105 lines
3.0 KiB
C
105 lines
3.0 KiB
C
/*
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* Copyright (c) 2019 Peter Bigot Consulting, LLC
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*
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* SPDX-License-Identifier: Apache-2.0
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*/
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/*
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* The time_civil_from_days function is derived directly from public
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* domain content written by Howard Hinnant and available at:
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* http://howardhinnant.github.io/date_algorithms.html#civil_from_days
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*/
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#include <time.h>
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/* A signed type with the representation of time_t without its
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* impliciations.
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*/
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typedef time_t bigint_type;
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/** Convert a UNIX time to civil time.
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*
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* This converts integral seconds since (before) 1970-01-01T00:00:00
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* to the POSIX standard civil time representation. Any adjustments
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* due to time zone, leap seconds, or a different epoch must be
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* applied to @p time before invoking this function.
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*
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* @param time the time represented as seconds.
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*
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* @return the time information for corresponding to the provided
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* instant.
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*
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* @see http://howardhinnant.github.io/date_algorithms.html#civil_from_days
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*/
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static void time_civil_from_days(bigint_type z,
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struct tm *_MLIBC_RESTRICT tp)
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{
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tp->tm_wday = (z >= -4) ? ((z + 4) % 7) : ((z + 5) % 7 + 6);
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z += 719468;
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bigint_type era = ((z >= 0) ? z : (z - 146096)) / 146097;
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unsigned int doe = (z - era * (bigint_type)146097);
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unsigned int yoe = (doe - doe / 1460U + doe / 36524U - doe / 146096U)
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/ 365U;
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bigint_type y = (time_t)yoe + era * 400;
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unsigned int doy = doe - (365U * yoe + yoe / 4U - yoe / 100U);
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unsigned int mp = (5U * doy + 2U) / 153U;
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unsigned int d = doy - (153U * mp + 2U) / 5U + 1U;
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unsigned int m = mp + ((mp < 10) ? 3 : -9);
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tp->tm_year = y + (m <= 2) - 1900;
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tp->tm_mon = m - 1;
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tp->tm_mday = d;
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/* Everything above is explained on the referenced page, but
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* doy is relative to --03-01 and we need it relative to
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* --01-01.
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*
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* doy=306 corresponds to --01-01, doy=364 to --02-28, and
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* doy=365 to --02-29. So we can just subtract 306 to handle
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* January and February.
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*
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* For doy<306 we have to add the number of days before
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* --03-01, which is 59 in a common year and 60 in a leap
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* year. Note that the first year in the era is a leap year.
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*/
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if (doy >= 306U) {
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tp->tm_yday = doy - 306U;
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} else {
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tp->tm_yday = doy + 59U + (((yoe % 4U == 0U) && (yoe % 100U != 0U)) || (yoe == 0U));
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}
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}
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/* Convert a UNIX time to civil time.
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*
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* This converts integral seconds since (before) 1970-01-01T00:00:00
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* to the POSIX standard civil time representation. Any adjustments
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* due to time zone, leap seconds, or a different epoch must be
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* applied to @p time before invoking this function.
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*/
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struct tm *gmtime_r(const time_t *_MLIBC_RESTRICT timep,
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struct tm *_MLIBC_RESTRICT result)
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{
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time_t z = *timep;
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bigint_type days = (z >= 0 ? z : z - 86399) / 86400;
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unsigned int rem = z - days * 86400;
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*result = (struct tm){ 0 };
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time_civil_from_days(days, result);
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result->tm_hour = rem / 60U / 60U;
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rem -= result->tm_hour * 60 * 60;
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result->tm_min = rem / 60;
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result->tm_sec = rem - result->tm_min * 60;
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return result;
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}
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struct tm *gmtime(const time_t *timep)
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{
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static struct tm shared;
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return gmtime_r(timep, &shared);
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}
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