mkgmtime.c

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/* mkgmtime.c - make time corresponding to a GMT timeval struct
 $Id: mkgmtime.c,v 1.10 2003/10/22 18:50:12 rjs3 Exp $
 
 * Copyright (c) 1998-2003 Carnegie Mellon University.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer. 
 *
 * 2. 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.
 *
 * 3. The name "Carnegie Mellon University" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For permission or any other legal
 *    details, please contact  
 *      Office of Technology Transfer
 *      Carnegie Mellon University
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 *      tech-transfer@andrew.cmu.edu
 *
 * 4. Redistributions of any form whatsoever must retain the following
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 *    "This product includes software developed by Computing Services
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/*
 * Copyright (c) 1987, 1989, 1993
 *  The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Arthur David Olson of the National Cancer Institute.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. 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.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *  This product includes software developed by the University of
 *  California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
 */

/*
** Adapted from code provided by Robert Elz, who writes:
**  The "best" way to do mktime I think is based on an idea of Bob
**  Kridle's (so its said...) from a long time ago. (mtxinu!kridle now).
**  It does a binary search of the time_t space.  Since time_t's are
**  just 32 bits, its a max of 32 iterations (even at 64 bits it
**  would still be very reasonable).
*/

#include "ublox_gps/mkgmtime.h"
#ifndef WRONG
#define WRONG   (-1)
#endif /* !defined WRONG */

static int tmcomp(register const struct tm * const  atmp, 
                  register const struct tm * const btmp)
{
    register int    result;

    if ((result = (atmp->tm_year - btmp->tm_year)) == 0 &&
        (result = (atmp->tm_mon - btmp->tm_mon)) == 0 &&
        (result = (atmp->tm_mday - btmp->tm_mday)) == 0 &&
        (result = (atmp->tm_hour - btmp->tm_hour)) == 0 &&
        (result = (atmp->tm_min - btmp->tm_min)) == 0)
            result = atmp->tm_sec - btmp->tm_sec;
    return result;
}

time_t mkgmtime(struct tm * const tmp) {
    register int            dir;
    register int            bits;
    register int            saved_seconds;
    time_t              t;
    struct tm           yourtm, *mytm;

    yourtm = *tmp;
    saved_seconds = yourtm.tm_sec;
    yourtm.tm_sec = 0;
    /*
    ** Calculate the number of magnitude bits in a time_t
    ** (this works regardless of whether time_t is
    ** signed or unsigned, though lint complains if unsigned).
    */
    for (bits = 0, t = 1; t > 0; ++bits, t <<= 1)
        ;
    /*
    ** If time_t is signed, then 0 is the median value,
    ** if time_t is unsigned, then 1 << bits is median.
    */
    t = (t < 0) ? 0 : ((time_t) 1 << bits);

    /* Some gmtime() implementations are broken and will return
     * NULL for time_ts larger than 40 bits even on 64-bit platforms
     * so we'll just cap it at 40 bits */
    if(bits > 40) bits = 40;

    for ( ; ; ) {
        mytm = gmtime(&t);

        if(!mytm) return WRONG;

        dir = tmcomp(mytm, &yourtm);
        if (dir != 0) {
            if (bits-- < 0)
                return WRONG;
            if (bits < 0)
                --t;
            else if (dir > 0)
                t -= (time_t) 1 << bits;
            else    t += (time_t) 1 << bits;
            continue;
        }
        break;
    }
    t += saved_seconds;
    return t;
}