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ARROW-6049: [C++] Support view from one dictionary type to another in Array::View #5129

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ARROW-6049: [C++] Support view from one dictionary type to another in Array::View #5129

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28 changes: 15 additions & 13 deletions cpp/src/arrow/array.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1044,15 +1044,6 @@ struct ViewDataImpl {
return Status::OK();
}

Status CheckInputHasNoDictionaries() {
for (const auto& layout : in_layouts) {
if (layout.has_dictionary) {
return InvalidView("input has dictionary");
}
}
return Status::OK();
}

Status CheckInputAtZeroOffset() {
for (const auto& data : in_data) {
if (data->offset != 0) {
Expand All @@ -1062,18 +1053,28 @@ struct ViewDataImpl {
return Status::OK();
}

Status GetDictionaryView(const DataType& out_type, std::shared_ptr<Array>* out) {
if (in_data[in_layout_idx]->type->id() != Type::DICTIONARY) {
return InvalidView("Cannot get view as dictionary type");
}
const auto& dict_out_type = static_cast<const DictionaryType&>(out_type);
return in_data[in_layout_idx]->dictionary->View(dict_out_type.value_type(), out);
}

Status MakeDataView(const std::shared_ptr<Field>& out_field,
std::shared_ptr<ArrayData>* out) {
const auto out_type = out_field->type();
const auto out_layout = out_type->layout();
if (out_layout.has_dictionary) {
return InvalidView("view type requires dictionary");
}

AdjustInputPointer();
int64_t out_length = in_data_length;
int64_t out_null_count;

std::shared_ptr<Array> dictionary;
if (out_type->id() == Type::DICTIONARY) {
RETURN_NOT_OK(GetDictionaryView(*out_type, &dictionary));
}

// No type has a purely empty layout
DCHECK_GT(out_layout.bit_widths.size(), 0);

Expand Down Expand Up @@ -1143,6 +1144,8 @@ struct ViewDataImpl {

std::shared_ptr<ArrayData> out_data =
ArrayData::Make(out_type, out_length, std::move(out_buffers), out_null_count);
out_data->dictionary = dictionary;

// Process children recursively, depth-first
for (const auto& child_field : out_type->children()) {
std::shared_ptr<ArrayData> child_data;
Expand All @@ -1166,7 +1169,6 @@ Status Array::View(const std::shared_ptr<DataType>& out_type,
impl.in_data_length = data_->length;

std::shared_ptr<ArrayData> out_data;
RETURN_NOT_OK(impl.CheckInputHasNoDictionaries());
RETURN_NOT_OK(impl.CheckInputAtZeroOffset());
// Dummy field for output type
auto out_field = field("", out_type);
Expand Down
31 changes: 27 additions & 4 deletions cpp/src/arrow/array_view_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -355,14 +355,37 @@ TEST(TestArrayView, DecimalRoundTrip) {
}

TEST(TestArrayView, Dictionaries) {
// Can't view dictionaries
// ARROW-6049
auto ty1 = dictionary(int8(), float32());
auto ty2 = dictionary(int8(), int32());

auto indices = ArrayFromJSON(int8(), "[0, 2, null, 1]");
auto values = ArrayFromJSON(float32(), "[0.0, 1.5, -2.5]");
std::shared_ptr<Array> arr;

std::shared_ptr<Array> arr, expected, expected_dict;
ASSERT_OK(values->View(int32(), &expected_dict));
ASSERT_OK(DictionaryArray::FromArrays(ty1, indices, values, &arr));
CheckViewFails(arr, int8());
CheckViewFails(indices, ty1);
ASSERT_OK(DictionaryArray::FromArrays(ty2, indices, expected_dict, &expected));

CheckView(arr, expected);
CheckView(expected, arr);

// Incompatible index type
auto ty3 = dictionary(int16(), int32());
CheckViewFails(arr, ty3);

// Incompatible dictionary type
auto ty4 = dictionary(int16(), float64());
CheckViewFails(arr, ty4);

// Check dictionary-encoded child
auto offsets = ArrayFromJSON(int32(), "[0, 2, 2, 4]");
std::shared_ptr<Array> list_arr, expected_list_arr;
ASSERT_OK(ListArray::FromArrays(*offsets, *arr, default_memory_pool(), &list_arr));
ASSERT_OK(ListArray::FromArrays(*offsets, *expected, default_memory_pool(),
&expected_list_arr));
CheckView(list_arr, expected_list_arr);
CheckView(expected_list_arr, list_arr);
}

TEST(TestArrayView, ExtensionType) {
Expand Down