[GitHub] [arrow] bkietz commented on a change in pull request #8894: ARROW-10322: [C++][Dataset] Minimize Expression

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bkietz commented on a change in pull request #8894:
URL: https://github.com/apache/arrow/pull/8894#discussion_r551574884



##########
File path: cpp/src/arrow/dataset/expression_internal.h
##########
@@ -0,0 +1,465 @@
+// 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/dataset/expression.h"
+
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include "arrow/compute/api_vector.h"
+#include "arrow/compute/registry.h"
+#include "arrow/record_batch.h"
+#include "arrow/table.h"
+#include "arrow/util/logging.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace dataset {
+
+bool Identical(const Expression& l, const Expression& r) { return l.impl_ == r.impl_; }
+
+const Expression::Call* CallNotNull(const Expression& expr) {
+  auto call = expr.call();
+  DCHECK_NE(call, nullptr);
+  return call;
+}
+
+inline void GetAllFieldRefs(const Expression& expr,
+                            std::unordered_set<FieldRef, FieldRef::Hash>* refs) {
+  if (auto lit = expr.literal()) return;
+
+  if (auto ref = expr.field_ref()) {
+    refs->emplace(*ref);
+    return;
+  }
+
+  for (const Expression& arg : CallNotNull(expr)->arguments) {
+    GetAllFieldRefs(arg, refs);
+  }
+}
+
+inline std::vector<ValueDescr> GetDescriptors(const std::vector<Expression>& exprs) {
+  std::vector<ValueDescr> descrs(exprs.size());
+  for (size_t i = 0; i < exprs.size(); ++i) {
+    DCHECK(exprs[i].IsBound());
+    descrs[i] = exprs[i].descr();
+  }
+  return descrs;
+}
+
+inline std::vector<ValueDescr> GetDescriptors(const std::vector<Datum>& values) {
+  std::vector<ValueDescr> descrs(values.size());
+  for (size_t i = 0; i < values.size(); ++i) {
+    descrs[i] = values[i].descr();
+  }
+  return descrs;
+}
+
+struct FieldPathGetDatumImpl {

Review comment:
       Since this is only valid for some Datum variant members, I think it should remain a special case for datasets at least for now




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