[z180-stamp.git] / time / gm_sidereal.c

/*
 * (C)2012 Michael Duane Rice All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 * Redistributions of source code must retain the above copyright notice, this
 * list of conditions and the following disclaimer. Redistributions in binary
 * form must reproduce the above copyright notice, this list of conditions
 * and the following disclaimer in the documentation and/or other materials
 * provided with the distribution. Neither the name of the copyright holders
 * nor the names of contributors may be used to endorse or promote products
 * derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/* $Id: gm_sidereal.c 2369 2013-04-28 14:19:35Z swfltek $ */

/*
    Greenwich Mean Sidereal Time. A sidereal second is somewhat shorter than a standard second,
    about 1.002737909350795 sidereal seconds per standard second.

    We resort to fixed point math due to the insufficient resolution of a 'double', using...

        timestamp * ( 1.002737909350795 << 31 )
        --------------------------------------- + Te
                        1 << 31

    Where Te is the sidereal time at the epoch.

*/

#include <time.h>
#include <stdint.h>

unsigned long
gm_sidereal(const time_t * timer)
{
    uint64_t        tmp;

    tmp = *timer;
    tmp *= 0x8059B740;
    tmp /= 0x80000000;
    tmp += (uint64_t) 23991;

    tmp %= ONE_DAY;
    return tmp;
}
Commit	Line	Data
e63b2f75 L	1	/*
	2	* (C)2012 Michael Duane Rice All rights reserved.
	3	*
	4	* Redistribution and use in source and binary forms, with or without
	5	* modification, are permitted provided that the following conditions are
	6	* met:
	7	*
	8	* Redistributions of source code must retain the above copyright notice, this
	9	* list of conditions and the following disclaimer. Redistributions in binary
	10	* form must reproduce the above copyright notice, this list of conditions
	11	* and the following disclaimer in the documentation and/or other materials
	12	* provided with the distribution. Neither the name of the copyright holders
	13	* nor the names of contributors may be used to endorse or promote products
	14	* derived from this software without specific prior written permission.
	15	*
	16	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
	17	* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	18	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	19	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
	20	* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	21	* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	22	* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	23	* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	24	* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	25	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	26	* POSSIBILITY OF SUCH DAMAGE.
	27	*/
	28
	29	/* $Id: gm_sidereal.c 2369 2013-04-28 14:19:35Z swfltek $ */
	30
	31	/*
	32	Greenwich Mean Sidereal Time. A sidereal second is somewhat shorter than a standard second,
	33	about 1.002737909350795 sidereal seconds per standard second.
	34
	35	We resort to fixed point math due to the insufficient resolution of a 'double', using...
	36
	37	timestamp * ( 1.002737909350795 << 31 )
	38	--------------------------------------- + Te
	39	1 << 31
	40
	41	Where Te is the sidereal time at the epoch.
	42
	43	*/
	44
	45	#include <time.h>
	46	#include <stdint.h>
	47
	48	unsigned long
	49	gm_sidereal(const time_t * timer)
	50	{
	51	uint64_t tmp;
	52
	53	tmp = *timer;
	54	tmp *= 0x8059B740;
	55	tmp /= 0x80000000;
	56	tmp += (uint64_t) 23991;
	57
	58	tmp %= ONE_DAY;
	59	return tmp;
	60	}