[GitHub] [calcite] vlsi commented on a change in pull request #2329: [CALCITE-3221] Add MergeUnion operator in Enumerable convention

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

vlsi commented on a change in pull request #2329:
URL: https://github.com/apache/calcite/pull/2329#discussion_r572026365



##########
File path: linq4j/src/main/java/org/apache/calcite/linq4j/MergeUnionEnumerator.java
##########
@@ -0,0 +1,206 @@
+/*
+ * 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.calcite.linq4j;
+
+import org.apache.calcite.linq4j.function.EqualityComparer;
+import org.apache.calcite.linq4j.function.Function1;
+
+import org.checkerframework.checker.initialization.qual.UnknownInitialization;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.RequiresNonNull;
+
+import java.util.Comparator;
+import java.util.HashSet;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Set;
+
+/**
+ * Performs a union (or union all) of all its inputs (which must be already sorted),
+ * respecting the order.
+ * @param <TSource> record type
+ * @param <TKey> sort key
+ */
+final class MergeUnionEnumerator<TSource, TKey> implements Enumerator<TSource> {
+  private final Enumerator<TSource>[] inputs;
+  private final TSource[] currentInputsValues;
+  private final boolean[] inputsFinished;
+  private final Function1<TSource, TKey> sortKeySelector;
+  private final Comparator<TKey> sortComparator;
+  private TSource currentValue;
+  private int activeInputs;
+
+  // Set to control duplicates, only used if "all" is false
+  private final @Nullable Set<EnumerableDefaults.Wrapped<TSource>> processed;
+  private final @Nullable Function1<TSource, EnumerableDefaults.Wrapped<TSource>> wrapper;
+  private @Nullable TKey currentKeyInProcessedSet;
+
+  private static final Object NOT_INIT = new Object();
+
+  MergeUnionEnumerator(
+      List<Enumerable<TSource>> sources,
+      Function1<TSource, TKey> sortKeySelector,
+      Comparator<TKey> sortComparator,
+      boolean all,
+      EqualityComparer<TSource> equalityComparer) {
+    this.sortKeySelector = sortKeySelector;
+    this.sortComparator = sortComparator;
+
+    if (all) {
+      this.processed = null;
+      this.wrapper = null;
+    } else {
+      this.processed = new HashSet<>();
+      this.wrapper = EnumerableDefaults.wrapperFor(equalityComparer);
+    }
+
+    final int size = sources.size();
+    //noinspection unchecked
+    this.inputs = new Enumerator[size];
+    int i = 0;
+    for (Enumerable<TSource> source : sources) {
+      this.inputs[i++] = source.enumerator();
+    }
+
+    //noinspection unchecked
+    this.currentInputsValues = (TSource[]) new Object[size];
+    this.activeInputs = this.currentInputsValues.length;
+    this.inputsFinished = new boolean[size];
+    //noinspection unchecked
+    this.currentValue = (TSource) NOT_INIT;
+
+    initEnumerators();
+  }
+
+  @RequiresNonNull("inputs")
+  @SuppressWarnings("method.invocation.invalid")
+  private void initEnumerators(@UnknownInitialization MergeUnionEnumerator<TSource, TKey> this) {
+    for (int i = 0; i < inputs.length; i++) {
+      moveEnumerator(i);
+    }
+  }
+
+  private void moveEnumerator(int i) {
+    final Enumerator<TSource> enumerator = inputs[i];
+    if (!enumerator.moveNext()) {
+      activeInputs--;
+      inputsFinished[i] = true;

Review comment:
       A slight note: it might be a good time to perform `currentInputsValues[i] = null` to release memory.




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