[GitHub] [arrow] jorisvandenbossche commented on a diff in pull request #34408: GH-34361: [C++] Fix the handling of logical nulls for types without bitmaps like Unions and Run-End Encoded

["jorisvandenbossche (via GitHub)" <gi...@apache.org>]

jorisvandenbossche commented on code in PR #34408:
URL: https://github.com/apache/arrow/pull/34408#discussion_r1164143465


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cpp/src/arrow/util/union_util.cc:
##########
@@ -0,0 +1,58 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/util/union_util.h"
+
+#include <cstdint>
+
+#include "arrow/array/data.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/logging.h"
+
+namespace arrow::union_util {
+
+int64_t LogicalSparseUnionNullCount(const ArraySpan& span) {
+  const auto* sparse_union_type =
+      internal::checked_cast<const SparseUnionType*>(span.type);
+  DCHECK_LE(span.child_data.size(), 128);
+
+  const int8_t* types = span.GetValues<int8_t>(1);  // NOLINT
+  int64_t null_count = 0;
+  for (int64_t i = 0; i < span.length; i++) {
+    const int8_t child_id = sparse_union_type->child_ids()[types[span.offset + i]];
+
+    null_count += span.child_data[child_id].IsNull(i);

Review Comment:
   Did you test this with a sliced array? (don't directly see it in the tests) 
   Because in https://github.com/apache/arrow/pull/35036, I based my implementation on this function, and I am finding that I need to remove/add the offset like so:
   
   ```suggestion
       const int8_t child_id = sparse_union_type->child_ids()[types[i]];
   
       null_count += span.child_data[child_id].IsNull(span.offset + i);
   ```
   
   So `types`, which is the result of `Span.GetValues()` already takes into account the span's offset, but `IsNull()` is called on the child_data, and accessing child_data gives the original array data without offset already taken into account.



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