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GH-35521: [C++] Hash null bitmap only if null count is 0 #35522

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merged 4 commits into from
May 11, 2023
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GH-35521: [C++] Hash null bitmap only if null count is 0 #35522

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9cad50d
Only hash array data bitmaps if the null count is not 0
micah-white May 9, 2023
d10fe67
not working test
micah-white May 9, 2023
d55a593
fix nits and test
micah-white May 10, 2023
f4d0ac8
Testing nits: ensure better output on error, add comments
pitrou May 11, 2023
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5 changes: 3 additions & 2 deletions cpp/src/arrow/scalar.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -153,9 +153,10 @@ struct ScalarHashImpl {

Status ArrayHash(const ArrayData& a) {
RETURN_NOT_OK(StdHash(a.length) & StdHash(a.GetNullCount()));
if (a.buffers[0] != nullptr) {
if (a.GetNullCount() != 0 && a.buffers[0] != nullptr) {
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IIRC, it's also possible that a.buffers[0] == nullptr if all the elements are valid. Is it possible that we still get differing hashes in this case?

  • All elements valid and equal and validity bitmap present (would hash the validity bitmap)
  • All elements valid and equal and validity bitmap missing (would not hash the validity bitmap)

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@micah-white micah-white May 10, 2023 •
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Yes, that's the bug that I am trying to fix. Since both cases have the same semantic value, their hashes should be the same.

// We can't visit values without unboxing the whole array, so only hash
// the null bitmap for now.
// the null bitmap for now. Only hash the null bitmap if the null count
// is not 0 to ensure hash consistency.
RETURN_NOT_OK(BufferHash(*a.buffers[0]));
}
for (const auto& child : a.child_data) {
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21 changes: 20 additions & 1 deletion cpp/src/arrow/scalar_test.cc
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Expand Up @@ -1064,7 +1064,6 @@ class TestListScalar : public ::testing::Test {
using ScalarType = typename TypeTraits<T>::ScalarType;

void SetUp() {
// type_ = std::make_shared<T>(int16());
type_ = MakeListType<T>(int16(), 3);
value_ = ArrayFromJSON(int16(), "[1, 2, null]");
}
Expand Down Expand Up @@ -1106,6 +1105,24 @@ class TestListScalar : public ::testing::Test {
ASSERT_RAISES(Invalid, scalar.ValidateFull());
}

void TestHashing() {
// GH-35521: the hash value of a non-null list scalar should not
// depend on the presence or absence of a null bitmap in the underlying
// list values.
ScalarType empty_bitmap_scalar(ArrayFromJSON(int16(), "[1, 2, 3]"));
ASSERT_OK(empty_bitmap_scalar.ValidateFull());
// Underlying list array doesn't have a null bitmap
ASSERT_EQ(empty_bitmap_scalar.value->data()->buffers[0], nullptr);

auto list_array = ArrayFromJSON(type_, "[[1, 2, 3], [4, 5, null]]");
ASSERT_OK_AND_ASSIGN(auto set_bitmap_scalar_uncasted, list_array->GetScalar(0));
auto set_bitmap_scalar = checked_pointer_cast<ScalarType>(set_bitmap_scalar_uncasted);
// Underlying list array has a null bitmap
ASSERT_NE(set_bitmap_scalar->value->data()->buffers[0], nullptr);
// ... yet it's hashing equal to the other scalar
ASSERT_EQ(empty_bitmap_scalar.hash(), set_bitmap_scalar->hash());
}

protected:
std::shared_ptr<DataType> type_;
std::shared_ptr<Array> value_;
Expand All @@ -1119,6 +1136,8 @@ TYPED_TEST(TestListScalar, Basics) { this->TestBasics(); }

TYPED_TEST(TestListScalar, ValidateErrors) { this->TestValidateErrors(); }

TYPED_TEST(TestListScalar, TestHashing) { this->TestHashing(); }

TEST(TestFixedSizeListScalar, ValidateErrors) {
const auto ty = fixed_size_list(int16(), 3);
FixedSizeListScalar scalar(ArrayFromJSON(int16(), "[1, 2, 5]"), ty);
Expand Down