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LibWeb: Add propper rounding to PixelUnits::operator*

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Also moves related constants into the class to make them accessible for
tests.
This commit is contained in:
Hendiadyoin1 2023-07-26 15:02:03 +02:00 • committed by Alexander Kalenik
parent 4896ba11dc
commit f9fc0505fb
3 changed files with 36 additions and 6 deletions
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11
Tests/LibWeb/TestCSSPixels.cpp
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@ -39,6 +39,17 @@ TEST_CASE(multiplication1)
CSSPixels b(4);
CSSPixels c = a * b;
EXPECT_EQ(c, CSSPixels(12));
// Temporary overflow
a = CSSPixels::from_raw(0xFFFF'FFFF >> (CSSPixels::fractional_bits + 1));
b = 1;
EXPECT_EQ((a * b), a);
// Rounding
a = CSSPixels::from_raw(0b01'000001);
b = CSSPixels::from_raw(0b01'100000);
EXPECT_EQ(a * b, CSSPixels(a.to_double() * b.to_double()));
EXPECT_EQ(a * -b, CSSPixels(a.to_double() * -b.to_double()));
}
TEST_CASE(addition2)

26
Userland/Libraries/LibWeb/PixelUnits.cpp
View file

@ -9,9 +9,6 @@
namespace Web {
static i32 const fractional_bits = 6;
static constexpr i32 fixed_point_denominator = 1 << fractional_bits;
CSSPixels::CSSPixels(int value)
{
m_value = value * fixed_point_denominator;
@ -103,9 +100,26 @@ CSSPixels CSSPixels::operator-(CSSPixels const& other) const
CSSPixels CSSPixels::operator*(CSSPixels const& other) const
{
CSSPixels result;
result.set_raw_value((static_cast<i64>(raw_value()) * other.raw_value()) >> fractional_bits);
return result;
i64 value = raw_value();
value *= other.raw_value();
int int_value = AK::clamp_to_int(value >> fractional_bits);
// Rounding:
// If last bit cut off was 1:
if (value & (1u << (fractional_bits - 1))) {
// If the bit after was 1 as well
if (value & (radix_mask >> 2u)) {
// We need to round away from 0
int_value = Checked<int>::saturating_add(int_value, 1);
} else {
// Otherwise we round to the next even value
// Which means we add the least significant bit of the raw integer value
int_value = Checked<int>::saturating_add(int_value, int_value & 1);
}
}
return from_raw(int_value);
}
CSSPixels CSSPixels::operator/(CSSPixels const& other) const

5
Userland/Libraries/LibWeb/PixelUnits.h
View file

@ -52,6 +52,11 @@ constexpr DevicePixels operator%(DevicePixels left, T right) { return left.value
/// See https://www.w3.org/TR/css-values-3/#reference-pixel
class CSSPixels {
public:
static constexpr i32 fractional_bits = 6;
static constexpr i32 fixed_point_denominator = 1 << fractional_bits;
static constexpr i32 radix_mask = fixed_point_denominator - 1;
CSSPixels() = default;
CSSPixels(int value);
CSSPixels(unsigned int value);