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

source code of libc/src/time/time_utils.cpp