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sfeed.c: datetounix: add comments - sfeed - RSS and Atom parser |
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git clone git://git.codemadness.org/sfeed (git://git.codemadness.org) |
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--- |
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commit a15c98f32d8a768fee7b6783f7cea710cc5878a9 |
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parent 0c938df27bf335fb290d6ea3612f5b397ed3da56 |
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Author: Hiltjo Posthuma <hiltjo@codemadness.org> (mailto://) |
application/vnd.lotus-organizer |
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Date: Fri, 7 Apr 2023 18:43:06 +0200 |
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sfeed.c: datetounix: add comments |
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Diffstat: |
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M sfeed.c | 17 ++++++++++++++--- |
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1 file changed, 14 insertions(+), 3 deletions(-) |
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--- |
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diff --git a/sfeed.c b/sfeed.c |
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@@ -432,10 +432,13 @@ string_print_timestamp(String *s) |
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printf("%lld", t); |
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} |
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-/* Convert time fields. Returns a UNIX timestamp. */ |
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+/* Convert time fields. Returns a signed (atleast) 64-bit UNIX timestamp. |
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+ Parameters should be passed as they are in a struct tm: |
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+ that is: year = year - 1900, month = month - 1. */ |
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static long long |
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datetounix(long long year, int mon, int day, int hour, int min, int sec) |
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{ |
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+ /* seconds in a month in a regular (non-leap) year */ |
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static const long secs_through_month[] = { |
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0, 31 * 86400, 59 * 86400, 90 * 86400, |
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120 * 86400, 151 * 86400, 181 * 86400, 212 * 86400, |
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@@ -443,7 +446,9 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) |
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int is_leap = 0, cycles, centuries = 0, leaps = 0, rem; |
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long long t; |
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+ /* optimization: handle common range year 1902 up to and including 2038 */ |
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if (year - 2ULL <= 136) { |
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+ /* amount of leap days relative to 1970: every 4 years */ |
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leaps = (year - 68) >> 2; |
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if (!((year - 68) & 3)) { |
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leaps--; |
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@@ -451,8 +456,11 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) |
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} else { |
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is_leap = 0; |
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} |
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- t = 31536000 * (year - 70) + 86400 * leaps; |
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+ t = 31536000 * (year - 70) + (86400 * leaps); /* 365 * 86400 = 31536000 */ |
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} else { |
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+ /* general leap year calculation: |
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+ leap years occur mostly every 4 years but every 100 years |
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+ a leap year is skipped unless the year is divisible by 400 */ |
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cycles = (year - 100) / 400; |
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rem = (year - 100) % 400; |
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if (rem < 0) { |
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@@ -474,7 +482,10 @@ datetounix(long long year, int mon, int day, int hour, int min, int sec) |
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is_leap = !rem; |
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} |
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} |
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- leaps += 97 * cycles + 24 * centuries - is_leap; |
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+ leaps += (97 * cycles) + (24 * centuries) - is_leap; |
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+ |
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+ /* adjust 8 leap days from 1970 up to and including 2000: |
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+ ((30 * 365) + 8) * 86400 = 946771200 */ |
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t = ((year - 100) * 31536000LL) + (leaps * 86400LL) + 946771200LL; |
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} |
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t += secs_through_month[mon]; |
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