libc/src/time/time_utils.cpp

   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/common.h"
  11
  12 #include <limits.h>
  13
  14 namespace __llvm_libc {
  15 namespace time_utils {
  16
  17 using __llvm_libc::time_utils::TimeConstants;
  18
  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.
  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   if (sizeof(time_t) == 4) {
  52     if (total_seconds < 0x80000000)
  53       return time_utils::out_of_range();
  54     if (total_seconds > 0x7FFFFFFF)
  55       return time_utils::out_of_range();
  56   } else {
  57     if (total_seconds <
  58             INT_MIN * static_cast<int64_t>(
  59                           TimeConstants::NUMBER_OF_SECONDS_IN_LEAP_YEAR) ||
  60         total_seconds >
  61             INT_MAX * static_cast<int64_t>(
  62                           TimeConstants::NUMBER_OF_SECONDS_IN_LEAP_YEAR))
  63       return time_utils::out_of_range();
  64   }
  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 = years + 2000 - TimeConstants::TIME_YEAR_BASE;
 135   tm->tm_mon = months + 2;
 136   tm->tm_mday = remainingDays + 1;
 137   tm->tm_wday = wday;
 138   tm->tm_yday = yday;
 139
 140   tm->tm_hour = remainingSeconds / TimeConstants::SECONDS_PER_HOUR;
 141   tm->tm_min = remainingSeconds / TimeConstants::SECONDS_PER_MIN %
 142                TimeConstants::SECONDS_PER_MIN;
 143   tm->tm_sec = remainingSeconds % TimeConstants::SECONDS_PER_MIN;
 144   // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
 145   tm->tm_isdst = 0;
 146
 147   return 0;
 148 }
 149
 150 } // namespace time_utils
 151 } // namespace __llvm_libc