248bff5eb8
Unlike the current code, it does no looping to count days or do table lookups, which means running time doesn't increase with year or month. A good thing if it's call a lot, especially if the algorithm were asked to compute dates centuries or more from the epoch start. As a bonus, handles negative time values. Change-Id: I198a23daf621e40623e6b44dacf2387078b4db9c
134 lines
4.1 KiB
C
134 lines
4.1 KiB
C
/***************************************************************************
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* __________ __ ___.
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* Open \______ \ ____ ____ | | _\_ |__ _______ ___
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* Source | _// _ \_/ ___\| |/ /| __ \ / _ \ \/ /
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* Jukebox | | ( <_> ) \___| < | \_\ ( <_> > < <
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* Firmware |____|_ /\____/ \___ >__|_ \|___ /\____/__/\_ \
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* \/ \/ \/ \/ \/
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* $Id$
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*
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* Copyright (C) 2017 by Michael Sevakis
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*
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* Copyright (C) 2012 by Bertrik Sikken
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*
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* Based on code from: rtc_as3514.c
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* Copyright (C) 2007 by Barry Wardell
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
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* KIND, either express or implied.
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*
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****************************************************************************/
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#include "time.h"
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/* Constants that mark Thursday, 1 January 1970 */
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#define UNIX_EPOCH_DAY_NUM 134774
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#define UNIX_EPOCH_YEAR (1601 - 1900)
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/* Last day number it can do */
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#define MAX_DAY_NUM 551879
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/* d is days since epoch start, Monday, 1 January 1601 (d = 0)
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*
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* That date is the beginning of a full 400 year cycle and so simplifies the
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* calculations a bit, not requiring offsets before divisions to shift the
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* leap year cycle.
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*
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* It can handle dates up through Sunday, 31 December 3111 (d = 551879).
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*
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* time_t can't get near the limits anyway for now but algorithm can be
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* altered slightly to increase range if even needed.
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*/
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static void get_tmdate(unsigned int d, struct tm *tm)
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{
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static const unsigned short mon_yday[13] =
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{
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/* year day of 1st of month (non-leap)
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+31 +28 +31 +30 +31 +30 +31 +31 +30 +31 +30 +31 +31 */
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0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365
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};
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unsigned int x0, x1, x2, x3; /* scratch variables */
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/* calculate year from day number */
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x0 = d / 1460;
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x1 = x0 / 25;
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x2 = x1 / 4;
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unsigned int y = (d - x0 + x1 - x2) / 365;
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tm->tm_year = y + UNIX_EPOCH_YEAR; /* year - 1900 */
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/* calculate year day from year number and day number */
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x0 = y / 4;
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x1 = x0 / 25;
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x2 = x1 / 4;
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unsigned int yday = d - x0 + x1 - x2 - y * 365;
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tm->tm_yday = x3 = yday; /* 0..364/365 */
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/* check if leap year; adjust February->March transition if so rather
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than keeping a leap year version of mon_yday[] */
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if (y - x0 * 4 == 3 && (x0 - x1 * 25 != 24 || x1 - x2 * 4 == 3)) {
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/* retard month lookup to make year day 59 into 29 Feb, both to make
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year day 60 into 01 March, lagging one day for remainder of year */
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if (x3 >= mon_yday[2] && --x3 >= mon_yday[2]) {
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yday--;
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}
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}
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/* stab approximately at current month based on year day; advance if
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it fell short (never initially more than 1 short). */
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x0 = x3 / 32;
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if (mon_yday[x0 + 1] <= x3) {
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x0++;
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}
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tm->tm_mon = x0; /* 0..11 */
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tm->tm_mday = yday - mon_yday[x0] + 1; /* 1..31 */
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tm->tm_wday = (d + 1) % 7; /* 0..6 */
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}
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struct tm * gmtime_r(const time_t *timep, struct tm *tm)
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{
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time_t t = *timep;
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int d = t / 86400; /* day number (-24856..24855) */
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int s = t - (time_t)d * 86400; /* second # of day (0..86399) */
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if (s < 0) {
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/* round towards 0 -> floored division */
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d--;
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s += 86400;
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}
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unsigned int x;
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x = s / 3600;
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tm->tm_hour = x; /* 0..23 */
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s -= x * 3600;
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x = s / 60;
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tm->tm_min = x; /* 0..59 */
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s -= x * 60;
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tm->tm_sec = s; /* 0..59 */
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/* not implemented right now */
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tm->tm_isdst = -1;
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get_tmdate(d + UNIX_EPOCH_DAY_NUM, tm);
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return tm;
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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 time;
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return gmtime_r(timep, &time);
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}
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