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Add function testing whether a bitmask is valid. 

This one verifies bitmasks where low bits are set to 1 (hence the name).
Any stray 0 among the lower ones or any stray 1 among the higher zeros
renders the mask invalid.

The edge cases of all zeros and all ones are considered valid masks.

It uses an efficient implementation. It's the counterpart of
https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
This commit is contained in:
Niels Boehm 2017-06-22 12:36:54 +02:00
parent e30484e8a7
commit df82adca43
2 changed files with 54 additions and 0 deletions
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22
Source/Core/Common/BitUtils.h
View File

@ -99,4 +99,26 @@ constexpr Result ExtractBits(const T src) noexcept
return ExtractBits<T, Result>(src, begin, end);
}
///
/// Verifies whether the supplied value is a valid bit mask of the form 0b00...0011...11.
/// Both edge cases of all zeros and all ones are considered valid masks, too.
///
/// @param mask The mask value to test for validity.
///
/// @tparam T The type of the value.
///
/// @return A bool indicating whether the mask is valid.
///
template <typename T>
constexpr bool IsValidLowMask(const T mask) noexcept
{
static_assert(std::is_integral<T>::value, "Mask must be an integral type.");
static_assert(std::is_unsigned<T>::value, "Signed masks can introduce hard to find bugs.");
// Can be efficiently determined without looping or bit counting. It's the counterpart
// to https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
// and doesn't require special casing either edge case.
return (mask & (mask + 1)) == 0;
}
} // namespace Common

32
Source/UnitTests/Common/BitUtilsTest.cpp
View File

@ -57,3 +57,35 @@ TEST(BitUtils, ExtractBits)
// Ensure bit extraction with type overriding works as expected
EXPECT_EQ((Common::ExtractBits<0, 31, s32, s32>(negative_one)), -1);
}
TEST(BitUtils, IsValidLowMask)
{
EXPECT_TRUE(Common::IsValidLowMask(0b0u));
EXPECT_TRUE(Common::IsValidLowMask(0b1u));
EXPECT_FALSE(Common::IsValidLowMask(0b10u));
EXPECT_TRUE(Common::IsValidLowMask(0b11u));
EXPECT_FALSE(Common::IsValidLowMask(0b1110u));
EXPECT_TRUE(Common::IsValidLowMask(0b1111u));
EXPECT_FALSE(Common::IsValidLowMask(0b10000u));
EXPECT_FALSE(Common::IsValidLowMask(0b101111u));
EXPECT_TRUE(Common::IsValidLowMask((u8)~0b0));
EXPECT_FALSE(Common::IsValidLowMask((u8)(~0b0 - 1)));
EXPECT_FALSE(Common::IsValidLowMask((u8)~(0b10000)));
EXPECT_FALSE(Common::IsValidLowMask((u8)(~((u8)(~0b0) >> 1) | 0b1111)));
EXPECT_TRUE(Common::IsValidLowMask((u16)~0b0));
EXPECT_FALSE(Common::IsValidLowMask((u16)(~0b0 - 1)));
EXPECT_FALSE(Common::IsValidLowMask((u16)~(0b10000)));
EXPECT_FALSE(Common::IsValidLowMask((u16)(~((u16)(~0b0) >> 1) | 0b1111)));
EXPECT_TRUE(Common::IsValidLowMask((u32)~0b0));
EXPECT_FALSE(Common::IsValidLowMask((u32)(~0b0 - 1)));
EXPECT_FALSE(Common::IsValidLowMask((u32)~(0b10000)));
EXPECT_FALSE(Common::IsValidLowMask((u32)(~((u32)(~0b0) >> 1) | 0b1111)));
EXPECT_TRUE(Common::IsValidLowMask((u64)~0b0));
EXPECT_FALSE(Common::IsValidLowMask((u64)(~0b0 - 1)));
EXPECT_FALSE(Common::IsValidLowMask((u64)~(0b10000)));
EXPECT_FALSE(Common::IsValidLowMask((u64)(~((u64)(~0b0) >> 1) | 0b1111)));
}