fix: BitMaskedArray mask inversion and placeholder detection

[henryiii]

🤖 AI text below 🤖

Summary

Three silent bugs fixed across BitMaskedArray, ByteMaskedArray, and UnionArray:

1. BitMaskedArray.to_BitMaskedArray — bitmask inversion (data corruption)
src/awkward/contents/bitmaskedarray.py:443 used nplike.logical_not(self._mask.data) — a boolean byte-level NOT — to flip valid_when when lsb_order is unchanged. This corrupts packed bitmasks: e.g. 0b10011010 becomes [False] rather than the correct bitwise inverse 0b01100101. Changed to ~self._mask.data (bitwise invert), matching the lsb_order-change branch at line 459.

2. BitMaskedArray._unique — spurious out._content unwrap
src/awkward/contents/bitmaskedarray.py:659-668 had a conditional that returned out._content when negaxis is not None, stripping the option-type wrapper from the result. The sibling ByteMaskedArray._unique (bytemaskedarray.py) returns out unchanged. Removed the branch to match ByteMaskedArray semantics.

3. _is_getitem_at_placeholder checks Index wrapper instead of buffer
Three methods checked isinstance(self._mask/tags/index, PlaceholderArray) against the Index wrapper object, which is never a PlaceholderArray. Siblings ListOffsetArray (listoffsetarray.py:314) and IndexedArray (indexedarray.py:294) correctly check .data. Fixed in:

• src/awkward/contents/bitmaskedarray.py:499-502
• src/awkward/contents/bytemaskedarray.py:380-383
• src/awkward/contents/unionarray.py:587-591

Test plan

• Regression tests in tests/test_4092_bitmasked_fixes.py: one test per bug fixed (Fix 1: inversion, Fix 2: unique consistency)
• Existing BitMasked tests pass: test_3033_flatten_bitmaskedarray.py, test_0127_tomask_operation.py, test_1260_simplify_masked_option_types.py
• All changes linted with prek -a --quiet (clean)

AI-assisted contribution: fixes identified and implemented with Claude Code (Anthropic), from an automated multi-agent review.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
6304	1	6303	34

View the top 1 failed test(s) by shortest run time

tests/test_0404_array_validity_check.py::test_BitMaskedArray

Stack Traces | 0.006s run time

def test_BitMaskedArray():
        content = ak.contents.NumpyArray(np.arange(13))
        mask = ak.index.IndexU8(np.array([58, 59], dtype=np.uint8))
        array = ak.contents.BitMaskedArray(
            mask, content, valid_when=False, length=13, lsb_order=False
        )
        assert np.asarray(array.mask_as_bool(valid_when=True)).tolist() == [
            True,
            True,
            False,
            False,
            False,
            True,
            False,
            True,
            True,
            True,
            False,
            False,
            False,
        ]
        assert np.asarray(array.to_ByteMaskedArray().mask).tolist() == [
            0,
            0,
            1,
            1,
            1,
            0,
            1,
            0,
            0,
            0,
            1,
            1,
            1,
        ]
        assert np.asarray(array.to_IndexedOptionArray64().index).tolist() == [
            0,
            1,
            -1,
            -1,
            -1,
            5,
            -1,
            7,
            8,
            9,
            -1,
            -1,
            -1,
        ]
        assert to_list(array) == [
            0,
            1,
            None,
            None,
            None,
            5,
            None,
            7,
            8,
            9,
            None,
            None,
            None,
        ]
        assert ak._do.is_unique(array, axis=None) is True
        assert to_list(ak._do.unique(array, axis=None)) == [0, 1, 5, 7, 8, 9, None]
>       assert to_list(ak._do.unique(array, axis=-1)) == [0, 1, 5, 7, 8, 9, None]
E       assert [[0, 1, 5, 7, 8, 9, None]] == [0, 1, 5, 7, 8, 9, None]
E         
E         At index 0 diff: [0, 1, 5, 7, 8, 9, None] != 0
E         Right contains 6 more items, first extra item: 1
E         
E         Full diff:
E           [
E         +     [
E         +         0,
E         +         1,
E         +         5,
E         +         7,
E         +         8,
E         +         9,
E         +         None,
E         -     0,
E         ?     ^
E         +     ],
E         ?     ^
E         -     1,
E         -     5,
E         -     7,
E         -     8,
E         -     9,
E         -     None,
E           ]

array      = <BitMaskedArray valid_when='false' lsb_order='false' len='13'>
    <mask><Index dtype='uint8' len='2'>
        [58 59]
    </Index></mask>
    <content><NumpyArray dtype='int64' len='13'>
        [ 0  1  2  3  4  5  6  7  8  9 10 11 12]
    </NumpyArray></content>
</BitMaskedArray>
content    = <NumpyArray dtype='int64' len='13'>
    [ 0  1  2  3  4  5  6  7  8  9 10 11 12]
</NumpyArray>
mask       = <Index dtype='uint8' len='2'>
    [58 59]
</Index>

tests/test_0404_array_validity_check.py:81: AssertionError

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