xref: /llvm-project/libc/src/time/time_utils.cpp (revision f9c2377fb68e5051b3061186c507f7b87db2a8b2)

//===-- Implementation of mktime function ---------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "src/time/time_utils.h"
#include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
#include "src/__support/common.h"
#include "src/__support/macros/config.h"
#include "src/time/time_constants.h"

namespace LIBC_NAMESPACE_DECL {
namespace time_utils {

static int64_t computeRemainingYears(int64_t daysPerYears,
                                     int64_t quotientYears,
                                     int64_t *remainingDays) {
  int64_t years = *remainingDays / daysPerYears;
  if (years == quotientYears)
    years--;
  *remainingDays -= years * daysPerYears;
  return years;
}

// First, divide "total_seconds" by the number of seconds in a day to get the
// number of days since Jan 1 1970. The remainder will be used to calculate the
// number of Hours, Minutes and Seconds.
//
// Then, adjust that number of days by a constant to be the number of days
// since Mar 1 2000. Year 2000 is a multiple of 400, the leap year cycle. This
// makes it easier to count how many leap years have passed using division.
//
// While calculating numbers of years in the days, the following algorithm
// subdivides the days into the number of 400 years, the number of 100 years and
// the number of 4 years. These numbers of cycle years are used in calculating
// leap day. This is similar to the algorithm used in  getNumOfLeapYearsBefore()
// and isLeapYear(). Then compute the total number of years in days from these
// subdivided units.
//
// Compute the number of months from the remaining days. Finally, adjust years
// to be 1900 and months to be from January.
int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
  // Days in month starting from March in the year 2000.
  static const char daysInMonth[] = {31 /* Mar */, 30, 31, 30, 31, 31,
                                     30,           31, 30, 31, 31, 29};

  constexpr time_t time_min =
      (sizeof(time_t) == 4)
          ? INT_MIN
          : INT_MIN * static_cast<int64_t>(
                          time_constants::NUMBER_OF_SECONDS_IN_LEAP_YEAR);
  constexpr time_t time_max =
      (sizeof(time_t) == 4)
          ? INT_MAX
          : INT_MAX * static_cast<int64_t>(
                          time_constants::NUMBER_OF_SECONDS_IN_LEAP_YEAR);

  time_t ts = static_cast<time_t>(total_seconds);
  if (ts < time_min || ts > time_max)
    return time_utils::out_of_range();

  int64_t seconds =
      total_seconds - time_constants::SECONDS_UNTIL2000_MARCH_FIRST;
  int64_t days = seconds / time_constants::SECONDS_PER_DAY;
  int64_t remainingSeconds = seconds % time_constants::SECONDS_PER_DAY;
  if (remainingSeconds < 0) {
    remainingSeconds += time_constants::SECONDS_PER_DAY;
    days--;
  }

  int64_t wday = (time_constants::WEEK_DAY_OF2000_MARCH_FIRST + days) %
                 time_constants::DAYS_PER_WEEK;
  if (wday < 0)
    wday += time_constants::DAYS_PER_WEEK;

  // Compute the number of 400 year cycles.
  int64_t numOfFourHundredYearCycles = days / time_constants::DAYS_PER400_YEARS;
  int64_t remainingDays = days % time_constants::DAYS_PER400_YEARS;
  if (remainingDays < 0) {
    remainingDays += time_constants::DAYS_PER400_YEARS;
    numOfFourHundredYearCycles--;
  }

  // The remaining number of years after computing the number of
  // "four hundred year cycles" will be 4 hundred year cycles or less in 400
  // years.
  int64_t numOfHundredYearCycles = computeRemainingYears(
      time_constants::DAYS_PER100_YEARS, 4, &remainingDays);

  // The remaining number of years after computing the number of
  // "hundred year cycles" will be 25 four year cycles or less in 100 years.
  int64_t numOfFourYearCycles = computeRemainingYears(
      time_constants::DAYS_PER4_YEARS, 25, &remainingDays);

  // The remaining number of years after computing the number of
  // "four year cycles" will be 4 one year cycles or less in 4 years.
  int64_t remainingYears = computeRemainingYears(
      time_constants::DAYS_PER_NON_LEAP_YEAR, 4, &remainingDays);

  // Calculate number of years from year 2000.
  int64_t years = remainingYears + 4 * numOfFourYearCycles +
                  100 * numOfHundredYearCycles +
                  400LL * numOfFourHundredYearCycles;

  int leapDay =
      !remainingYears && (numOfFourYearCycles || !numOfHundredYearCycles);

  // We add 31 and 28 for the number of days in January and February, since our
  // starting point was March 1st.
  int64_t yday = remainingDays + 31 + 28 + leapDay;
  if (yday >= time_constants::DAYS_PER_NON_LEAP_YEAR + leapDay)
    yday -= time_constants::DAYS_PER_NON_LEAP_YEAR + leapDay;

  int64_t months = 0;
  while (daysInMonth[months] <= remainingDays) {
    remainingDays -= daysInMonth[months];
    months++;
  }

  if (months >= time_constants::MONTHS_PER_YEAR - 2) {
    months -= time_constants::MONTHS_PER_YEAR;
    years++;
  }

  if (years > INT_MAX || years < INT_MIN)
    return time_utils::out_of_range();

  // All the data (years, month and remaining days) was calculated from
  // March, 2000. Thus adjust the data to be from January, 1900.
  tm->tm_year = static_cast<int>(years + 2000 - time_constants::TIME_YEAR_BASE);
  tm->tm_mon = static_cast<int>(months + 2);
  tm->tm_mday = static_cast<int>(remainingDays + 1);
  tm->tm_wday = static_cast<int>(wday);
  tm->tm_yday = static_cast<int>(yday);

  tm->tm_hour =
      static_cast<int>(remainingSeconds / time_constants::SECONDS_PER_HOUR);
  tm->tm_min =
      static_cast<int>(remainingSeconds / time_constants::SECONDS_PER_MIN %
                       time_constants::SECONDS_PER_MIN);
  tm->tm_sec =
      static_cast<int>(remainingSeconds % time_constants::SECONDS_PER_MIN);
  // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
  tm->tm_isdst = 0;

  return 0;
}

} // namespace time_utils
} // namespace LIBC_NAMESPACE_DECL