mk_gmtime.cpp

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/*
 * (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.
 */

#include "time.h"

time_t
mk_gmtime(const struct tm * timeptr)
{
  time_t res;
  uint32_t tmp;
  int n, m, d, leaps;

  /*
   * Determine elapsed whole days since the epoch to the beginning of
   * this year. Since our epoch is at a conjunction of the leap
   * cycles, we can do this rather quickly.
   */
  n = timeptr->tm_year - 100;
  leaps = 0;
  if (n) {
    m = n - 1;
    leaps = m / 4;
    leaps -= m / 100;
    leaps++;
  }
  tmp = 365UL * n + leaps;

  /*
   * Derive the day of year from month and day of month. We use the
   * pattern of 31 day months followed by 30 day months to our
   * advantage, but we must 'special case' Jan/Feb, and account for a
   * 'phase change' between July and August (153 days after March 1).
   * Note: tm_mday is one based.
   */
  d = timeptr->tm_mday - 1;

  // Handle Jan/Feb as a special case
  if (timeptr->tm_mon < 2) {
    if (timeptr->tm_mon)
      d += 31;
  }
  else {
    n = 59 + is_leap_year(timeptr->tm_year + 1900);
    d += n;
    n = timeptr->tm_mon - MARCH;

    // Account for phase change
    if (n > (JULY - MARCH))
      d += 153;
    n %= 5;

    // N is now an index into a group of alternating 31 and 30
    // day months... 61 day pairs
    m = n / 2;
    m *= 61;
    d += m;

    // If n is odd, we are in the second half of the month pair
    if (n & 1)
      d += 31;
  }

  // Add day of year to elapsed days, and convert to seconds
  tmp += d;
  tmp *= ONE_DAY;
  res = tmp;

  // Compute 'fractional' day
  tmp = timeptr->tm_hour;
  tmp *= ONE_HOUR;
  tmp += timeptr->tm_min * 60UL;
  tmp += timeptr->tm_sec;
  res += tmp;

  return res;
}