[GitHub] [arrow] westonpace commented on a diff in pull request #34137: GH-34136: [C++] Add a concept of ordering to ExecPlan

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

westonpace commented on code in PR #34137:
URL: https://github.com/apache/arrow/pull/34137#discussion_r1110528592


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cpp/src/arrow/compute/ordering.h:
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@@ -0,0 +1,120 @@
+// 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 <string>
+#include <vector>
+
+#include "arrow/type.h"
+#include "arrow/util/compare.h"
+#include "arrow/util/visibility.h"
+
+namespace arrow {
+namespace compute {
+
+enum class SortOrder {
+  /// Arrange values in increasing order
+  Ascending,
+  /// Arrange values in decreasing order
+  Descending,
+};
+
+enum class NullPlacement {
+  /// Place nulls and NaNs before any non-null values.
+  /// NaNs will come after nulls.
+  AtStart,
+  /// Place nulls and NaNs after any non-null values.
+  /// NaNs will come before nulls.
+  AtEnd,
+};
+
+/// \brief One sort key for PartitionNthIndices (TODO) and SortIndices
+class ARROW_EXPORT SortKey : public util::EqualityComparable<SortKey> {
+ public:
+  explicit SortKey(FieldRef target, SortOrder order = SortOrder::Ascending)
+      : target(std::move(target)), order(order) {}
+
+  bool Equals(const SortKey& other) const;
+  std::string ToString() const;
+
+  /// A FieldRef targetting the sort column.
+  FieldRef target;
+  /// How to order by this sort key.
+  SortOrder order;
+};
+
+class ARROW_EXPORT Ordering : public util::EqualityComparable<Ordering> {
+ public:
+  Ordering(std::vector<SortKey> sort_keys,
+           NullPlacement null_placement = NullPlacement::AtStart)
+      : sort_keys_(std::move(sort_keys)), null_placement_(null_placement) {}
+  /// true if data ordered by other is also ordered by this
+  ///
+  /// For example, if data is ordered by [a, b, c] then it is also ordered
+  /// by [a, b] but not by [b, c] or [a, b, c, d].
+  ///
+  /// [a, b].IsSuborderOf([a, b, c]) - true
+  /// [a, b, c].IsSuborderOf([a, b, c]) - true
+  /// [b, c].IsSuborderOf([a, b, c]) - false
+  /// [a, b, c, d].IsSuborderOf([a, b, c]) - false
+  ///
+  /// The implicit ordering is not a suborder of any other ordering and
+  /// no other ordering is a suborder of it.  The implicit ordering is not a
+  /// suborder of itself.
+  ///
+  /// The unordered ordering is a suborder of all other orderings but no
+  /// other ordering is a suborder of it.  The unordered ordering is a suborder
+  /// of itself.
+  ///
+  /// The unordered ordering is a suborder of the implicit ordering.
+  bool IsSuborderOf(const Ordering& other) const;
+
+  bool Equals(const Ordering& other) const;
+  std::string ToString() const;
+
+  bool is_implicit() const { return is_implicit_; }
+  bool is_unordered() const { return !is_implicit_ && sort_keys_.empty(); }
+
+  std::vector<SortKey> sort_keys() { return sort_keys_; }
+  NullPlacement null_placement() { return null_placement_; }
+
+  static const Ordering& Imiplicit() {

Review Comment:
   :facepalm: 



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