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AK+Kernel: Eliminate UB (signed overflow) from days_since_epoch

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Ben Wiederhake 2023-01-01 20:07:27 +01:00 • committed by Tim Flynn
parent 7a69219a35
commit 3334cf675a
Notes: sideshowbarker 2024-07-17 22:09:47 +09:00
3 changed files with 20 additions and 16 deletions
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15
AK/Time.h
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@ -70,9 +70,9 @@ 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)
constexpr i64 floor_div_by(i64 dividend)
{
static_assert(divisor >= 1);
static_assert(divisor > 1);
int is_negative = dividend < 0;
return (dividend + is_negative) / divisor - is_negative;
}
@ -80,13 +80,13 @@ constexpr int floor_div_by(int dividend)
// 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)
constexpr i64 mod_zeros_in_range(i64 begin, i64 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)
constexpr i64 years_to_days_since_epoch(int year)
{
int begin_year, end_year, leap_sign;
if (year < 1970) {
@ -98,17 +98,18 @@ constexpr int years_to_days_since_epoch(int year)
end_year = year;
leap_sign = +1;
}
i64 year_i64 = year;
// 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;
i64 days = 365 * (year_i64 - 1970);
i64 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)
constexpr i64 days_since_epoch(int year, int month, int day)
{
return years_to_days_since_epoch(year) + day_of_year(year, month, day);
}