libc/src/time/mktime.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/mktime.h"
  10 #include "src/__support/common.h"
  11 #include "src/time/time_utils.h"
  12
  13 #include <limits.h>
  14
  15 namespace __llvm_libc {
  16
  17 using __llvm_libc::time_utils::TimeConstants;
  18
  19 static constexpr int NON_LEAP_YEAR_DAYS_IN_MONTH[] = {31, 28, 31, 30, 31, 30,
  20                                                       31, 31, 30, 31, 30, 31};
  21
  22 // Returns number of years from (1, year).
  23 static constexpr int64_t get_num_of_leap_years_before(int64_t year) {
  24   return (year / 4) - (year / 100) + (year / 400);
  25 }
  26
  27 // Returns True if year is a leap year.
  28 static constexpr bool is_leap_year(const int64_t year) {
  29   return (((year) % 4) == 0 && (((year) % 100) != 0 || ((year) % 400) == 0));
  30 }
  31
  32 LLVM_LIBC_FUNCTION(time_t, mktime, (struct tm * tm_out)) {
  33   // Unlike most C Library functions, mktime doesn't just die on bad input.
  34   // TODO(rtenneti); Handle leap seconds.
  35   int64_t tm_year_from_base = tm_out->tm_year + TimeConstants::TIME_YEAR_BASE;
  36
  37   // 32-bit end-of-the-world is 03:14:07 UTC on 19 January 2038.
  38   if (sizeof(time_t) == 4 &&
  39       tm_year_from_base >= TimeConstants::END_OF32_BIT_EPOCH_YEAR) {
  40     if (tm_year_from_base > TimeConstants::END_OF32_BIT_EPOCH_YEAR)
  41       return time_utils::out_of_range();
  42     if (tm_out->tm_mon > 0)
  43       return time_utils::out_of_range();
  44     if (tm_out->tm_mday > 19)
  45       return time_utils::out_of_range();
  46     if (tm_out->tm_hour > 3)
  47       return time_utils::out_of_range();
  48     if (tm_out->tm_min > 14)
  49       return time_utils::out_of_range();
  50     if (tm_out->tm_sec > 7)
  51       return time_utils::out_of_range();
  52   }
  53
  54   // Years are ints.  A 32-bit year will fit into a 64-bit time_t.
  55   // A 64-bit year will not.
  56   static_assert(sizeof(int) == 4,
  57                 "ILP64 is unimplemented.  This implementation requires "
  58                 "32-bit integers.");
  59
  60   // Calculate number of months and years from tm_mon.
  61   int64_t month = tm_out->tm_mon;
  62   if (month < 0 || month >= TimeConstants::MONTHS_PER_YEAR - 1) {
  63     int64_t years = month / 12;
  64     month %= 12;
  65     if (month < 0) {
  66       years--;
  67       month += 12;
  68     }
  69     tm_year_from_base += years;
  70   }
  71   bool tm_year_is_leap = is_leap_year(tm_year_from_base);
  72
  73   // Calculate total number of days based on the month and the day (tm_mday).
  74   int64_t total_days = tm_out->tm_mday - 1;
  75   for (int64_t i = 0; i < month; ++i)
  76     total_days += NON_LEAP_YEAR_DAYS_IN_MONTH[i];
  77   // Add one day if it is a leap year and the month is after February.
  78   if (tm_year_is_leap && month > 1)
  79     total_days++;
  80
  81   // Calculate total numbers of days based on the year.
  82   total_days += (tm_year_from_base - TimeConstants::EPOCH_YEAR) *
  83                 TimeConstants::DAYS_PER_NON_LEAP_YEAR;
  84   if (tm_year_from_base >= TimeConstants::EPOCH_YEAR) {
  85     total_days += get_num_of_leap_years_before(tm_year_from_base - 1) -
  86                   get_num_of_leap_years_before(TimeConstants::EPOCH_YEAR);
  87   } else if (tm_year_from_base >= 1) {
  88     total_days -= get_num_of_leap_years_before(TimeConstants::EPOCH_YEAR) -
  89                   get_num_of_leap_years_before(tm_year_from_base - 1);
  90   } else {
  91     // Calculate number of leap years until 0th year.
  92     total_days -= get_num_of_leap_years_before(TimeConstants::EPOCH_YEAR) -
  93                   get_num_of_leap_years_before(0);
  94     if (tm_year_from_base <= 0) {
  95       total_days -= 1; // Subtract 1 for 0th year.
  96       // Calculate number of leap years until -1 year
  97       if (tm_year_from_base < 0) {
  98         total_days -= get_num_of_leap_years_before(-tm_year_from_base) -
  99                       get_num_of_leap_years_before(1);
 100       }
 101     }
 102   }
 103
 104   // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
 105   int64_t seconds = tm_out->tm_sec +
 106                     tm_out->tm_min * TimeConstants::SECONDS_PER_MIN +
 107                     tm_out->tm_hour * TimeConstants::SECONDS_PER_HOUR +
 108                     total_days * TimeConstants::SECONDS_PER_DAY;
 109
 110   // Update the tm structure's year, month, day, etc. from seconds.
 111   if (time_utils::update_from_seconds(seconds, tm_out) < 0)
 112     return time_utils::out_of_range();
 113
 114   return static_cast<time_t>(seconds);
 115 }
 116
 117 } // namespace __llvm_libc