "src/__support/common.h"
12 #include "src/__support/macros/config.h"
13
14 namespace LIBC_NAMESPACE_DECL {
15 namespace time_utils {
16
17 using LIBC_NAMESPACE::time_utils::TimeConstants;
18
computeRemainingYears(int64_t daysPerYears,int64_t quotientYears,int64_t * remainingDays)19 static int64_t computeRemainingYears(int64_t daysPerYears,
20 int64_t quotientYears,
21 int64_t *remainingDays) {
22 int64_t years = *remainingDays / daysPerYears;
23 if (years == quotientYears)
24 years--;
25 *remainingDays -= years * daysPerYears;
26 return years;
27 }
28
29 // First, divide "total_seconds" by the number of seconds in a day to get the
30 // number of days since Jan 1 1970. The remainder will be used to calculate the
31 // number of Hours, Minutes and Seconds.
32 //
33 // Then, adjust that number of days by a constant to be the number of days
34 // since Mar 1 2000. Year 2000 is a multiple of 400, the leap year cycle. This
35 // makes it easier to count how many leap years have passed using division.
36 //
37 // While calculating numbers of years in the days, the following algorithm
38 // subdivides the days into the number of 400 years, the number of 100 years and
39 // the number of 4 years. These numbers of cycle years are used in calculating
40 // leap day. This is similar to the algorithm used in getNumOfLeapYearsBefore()
41 // and isLeapYear(). Then compute the total number of years in days from these
42 // subdivided units.
43 //
44 // Compute the number of months from the remaining days. Finally, adjust years
45 // to be 1900 and months to be from January.
update_from_seconds(int64_t total_seconds,struct tm * tm)46 int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
47 // Days in month starting from March in the year 2000.
48 static const char daysInMonth[] = {31 /* Mar */, 30, 31, 30, 31, 31,
49 30, 31, 30, 31, 31, 29};
50
51 constexpr time_t time_min =
52 (sizeof(time_t) == 4)
53 ? INT_MIN
54 : INT_MIN * static_cast<int64_t>(
55 TimeConstants::NUMBER_OF_SECONDS_IN_LEAP_YEAR);
56 constexpr time_t time_max =
57 (sizeof(time_t) == 4)
58 ? INT_MAX
59 : INT_MAX * static_cast<int64_t>(
60 TimeConstants::NUMBER_OF_SECONDS_IN_LEAP_YEAR);
61
62 time_t ts = static_cast<time_t>(total_seconds);
63 if (ts < time_min || ts > time_max)
64 return time_utils::out_of_range();
65
66 int64_t seconds =
67 total_seconds - TimeConstants::SECONDS_UNTIL2000_MARCH_FIRST;
68 int64_t days = seconds / TimeConstants::SECONDS_PER_DAY;
69 int64_t remainingSeconds = seconds % TimeConstants::SECONDS_PER_DAY;
70 if (remainingSeconds < 0) {
71 remainingSeconds += TimeConstants::SECONDS_PER_DAY;
72 days--;
73 }
74
75 int64_t wday = (TimeConstants::WEEK_DAY_OF2000_MARCH_FIRST + days) %
76 TimeConstants::DAYS_PER_WEEK;
77 if (wday < 0)
78 wday += TimeConstants::DAYS_PER_WEEK;
79
80 // Compute the number of 400 year cycles.
81 int64_t numOfFourHundredYearCycles = days / TimeConstants::DAYS_PER400_YEARS;
82 int64_t remainingDays = days % TimeConstants::DAYS_PER400_YEARS;
83 if (remainingDays < 0) {
84 remainingDays += TimeConstants::DAYS_PER400_YEARS;
85 numOfFourHundredYearCycles--;
86 }
87
88 // The remaining number of years after computing the number of
89 // "four hundred year cycles" will be 4 hundred year cycles or less in 400
90 // years.
91 int64_t numOfHundredYearCycles = computeRemainingYears(
92 TimeConstants::DAYS_PER100_YEARS, 4, &remainingDays);
93
94 // The remaining number of years after computing the number of
95 // "hundred year cycles" will be 25 four year cycles or less in 100 years.
96 int64_t numOfFourYearCycles =
97 computeRemainingYears(TimeConstants::DAYS_PER4_YEARS, 25, &remainingDays);
98
99 // The remaining number of years after computing the number of
100 // "four year cycles" will be 4 one year cycles or less in 4 years.
101 int64_t remainingYears = computeRemainingYears(
102 TimeConstants::DAYS_PER_NON_LEAP_YEAR, 4, &remainingDays);
103
104 // Calculate number of years from year 2000.
105 int64_t years = remainingYears + 4 * numOfFourYearCycles +
106 100 * numOfHundredYearCycles +
107 400LL * numOfFourHundredYearCycles;
108
109 int leapDay =
110 !remainingYears && (numOfFourYearCycles || !numOfHundredYearCycles);
111
112 // We add 31 and 28 for the number of days in January and February, since our
113 // starting point was March 1st.
114 int64_t yday = remainingDays + 31 + 28 + leapDay;
115 if (yday >= TimeConstants::DAYS_PER_NON_LEAP_YEAR + leapDay)
116 yday -= TimeConstants::DAYS_PER_NON_LEAP_YEAR + leapDay;
117
118 int64_t months = 0;
119 while (daysInMonth[months] <= remainingDays) {
120 remainingDays -= daysInMonth[months];
121 months++;
122 }
123
124 if (months >= TimeConstants::MONTHS_PER_YEAR - 2) {
125 months -= TimeConstants::MONTHS_PER_YEAR;
126 years++;
127 }
128
129 if (years > INT_MAX || years < INT_MIN)
130 return time_utils::out_of_range();
131
132 // All the data (years, month and remaining days) was calculated from
133 // March, 2000. Thus adjust the data to be from January, 1900.
134 tm->tm_year = static_cast<int>(years + 2000 - TimeConstants::TIME_YEAR_BASE);
135 tm->tm_mon = static_cast<int>(months + 2);
136 tm->tm_mday = static_cast<int>(remainingDays + 1);
137 tm->tm_wday = static_cast<int>(wday);
138 tm->tm_yday = static_cast<int>(yday);
139
140 tm->tm_hour =
141 static_cast<int>(remainingSeconds / TimeConstants::SECONDS_PER_HOUR);
142 tm->tm_min =
143 static_cast<int>(remainingSeconds / TimeConstants::SECONDS_PER_MIN %
144 TimeConstants::SECONDS_PER_MIN);
145 tm->tm_sec =
146 static_cast<int>(remainingSeconds % TimeConstants::SECONDS_PER_MIN);
147 // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
148 tm->tm_isdst = 0;
149
150 return 0;
151 }
152
153 } // namespace time_utils
154 } // namespace LIBC_NAMESPACE_DECL
155