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LibC: Make localtime() and gmtime() handle years before 1970

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Year computation has to be based on seconds, not days, in case
t is < 0 but t / __seconds_per_day is 0.

Year computation also has to consider negative timestamps.

With this, days is always positive and <= the number of days in the
year, so base the tm_wday computation directly on the timestamp,
and do it first, before t is modified in the year computation.
In C, % can return a negative number if the left operand is negative,
compensate for that.

Tested via test-js. (Except for tm_wday, since we don't implement
Date.prototype.getUTCDate() yet.)
This commit is contained in:
Nico Weber 2020-08-22 22:15:00 -04:00 • committed by Andreas Kling
parent fc28c9b085
commit 5f595e7e1b
2 changed files with 16 additions and 10 deletions
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16
Libraries/LibC/time.cpp
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@ -69,16 +69,24 @@ static const int __seconds_per_day = 60 * 60 * 24;
static void time_to_tm(struct tm* tm, time_t t)
{
tm->tm_wday = (4 * __seconds_per_day + t) % (7 * __seconds_per_day); // 1970-01-01 was a Thursday.
if (tm->tm_wday < 0)
tm->tm_wday += 7 * __seconds_per_day;
tm->tm_wday /= __seconds_per_day;
int year = 1970;
for (; t >= (365 + __is_leap_year(year)) * __seconds_per_day; ++year)
t -= (365 + __is_leap_year(year)) * __seconds_per_day;
for (; t < 0; --year)
t += (365 + __is_leap_year(year - 1)) * __seconds_per_day;
ASSERT(t >= 0);
int days = t / __seconds_per_day;
int remaining = t % __seconds_per_day;
tm->tm_sec = remaining % 60;
remaining /= 60;
tm->tm_min = remaining % 60;
tm->tm_hour = remaining / 60;
tm->tm_wday = (4 + days) % 7;
int year;
for (year = 1970; days >= 365 + __is_leap_year(year); ++year)
days -= 365 + __is_leap_year(year);
tm->tm_year = year - 1900;
tm->tm_yday = days;
tm->tm_mday = 1;