[GitHub] [spark] dongjoon-hyun commented on a change in pull request #26728: [SPARK-30012][CORE][SQL] Change classes extending scala collection classes to work with 2.13

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dongjoon-hyun commented on a change in pull request #26728: [SPARK-30012][CORE][SQL] Change classes extending scala collection classes to work with 2.13
URL: https://github.com/apache/spark/pull/26728#discussion_r352930777
 
 

 ##########
 File path: sql/catalyst/src/main/scala-2.13/org/apache/spark/sql/catalyst/expressions/ExpressionSet.scala
 ##########
 @@ -0,0 +1,100 @@
+/*
+ * 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.
+ */
+
+package org.apache.spark.sql.catalyst.expressions
+
+import scala.collection.mutable
+import scala.collection.mutable.ArrayBuffer
+
+object ExpressionSet {
+  /** Constructs a new [[ExpressionSet]] by applying [[Canonicalize]] to `expressions`. */
+  def apply(expressions: TraversableOnce[Expression]): ExpressionSet = {
+    val set = new ExpressionSet()
+    expressions.foreach(set.add)
+    set
+  }
+}
+
+/**
+ * A [[Set]] where membership is determined based on determinacy and a canonical representation of
+ * an [[Expression]] (i.e. one that attempts to ignore cosmetic differences).
+ * See [[Canonicalize]] for more details.
+ *
+ * Internally this set uses the canonical representation, but keeps also track of the original
+ * expressions to ease debugging.  Since different expressions can share the same canonical
+ * representation, this means that operations that extract expressions from this set are only
+ * guaranteed to see at least one such expression.  For example:
+ *
+ * {{{
+ *   val set = ExpressionSet(a + 1, 1 + a)
+ *
+ *   set.iterator => Iterator(a + 1)
+ *   set.contains(a + 1) => true
+ *   set.contains(1 + a) => true
+ *   set.contains(a + 2) => false
+ * }}}
+ *
+ * For non-deterministic expressions, they are always considered as not contained in the [[Set]].
+ * On adding a non-deterministic expression, simply append it to the original expressions.
+ * This is consistent with how we define `semanticEquals` between two expressions.
+ */
+class ExpressionSet protected(
+    protected val baseSet: mutable.Set[Expression] = new mutable.HashSet,
+    protected val originals: mutable.Buffer[Expression] = new ArrayBuffer)
+  extends Set[Expression] {
+
+  //  Note: this class supports Scala 2.13. A parallel source tree has a 2.12 implementation.
+
+  protected def add(e: Expression): Unit = {
+    if (!e.deterministic) {
+      originals += e
+    } else if (!baseSet.contains(e.canonicalized) ) {
+      baseSet.add(e.canonicalized)
+      originals += e
+    }
+  }
+
+  override def contains(elem: Expression): Boolean = baseSet.contains(elem.canonicalized)
+
+  override def incl(elem: Expression): ExpressionSet = {
+    val newSet = new ExpressionSet(baseSet.clone(), originals.clone())
+    newSet.add(elem)
+    newSet
+  }
+
 
 Review comment:
   So, `++` exists only at `scala-2.12`. We are using the underlying `concat` via alias `++`?

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