[GitHub] [lucene-solr] mayya-sharipova commented on a change in pull request #1725: LUCENE-9449 Skip docs with _doc sort and "after"

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

mayya-sharipova commented on a change in pull request #1725:
URL: https://github.com/apache/lucene-solr/pull/1725#discussion_r481011126



##########
File path: lucene/core/src/java/org/apache/lucene/search/comparators/DocComparator.java
##########
@@ -0,0 +1,185 @@
+/*
+ * 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.lucene.search.comparators;
+
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.search.DocIdSetIterator;
+import org.apache.lucene.search.FieldComparator;
+import org.apache.lucene.search.FilteringLeafFieldComparator;
+import org.apache.lucene.search.LeafFieldComparator;
+import org.apache.lucene.search.Scorable;
+
+import java.io.IOException;
+
+/**
+ * Comparator that sorts by asc _doc
+ */
+public class DocComparator extends FieldComparator<Integer> {
+    private final int[] docIDs;
+    private int topValue;
+    private boolean topValueSet;
+    private boolean reverse = false; // only used to check if skipping functionality should be enabled
+
+    /** Creates a new comparator based on document ids for {@code numHits} */
+    public DocComparator(int numHits) {
+        docIDs = new int[numHits];
+    }
+
+    @Override
+    public int compare(int slot1, int slot2) {
+        // No overflow risk because docIDs are non-negative
+        return docIDs[slot1] - docIDs[slot2];
+    }
+
+
+    @Override
+    public LeafFieldComparator getLeafComparator(LeafReaderContext context) {
+        // TODO: can we "map" our docIDs to the current
+        // reader? saves having to then subtract on every
+        // compare call
+        return new DocLeafComparator(context);
+    }
+
+    @Override
+    public void setTopValue(Integer value) {
+        topValue = value;
+        topValueSet = true;
+    }
+
+    @Override
+    public Integer value(int slot) {
+        return Integer.valueOf(docIDs[slot]);
+    }
+
+    @Override
+    public void setReverse() {
+        reverse = true;
+    }
+
+
+    /**
+     * DocLeafComparator with skipping functionality.
+     * When sort by _doc asc and "after" document is set,
+     * the comparator provides an iterator that can quickly skip to the desired "after" document.
+     */
+    private class DocLeafComparator implements FilteringLeafFieldComparator {
+        private final int docBase;
+        private int bottom;
+
+        private final boolean enableSkipping;
+        private final int minDoc;
+        private final int maxDoc;
+        private DocIdSetIterator topValueIterator; // iterator that starts from topValue
+
+        private boolean iteratorUpdated = false;
+
+        public DocLeafComparator(LeafReaderContext context) {
+            this.docBase = context.docBase;
+            // skipping functionality is enabled if topValue is set and sort is asc
+            this.enableSkipping = topValueSet && reverse == false ? true: false;
+            if (enableSkipping) {
+                this.minDoc = topValue + 1;
+                this.maxDoc = context.reader().maxDoc();
+                this.topValueIterator = DocIdSetIterator.all(maxDoc);
+            } else {
+                this.minDoc = -1;
+                this.maxDoc = -1;
+                this.topValueIterator = null;
+            }
+        }
+
+        @Override
+        public void setBottom(int slot) {
+            bottom = docIDs[slot];

Review comment:
       @jimczi That sounds good to me.  Thank you for the feedback.  Do you have any further comments for this PR? 




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