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GH-39635: [C++] Keep null in unique/dictionary output for view types #53256

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GH-39635: [C++] Keep null in unique/dictionary output for view types #53256

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11 changes: 9 additions & 2 deletions cpp/src/arrow/array/dict_internal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -166,8 +166,15 @@ struct DictionaryTraits<T, enable_if_binary_view_like<T>> {
BinaryViewBuilder builder(pool);
RETURN_NOT_OK(builder.Resize(memo_table.size() - start_offset));
RETURN_NOT_OK(builder.ReserveData(memo_table.values_size()));
memo_table.VisitValues(static_cast<int32_t>(start_offset),
[&](std::string_view s) { builder.UnsafeAppend(s); });
const int32_t null_index = memo_table.GetNull();
int32_t index = static_cast<int32_t>(start_offset);
memo_table.VisitValues(static_cast<int32_t>(start_offset), [&](std::string_view s) {
if (index++ == null_index) {
builder.UnsafeAppendNull();
} else {
builder.UnsafeAppend(s);
}
});

std::shared_ptr<ArrayData> out;
RETURN_NOT_OK(builder.FinishInternal(&out));
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12 changes: 12 additions & 0 deletions cpp/src/arrow/compute/kernels/vector_hash_test.cc
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Expand Up @@ -538,6 +538,18 @@ TYPED_TEST(TestHashKernelBinaryTypes, BinaryResizeTable) {
this->CheckDictEncodeP(values, {}, uniques, {}, indices);
}

TEST_F(TestHashKernel, UniqueBinaryViewWithNull) {
// GH-39635: the null slot must stay null in the unique values
for (const auto& type : {binary_view(), utf8_view()}) {
ARROW_SCOPED_TRACE(type->ToString());
CheckUnique(ArrayFromJSON(type, R"(["test", "a string that is not inlined", null,
"test"])"),
ArrayFromJSON(type, R"(["test", "a string that is not inlined", null])"));
CheckUnique(ArrayFromJSON(type, R"(["ab", null, "cd", "ab", null])")->Slice(1, 3),
ArrayFromJSON(type, R"([null, "cd", "ab"])"));
}
}

TEST_F(TestHashKernel, UniqueFixedSizeBinary) {
auto type = fixed_size_binary(3);

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