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Add function testing whether a bitmask is valid.

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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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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)));
}