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	sfeed.c: datetounix: add comments - sfeed - RSS and Atom parser
	git clone git://git.codemadness.org/sfeed
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	---
	commit a15c98f32d8a768fee7b6783f7cea710cc5878a9
	parent 0c938df27bf335fb290d6ea3612f5b397ed3da56
	Author: Hiltjo Posthuma <[email protected]>
	Date: Fri, 7 Apr 2023 18:43:06 +0200

	sfeed.c: datetounix: add comments

	Diffstat:
	M sfeed.c \| 17 ++++++++++++++---

	1 file changed, 14 insertions(+), 3 deletions(-)
	---
	diff --git a/sfeed.c b/sfeed.c
	@@ -432,10 +432,13 @@ string_print_timestamp(String *s)
	printf("%lld", t);
	}

	-/* Convert time fields. Returns a UNIX timestamp. */
	+/* Convert time fields. Returns a signed (atleast) 64-bit UNIX timestamp.
	+ Parameters should be passed as they are in a struct tm:
	+ that is: year = year - 1900, month = month - 1. */
	static long long
	datetounix(long long year, int mon, int day, int hour, int min, int sec)
	{
	+ /* seconds in a month in a regular (non-leap) year */
	static const long secs_through_month[] = {
	0, 31 * 86400, 59 * 86400, 90 * 86400,
	120 * 86400, 151 * 86400, 181 * 86400, 212 * 86400,
	@@ -443,7 +446,9 @@ datetounix(long long year, int mon, int day, int hour, int …
	int is_leap = 0, cycles, centuries = 0, leaps = 0, rem;
	long long t;

	+ /* optimization: handle common range year 1902 up to and including 203…
	if (year - 2ULL <= 136) {
	+ /* amount of leap days relative to 1970: every 4 years */
	leaps = (year - 68) >> 2;
	if (!((year - 68) & 3)) {
	leaps--;
	@@ -451,8 +456,11 @@ datetounix(long long year, int mon, int day, int hour, int…
	} else {
	is_leap = 0;
	}
	- t = 31536000 * (year - 70) + 86400 * leaps;
	+ t = 31536000 * (year - 70) + (86400 * leaps); /* 365 * 86400 =…
	} else {
	+ /* general leap year calculation:
	+ leap years occur mostly every 4 years but every 100 years
	+ a leap year is skipped unless the year is divisible by 400 …
	cycles = (year - 100) / 400;
	rem = (year - 100) % 400;
	if (rem < 0) {
	@@ -474,7 +482,10 @@ datetounix(long long year, int mon, int day, int hour, int…
	is_leap = !rem;
	}
	}
	- leaps += 97 * cycles + 24 * centuries - is_leap;
	+ leaps += (97 * cycles) + (24 * centuries) - is_leap;
	+
	+ /* adjust 8 leap days from 1970 up to and including 2000:
	+ ((30 * 365) + 8) * 86400 = 946771200 */
	t = ((year - 100) * 31536000LL) + (leaps * 86400LL) + 94677120…
	}
	t += secs_through_month[mon];