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✨ Count Zero Bytes #1412

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Merged
Vectorized merged 3 commits into from
May 7, 2025
Merged

✨ Count Zero Bytes #1412

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Count Zero Bytes
Vectorized May 7, 2025
945becf
T
Vectorized May 7, 2025
f9ad2d2
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Vectorized May 7, 2025
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13 changes: 13 additions & 0 deletions src/utils/LibBit.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -81,6 +81,19 @@ library LibBit {
}
}

/// @dev Returns the number of zero bits in `x`.
/// To get the number of non-zero bytes, simply do `32 - countZeroBytes(x)`
function countZeroBytes(uint256 x) internal pure returns (uint256 c) {
/// @solidity memory-safe-assembly
assembly {
c := 0x7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f7f
c := not(or(or(add(and(x, c), c), x), c)) // `.each(b => b == 0x00 ? 0x80 : 0x00)`.
c := shr(7, c)
c := mul(0x0101010101010101010101010101010101010101010101010101010101010101, c)
c := byte(0, c)
}
}

/// @dev Returns whether `x` is a power of 2.
function isPo2(uint256 x) internal pure returns (bool result) {
/// @solidity memory-safe-assembly
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19 changes: 19 additions & 0 deletions test/LibBit.t.sol
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Expand Up @@ -276,4 +276,23 @@ contract LibBitTest is SoladyTest {
m >>= 1;
}
}

function testCountZeroBytesDifferential(uint256 x) public {
assertEq(LibBit.countZeroBytes(x), _countZeroBytesOriginal(x));
}

function testCountZeroBytes() public {
uint256 x = 0xff00ffffffffffffffffffffffffffffffffffffffffffffffffffffffff00ff;
x |= uint256(uint160(address(this))) << 16;
assertEq(LibBit.countZeroBytes(x), 2);
}

function _countZeroBytesOriginal(uint256 x) internal pure returns (uint256 c) {
unchecked {
for (uint256 i; i < 32; ++i) {
c += ((x & (0xff << (i * 8))) == 0 ? 1 : 0);
}
return c;
}
}
}