milesgranger commented on code in PR #14395:
URL: https://github.com/apache/arrow/pull/14395#discussion_r1011543980
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cpp/src/arrow/compute/kernels/scalar_nested.cc:
##########
@@ -87,6 +89,199 @@ Status GetListElementIndex(const ExecValue& value, T* out) {
return Status::OK();
}
+template <typename Type, typename IndexType>
+struct ListSlice {
+ using offset_type = typename Type::offset_type;
+
+ static Status Exec(KernelContext* ctx, const ExecSpan& batch, ExecResult*
out) {
+ const auto opts = OptionsWrapper<ListSliceOptions>::Get(ctx);
+
+ // Invariants
+ if (opts.start < 0 || (opts.start >= opts.stop && opts.stop != -1)) {
+ // TODO: support start == stop which should give empty lists
+ return Status::Invalid("`start`(", opts.start,
+ ") should be greater than 0 and smaller than
`stop`(",
+ opts.stop, ")");
+ }
+ if (opts.step != 1) {
+ // TODO: support step in slicing
+ return Status::NotImplemented(
+ "Setting `step` to anything other than 1 is not supported; got
step=",
+ opts.step);
+ }
+ if (opts.stop == -1) {
+ // TODO: Support slicing to arbitrary end
+ // For variable size list, this would be the largest difference in
offsets
+ // For fixed size list, this would be the fixed size.
+ return Status::NotImplemented(
+ "Setting `stop==-1` to signify slicing to end, not yet
implemented.");
+ }
+
+ const ArraySpan& list_ = batch[0].array;
+ const Type* list_type = checked_cast<const Type*>(list_.type);
+ const auto value_type = list_type->value_type();
+
+ std::unique_ptr<ArrayBuilder> builder;
+
+ // construct array values
+ if (opts.return_fixed_size_list) {
+ RETURN_NOT_OK(MakeBuilder(
+ ctx->memory_pool(),
+ fixed_size_list(value_type, static_cast<int32_t>(opts.stop -
opts.start)),
+ &builder));
+ RETURN_NOT_OK(BuildArray<FixedSizeListBuilder>(batch, opts, *builder));
+ } else {
+ if constexpr (std::is_same_v<Type, LargeListType>) {
+ RETURN_NOT_OK(MakeBuilder(ctx->memory_pool(), large_list(value_type),
&builder));
+ RETURN_NOT_OK(BuildArray<LargeListBuilder>(batch, opts, *builder));
+ } else {
+ RETURN_NOT_OK(MakeBuilder(ctx->memory_pool(), list(value_type),
&builder));
+ RETURN_NOT_OK(BuildArray<ListBuilder>(batch, opts, *builder));
+ }
+ }
+
+ // build output arrays and set result
+ ARROW_ASSIGN_OR_RAISE(auto result, builder->Finish());
+ out->value = result->data();
+ return Status::OK();
+ }
+
+ template <typename BuilderType>
+ static Status BuildArray(const ExecSpan& batch, const ListSliceOptions& opts,
+ ArrayBuilder& builder) {
+ if constexpr (std::is_same_v<Type, FixedSizeListType>) {
+ RETURN_NOT_OK(BuildArrayFromFixedSizeListType<BuilderType>(batch, opts,
builder));
+ } else {
+ RETURN_NOT_OK(BuildArrayFromListType<BuilderType>(batch, opts, builder));
+ }
+ return Status::OK();
+ }
+
+ template <typename BuilderType>
+ static Status BuildArrayFromFixedSizeListType(const ExecSpan& batch,
+ const ListSliceOptions& opts,
+ ArrayBuilder& builder) {
+ const auto list_size =
+ checked_cast<const FixedSizeListType&>(*batch[0].type()).list_size();
+ const ArraySpan& list_ = batch[0].array;
+ const ArraySpan& list_values = list_.child_data[0];
+ const offset_type n_offsets = list_values.length / list_size;
+
+ auto list_builder = checked_cast<BuilderType*>(&builder);
+ for (offset_type offset = 0; offset < n_offsets * list_size;
+ offset = offset + list_size) {
+ auto next_offset = offset + list_size;
+ if (list_values.IsNull(offset)) {
+ RETURN_NOT_OK(list_builder->AppendNull());
+ } else {
+ RETURN_NOT_OK(SetValues<BuilderType>(list_builder, offset,
next_offset, &opts,
+ &list_values));
+ }
+ }
+ return Status::OK();
+ }
+
+ template <typename BuilderType>
+ static Status BuildArrayFromListType(const ExecSpan& batch,
+ const ListSliceOptions& opts,
+ ArrayBuilder& builder) {
+ const ArraySpan& list_ = batch[0].array;
+ const offset_type* offsets = list_.GetValues<offset_type>(1);
+ const auto n_offsets = static_cast<offset_type>(
+ static_cast<size_t>(list_.GetBuffer(1)->size()) / sizeof(offset_type));
+
+ // validity bitmap if set.
+ std::unique_ptr<arrow::internal::Bitmap> validity_bitmap;
+ if (list_.buffers[0].data != nullptr) {
+ arrow::internal::Bitmap bitmap{list_.buffers[0].data, list_.offset,
list_.length};
+ validity_bitmap = std::make_unique<arrow::internal::Bitmap>(bitmap);
Review Comment:
I was using it to identify nulls in the `list_values` but fixing the offset
removed this need, thank you.
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