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AK+Tests: Replace years_to_days_since_epoch by near-instant function

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This solves half the problem of #12729. Note that the inverse function
time_to_tm() in LibC/time.cpp still uses a slow for-loop.

See also #13138
This commit is contained in:
Ben Wiederhake 2023-01-01 17:08:05 +01:00 • committed by Tim Flynn
parent 60ee695287
commit 7a69219a35
2 changed files with 103 additions and 8 deletions
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44
AK/Time.h
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@ -66,14 +66,46 @@ constexpr int days_in_year(int year)
return 365 + (is_leap_year(year) ? 1 : 0);
}
namespace Detail {
// Integer division rounding towards negative infinity.
// TODO: This feels like there should be an easier way to do this.
template<int divisor>
constexpr int floor_div_by(int dividend)
{
static_assert(divisor >= 1);
int is_negative = dividend < 0;
return (dividend + is_negative) / divisor - is_negative;
}
// Counts how many integers n are in the interval [begin, end) with n % positive_mod == 0.
// NOTE: "end" is not considered to be part of the range, hence "[begin, end)".
template<int positive_mod>
constexpr int mod_zeros_in_range(int begin, int end)
{
return floor_div_by<positive_mod>(end - 1) - floor_div_by<positive_mod>(begin - 1);
}
}
constexpr int years_to_days_since_epoch(int year)
{
int days = 0;
for (int current_year = 1970; current_year < year; ++current_year)
days += days_in_year(current_year);
for (int current_year = year; current_year < 1970; ++current_year)
days -= days_in_year(current_year);
return days;
int begin_year, end_year, leap_sign;
if (year < 1970) {
begin_year = year;
end_year = 1970;
leap_sign = -1;
} else {
begin_year = 1970;
end_year = year;
leap_sign = +1;
}
// This duplicates the logic of 'is_leap_year', with the advantage of not needing any loops.
// Given that the definition of leap years is not expected to change, this should be a good trade-off.
int days = 365 * (year - 1970);
int extra_leap_days = 0;
extra_leap_days += Detail::mod_zeros_in_range<4>(begin_year, end_year);
extra_leap_days -= Detail::mod_zeros_in_range<100>(begin_year, end_year);
extra_leap_days += Detail::mod_zeros_in_range<400>(begin_year, end_year);
return days + extra_leap_days * leap_sign;
}
constexpr int days_since_epoch(int year, int month, int day)