text: /*
   * The MIT License (MIT)
   *
   * Copyright (c) 2015 Derick Rethans
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   * THE SOFTWARE.
   */
  
  #include "timelib.h"
  #include "timelib_private.h"
  #include <math.h>
  
  timelib_rel_time *timelib_diff(timelib_time *one, timelib_time *two)
  {
  	timelib_rel_time *rt;
  	timelib_time *swp;
  	timelib_sll dst_corr = 0 ,dst_h_corr = 0, dst_m_corr = 0;
  	timelib_time one_backup, two_backup;
  
  	rt = timelib_rel_time_ctor();
  	rt->invert = 0;
  	if (
  		(one->sse > two->sse) ||
  		(one->sse == two->sse && one->us > two->us)
  	) {
  		swp = two;
  		two = one;
  		one = swp;
  		rt->invert = 1;
  	}
  
  	/* Calculate correction for DST change over, but only if the TZ type is ID
  	 * and it's the same */
  	if (one->zone_type == 3 && two->zone_type == 3
  		&& (strcmp(one->tz_info->name, two->tz_info->name) == 0)
  		&& (one->z != two->z))
  	{
  		dst_corr = two->z - one->z;
  		dst_h_corr = dst_corr / 3600;
  		dst_m_corr = (dst_corr % 3600) / 60;
  	}
  
  	/* Save old TZ info */
  	memcpy(&one_backup, one, sizeof(one_backup));
  	memcpy(&two_backup, two, sizeof(two_backup));
  
      timelib_apply_localtime(one, 0);
      timelib_apply_localtime(two, 0);
  
  	rt->y = two->y - one->y;
  	rt->m = two->m - one->m;
  	rt->d = two->d - one->d;
  	rt->h = two->h - one->h;
  	rt->i = two->i - one->i;
  	rt->s = two->s - one->s;
  	rt->us = two->us - one->us;
  	if (one_backup.dst == 0 && two_backup.dst == 1 && two->sse >= one->sse + 86400 - dst_corr) {
  		rt->h += dst_h_corr;
  		rt->i += dst_m_corr;
  	}
  
  	rt->days = fabs(floor((one->sse - two->sse - (dst_h_corr * 3600) - (dst_m_corr * 60)) / 86400));
  
  	timelib_do_rel_normalize(rt->invert ? one : two, rt);
  
  	/* We need to do this after normalisation otherwise we can't get "24H" */
  	if (one_backup.dst == 1 && two_backup.dst == 0 && two->sse >= one->sse + 86400) {
  		if (two->sse < one->sse + 86400 - dst_corr) {
  			rt->d--;
  			rt->h = 24;
  		} else {
  			rt->h += dst_h_corr;
  			rt->i += dst_m_corr;
  		}
  	}
  
  	/* Restore old TZ info */
  	memcpy(one, &one_backup, sizeof(one_backup));
  	memcpy(two, &two_backup, sizeof(two_backup));
  
  	return rt;
  }
  
  timelib_time *timelib_add(timelib_time *old_time, timelib_rel_time *interval)
  {
  	int bias = 1;
  	timelib_time *t = timelib_time_clone(old_time);
  
  	if (interval->have_weekday_relative || interval->have_special_relative) {
  		memcpy(&t->relative, interval, sizeof(timelib_rel_time));
  	} else {
  		if (interval->invert) {
  			bias = -1;
  		}
  		memset(&t->relative, 0, sizeof(timelib_rel_time));
  		t->relative.y = interval->y * bias;
  		t->relative.m = interval->m * bias;
  		t->relative.d = interval->d * bias;
  		t->relative.h = interval->h * bias;
  		t->relative.i = interval->i * bias;
  		t->relative.s = interval->s * bias;
  		t->relative.us = interval->us * bias;
  	}
  	t->have_relative = 1;
  	t->sse_uptodate = 0;
  
  	timelib_update_ts(t, NULL);
  
  //	printf("%lld %lld %d\n", old_time->dst, t->dst, (t->sse - old_time->sse));
  	/* Adjust for backwards DST changeover */
  	if (old_time->dst == 1 && t->dst == 0 && !interval->y && !interval->m && !interval->d) {
  		t->sse -= old_time->z;
  		t->sse += t->z;
  	}
  
  	timelib_update_from_sse(t);
  	t->have_relative = 0;
  
  	return t;
  }
  
  timelib_time *timelib_sub(timelib_time *old_time, timelib_rel_time *interval)
  {
  	int bias = 1;
  	timelib_time *t = timelib_time_clone(old_time);
  
  	if (interval->invert) {
  		bias = -1;
  	}
  
  	memset(&t->relative, 0, sizeof(timelib_rel_time));
  	t->relative.y = 0 - (interval->y * bias);
  	t->relative.m = 0 - (interval->m * bias);
  	t->relative.d = 0 - (interval->d * bias);
  	t->relative.h = 0 - (interval->h * bias);
  	t->relative.i = 0 - (interval->i * bias);
  	t->relative.s = 0 - (interval->s * bias);
  	t->relative.us = 0 - (interval->us * bias);
  	t->have_relative = 1;
  	t->sse_uptodate = 0;
  
  	timelib_update_ts(t, NULL);
  
  	/* Adjust for backwards DST changeover */
  	if (old_time->dst == 1 && t->dst == 0 && !interval->y && !interval->m && !interval->d) {
  		t->sse -= old_time->z;
  		t->sse += t->z;
  	}
  	/* Adjust for forwards DST changeover */
  	if (old_time->dst == 0 && t->dst == 1 && !interval->y && !interval->m && !interval->d ) {
  		t->sse -= old_time->z;
  		t->sse += t->z;
  	}
  
  	timelib_update_from_sse(t);
  
  	t->have_relative = 0;
  
  	return t;
  }
  