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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
Notes: sideshowbarker 2024-07-17 02:17:12 +09:00 
Author: https://github.com/BenWiederhake
Commit: https://github.com/SerenityOS/serenity/commit/7a69219a35
Pull-request: https://github.com/SerenityOS/serenity/pull/16760
Reviewed-by: https://github.com/nico ✅
Reviewed-by: https://github.com/trflynn89 ✅
2 changed files with 103 additions and 8 deletions
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44
AK/Time.h
View file

@ -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)

67
Tests/AK/TestTime.cpp
View file

@ -317,8 +317,7 @@ TEST_CASE(years_to_days_since_epoch_points)
BENCHMARK_CASE(years_to_days_since_epoch_benchmark)
{
// This benchmark takes consistently about 295±1 ms on Linux, and roughly 2300 ms on Serenity.
// TODO: Computing the amount of days should never take dozens of milliseconds.
// This benchmark takes consistently "0ms" on Linux, and "0ms" on Serenity.
for (size_t i = 0; i < 100; ++i) {
int actual_days = years_to_days_since_epoch(-5877640);
(void)actual_days;
@ -326,6 +325,70 @@ BENCHMARK_CASE(years_to_days_since_epoch_benchmark)
}
}
TEST_CASE(div_floor_by)
{
EXPECT_EQ(AK::Detail::floor_div_by<4>(-5), -2);
EXPECT_EQ(AK::Detail::floor_div_by<4>(-4), -1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(-3), -1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(-2), -1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(-1), -1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+0), +0);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+1), +0);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+2), +0);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+3), +0);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+4), +1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+5), +1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+6), +1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+7), +1);
EXPECT_EQ(AK::Detail::floor_div_by<4>(+8), +2);
}
TEST_CASE(mod_zeros_in_range)
{
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 0), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 1), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 2), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 3), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 4), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 5), 2);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(0, 6), 2);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 1), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 2), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 3), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 4), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 5), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(1, 6), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(2, 2), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(2, 3), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(2, 4), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(2, 5), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(2, 6), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(3, 3), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(3, 4), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(3, 5), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(3, 6), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(4, 4), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(4, 5), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(4, 6), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(5, 5), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(5, 6), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(6, 6), 0);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-5, 3), 2);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-4, 3), 2);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-3, 3), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-2, 3), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-1, 3), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(-0, 3), 1);
EXPECT_EQ(AK::Detail::mod_zeros_in_range<4>(+1, 3), 0);
}
TEST_CASE(years_to_days_since_epoch_span)
{
auto test_data_start_year = 1900;