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#include <stdbool.h>
#include <time.h>
/*
* Returns the number of leap years prior to the given year.
*/
static int leap_years(int year)
{
return (year-1)/4 + (year-1)/400 - (year-1)/100;
}
static int is_leap_year(int year)
{
return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}
static int days_in_month(int month, int year)
{
static int month_days[] = {
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
};
/* we may need to update this in the year 4000, pending a
* decision on whether or not it's a leap year */
if (month == 1)
return is_leap_year(year) ? 29 : 28;
return month_days[month];
}
static const int days_per_month[2][13] =
{{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
{0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366}};
#define SECS_PER_MIN 60
#define SECS_PER_HOUR (SECS_PER_MIN*60)
#define SECS_PER_DAY (24*SECS_PER_HOUR)
#define DAYS_PER_YEAR 365
struct tm *gmtime_r(const time_t *timep, struct tm *result)
{
int i;
int Y;
int M;
int D;
int h;
int m;
int s;
D = *timep / SECS_PER_DAY;
s = *timep % SECS_PER_DAY;
m = s / 60;
h = m / 60;
m %= 60;
s %= 60;
/*
* Work out the year. We subtract one day for every four years
* and every 400 years after 1969. However as leap years don't
* occur every 100 years we add one day back to counteract the
* the subtraction for every 4 years.
*/
Y = (D - (1+D/365)/4 + (69+D/365)/100 - (369+D/365)/400)/365;
/*
* Remember we're doing integer arithmetic here so
* leap_years(Y+1970) - leap_years(1970) != leap_years(Y)
*/
D = D - Y*365 - (leap_years(Y+1970) - leap_years(1970)) + 1;
Y += 1970;
M = 0;
for (i = 0; i < 13; i++)
if (D <= days_per_month[is_leap_year(Y) ? 1 : 0][i]) {
M = i;
break;
}
D -= days_per_month[is_leap_year(Y)][M-1];
result->tm_year = Y;
result->tm_mon = M - 1;
result->tm_mday = D;
result->tm_hour = h;
result->tm_min = m;
result->tm_sec = s;
return result;
}
time_t mktime(struct tm *tm)
{
unsigned long year, month, mday, hour, minute, second, d;
static const unsigned long sec_in_400_years =
((3903ul * 365) + (97 * 366)) * 24 * 60 * 60;
second = tm->tm_sec;
minute = tm->tm_min;
hour = tm->tm_hour;
mday = tm->tm_mday;
month = tm->tm_mon;
year = tm->tm_year;
/* There are the same number of seconds in any 400-year block; this
* limits the iterations in the loop below */
year += 400 * (second / sec_in_400_years);
second = second % sec_in_400_years;
if (second >= 60) {
minute += second / 60;
second = second % 60;
}
if (minute >= 60) {
hour += minute / 60;
minute = minute % 60;
}
if (hour >= 24) {
mday += hour / 24;
hour = hour % 24;
}
for (d = days_in_month(month, year); mday > d;
d = days_in_month(month, year)) {
month++;
if (month > 12) {
month = 0;
year++;
}
mday -= d;
}
tm->tm_year = year;
tm->tm_mon = month;
tm->tm_mday = mday;
tm->tm_hour = hour;
tm->tm_min = minute;
tm->tm_sec = second;
d = mday;
d += days_per_month[is_leap_year(year)][month];
d += (year-1970)*DAYS_PER_YEAR + leap_years(year) - leap_years(1970) - 1;
return d*SECS_PER_DAY + hour*SECS_PER_HOUR + minute*SECS_PER_MIN + second;
}
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