sfeed.c: datetounix: add comments - sfeed - RSS and Atom parser
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---
(DIR) commit a15c98f32d8a768fee7b6783f7cea710cc5878a9
(DIR) parent 0c938df27bf335fb290d6ea3612f5b397ed3da56
(HTM) Author: Hiltjo Posthuma <hiltjo@codemadness.org>
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(-)
---
(DIR) 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 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--;
@@ -451,8 +456,11 @@ 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) {
@@ -474,7 +482,10 @@ 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];