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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: [PATCHv2] Add support for leap seconds in zoneinfo files
Date: Thu, 5 Dec 2013 06:18:09 +0100
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* Laurent Bercot <ska-dietlibc@skarnet.org> [2013-12-05 01:13:30 +0000]:
>  
> +static int leapsecs_sub(long long *t)
> +{
> +	long long trans;
> +	int corr;
> +	unsigned int i = __leapsecs_num;
> +	int hit = 0;
> +	for (; i; i--) {
> +		__leapsecs_read(i-1, &trans, &corr);
> +		if (*t >= trans) break;
> +	}
> +	if (i) {
> +		if (*t == trans) hit = 1;
> +		*t -= corr;
> +	}
> +	return hit;
> +}
>  
> +static void leapsecs_add(long long *t, int hit)
> +{
> +	int oldcorr = 0;
> +	unsigned int i = 0;
> +	for (; i < __leapsecs_num; i++) {
> +		long long trans;
> +		int newcorr;
> +		__leapsecs_read(i, &trans, &newcorr);
> +		if (*t < trans) break;
> +		if (!hit || (*t > trans)) {
> +			*t += newcorr - oldcorr;
> +		}
> +		oldcorr = newcorr;
> +	}
> +}
> +

i assume

	hit=leapsecs_sub(&t);
	if(hit) t++;
	leapsecs_add(&t,hit);

should give back the original t,
then the add part can go backwards on the leapsec list instead of forward:

static void leapsecs_add(long long *t, int hit)
{
	long long trans;
	int corr;
	unsigned int i = __leapsecs_num;
	for (; i; i--) {
		__leapsecs_read(i-1, &trans, &corr);
		if (*t+corr > trans) {
			*t += corr;
			// only consider hit at a leapsecond
			if (hit && *t == trans+1) *t--;
			break;
		}
	}
}