sync datetounix from sfeed.c with more comments - jfconvert - JSON Feed (subset) to sfeed or Atom converter
(HTM) git clone git://git.codemadness.org/jfconvert
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---
(DIR) commit ff5861a0f777bfccd432d267edd466f9cc672872
(DIR) parent 1813cd84b007fe5497847508c6e9804396d774e1
(HTM) Author: Hiltjo Posthuma <hiltjo@codemadness.org>
Date: Mon, 10 Apr 2023 17:11:31 +0200
sync datetounix from sfeed.c with more comments
Diffstat:
M jf2sfeed.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
---
(DIR) diff --git a/jf2sfeed.c b/jf2sfeed.c
@@ -94,11 +94,13 @@ errx(int exitstatus, const char *fmt, ...)
exit(exitstatus);
}
-
-/* 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,
@@ -106,7 +108,9 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec)
int is_leap = 0, cycles, centuries = 0, leaps = 0, rem;
long long t;
+ /* optimization: handle common range year 1902 up to and including 2038 */
if (year - 2ULL <= 136) {
+ /* amount of leap days relative to 1970: every 4 years */
leaps = (year - 68) >> 2;
if (!((year - 68) & 3)) {
leaps--;
@@ -114,8 +118,11 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec)
} else {
is_leap = 0;
}
- t = 31536000 * (year - 70) + 86400 * leaps;
+ t = 31536000 * (year - 70) + (86400 * leaps); /* 365 * 86400 = 31536000 */
} 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) {
@@ -137,7 +144,10 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec)
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) + 946771200LL;
}
t += secs_through_month[mon];