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135 lines
3.4 KiB
C
135 lines
3.4 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <string.h>
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#include "dive.h"
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/*
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* The date handling internally works in seconds since
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* Jan 1, 1900. That avoids negative numbers which avoids
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* some silly problems.
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*
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* But we then use the same base epoch base (Jan 1, 1970)
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* that POSIX uses, so that we can use the normal date
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* handling functions for getting current time etc.
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*
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* There's 25567 dats from Jan 1, 1900 to Jan 1, 1970.
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*
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* NOTE! The SEC_PER_DAY is not so much because the
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* number is complicated, as to make sure we always
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* expand the type to "timestamp_t" in the arithmetic.
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*/
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#define SEC_PER_DAY ((timestamp_t) 24*60*60)
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#define EPOCH_OFFSET (25567 * SEC_PER_DAY)
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/*
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* Convert 64-bit timestamp to 'struct tm' in UTC.
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*
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* On 32-bit machines, only do 64-bit arithmetic for the seconds
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* part, after that we do everything in 'long'. 64-bit divides
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* are unnecessary once you're counting minutes (32-bit minutes:
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* 8000+ years).
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*/
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void utc_mkdate(timestamp_t timestamp, struct tm *tm)
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{
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static const unsigned int mdays[] = {
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31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
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};
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static const unsigned int mdays_leap[] = {
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31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
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};
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unsigned long val;
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unsigned int leapyears;
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int m;
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const unsigned int *mp;
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memset(tm, 0, sizeof(*tm));
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// Midnight at Jan 1, 1970 means "no date"
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if (!timestamp)
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return;
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/* Convert to seconds since 1900 */
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timestamp += EPOCH_OFFSET;
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/* minutes since 1900 */
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tm->tm_sec = timestamp % 60;
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val = timestamp /= 60;
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/* Do the simple stuff */
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tm->tm_min = val % 60;
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val /= 60;
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tm->tm_hour = val % 24;
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val /= 24;
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/* Jan 1, 1900 was a Monday (tm_wday=1) */
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tm->tm_wday = (val + 1) % 7;
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/*
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* Now we're in "days since Jan 1, 1900". To make things easier,
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* let's make it "days since Jan 1, 1904", since that's a leap-year.
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* 1900 itself was not. The following logic will get 1900-1903
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* wrong. If you were diving back then, you're kind of screwed.
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*/
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val -= 365*4;
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/* This only works up until 2099 (2100 isn't a leap-year) */
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leapyears = val / (365 * 4 + 1);
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val %= (365 * 4 + 1);
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tm->tm_year = 1904 + leapyears * 4;
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/* Handle the leap-year itself */
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mp = mdays_leap;
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if (val > 365) {
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tm->tm_year++;
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val -= 366;
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tm->tm_year += val / 365;
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val %= 365;
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mp = mdays;
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}
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for (m = 0; m < 12; m++) {
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if (val < *mp)
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break;
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val -= *mp++;
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}
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tm->tm_mday = val + 1;
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tm->tm_mon = m;
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}
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timestamp_t utc_mktime(struct tm *tm)
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{
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static const int mdays[] = {
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0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
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};
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int year = tm->tm_year;
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int month = tm->tm_mon;
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int day = tm->tm_mday;
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int days_since_1900;
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timestamp_t when;
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/* First normalize relative to 1900 */
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if (year < 50)
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year += 100;
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else if (year >= 1900)
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year -= 1900;
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if (year < 0 || year > 129) /* algo only works for 1900-2099 */
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return 0;
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if (month < 0 || month > 11) /* array bounds */
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return 0;
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if (month < 2 || (year && year % 4))
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day--;
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if (tm->tm_hour < 0 || tm->tm_min < 0 || tm->tm_sec < 0)
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return 0;
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/* This works until 2099 */
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days_since_1900 = year * 365 + (year - 1) / 4;
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/* Note the 'day' fixup for non-leapyears above */
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days_since_1900 += mdays[month] + day;
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/* Now add it all up, making sure to do this part in "timestamp_t" */
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when = days_since_1900 * SEC_PER_DAY;
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when += tm->tm_hour * 60 * 60 + tm->tm_min * 60 + tm->tm_sec;
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return when - EPOCH_OFFSET;
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}
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