76 lines
2.1 KiB
C
76 lines
2.1 KiB
C
#include <time.h>
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#include <stdbool.h>
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static struct tm g_time_struct = {};
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/* to the suprise of nobody, this is from stack overflow.
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* https://stackoverflow.com/questions/21593692/convert-unix-timestamp-to-date-without-system-libs
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*/
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struct tm *gmtime(const time_t *timer)
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{
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unsigned int seconds, minutes, hours, days, year, month;
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unsigned int day_of_week;
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seconds = *timer;
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/* calculate minutes */
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minutes = seconds / 60;
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seconds -= minutes * 60;
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/* calculate hours */
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hours = minutes / 60;
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minutes -= hours * 60;
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/* calculate days */
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days = hours / 24;
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hours -= days * 24;
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/* Unix time starts in 1970 on a Thursday */
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year = 1970;
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day_of_week = 4;
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while(1) {
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bool leap_year = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
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uint16_t days_in_year = leap_year ? 366 : 365;
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if (days >= days_in_year) {
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day_of_week += leap_year ? 2 : 1;
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days -= days_in_year;
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if (day_of_week >= 7) {
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day_of_week -= 7;
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}
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++year;
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} else {
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g_time_struct.tm_yday = days;
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day_of_week += days;
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day_of_week %= 7;
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/* calculate the month and day */
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static const uint8_t days_in_month[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
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for (month = 0; month < 12; ++month) {
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uint8_t dim = days_in_month[month];
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/* add a day to feburary if this is a leap year */
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if (month == 1 && leap_year) {
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++dim;
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}
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if (days >= dim) {
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days -= dim;
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} else {
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break;
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}
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}
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break;
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}
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}
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g_time_struct.tm_sec = seconds;
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g_time_struct.tm_min = minutes;
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g_time_struct.tm_hour = hours;
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g_time_struct.tm_mday = days + 1;
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g_time_struct.tm_mon = month;
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g_time_struct.tm_year = year - 1900;
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g_time_struct.tm_wday = day_of_week;
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return &g_time_struct;
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}
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