[GitHub] [arrow] pitrou commented on a diff in pull request #12055: ARROW-11989: [C++][Python] Improve ChunkedArray's complexity for the access of elements

[GitBox <gi...@apache.org>]

pitrou commented on code in PR #12055:
URL: https://github.com/apache/arrow/pull/12055#discussion_r849504481


##########
cpp/src/arrow/chunk_resolver.h:
##########
@@ -0,0 +1,93 @@
+// 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.
+
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+#include <vector>
+
+#include "arrow/type_fwd.h"
+#include "arrow/util/macros.h"
+
+namespace arrow {
+namespace internal {
+
+struct ChunkLocation {
+  int64_t chunk_index, index_in_chunk;
+};
+
+// An object that resolves an array chunk depending on the index
+struct ChunkResolver {
+  ChunkResolver(const ChunkResolver& chunks);

Review Comment:
   If a copy constructor is needed, can we add a move constructor as well?



##########
cpp/src/arrow/chunked_array.cc:
##########
@@ -44,18 +45,20 @@ class MemoryPool;
 
 ChunkedArray::ChunkedArray(ArrayVector chunks, std::shared_ptr<DataType> type)
     : chunks_(std::move(chunks)), type_(std::move(type)) {
-  length_ = 0;
-  null_count_ = 0;
-
   if (type_ == nullptr) {
     ARROW_CHECK_GT(chunks_.size(), 0)
         << "cannot construct ChunkedArray from empty vector and omitted type";
     type_ = chunks_[0]->type();
   }
+
+  length_ = 0;
+  null_count_ = 0;

Review Comment:
   Can we simply move those to the attribute initialization list above?



##########
cpp/src/arrow/chunked_array.h:
##########
@@ -182,6 +183,7 @@ class ARROW_EXPORT ChunkedArray {
   std::shared_ptr<DataType> type_;
 
  private:
+  std::unique_ptr<internal::ChunkResolver> chunk_resolver_;

Review Comment:
   Is this useful for this to be dynamically allocated, since the constructor always initializes it? If this is so as to defer initialization, this could use `util::optional` instead (but `ChunkResolver` could simply have a default constructor).



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