hugegraph-store/hg-store-client/src/main/java/org/apache/hugegraph/store/client/SequencedIterator.java

/*
 * 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.hugegraph.store.client;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Queue;

import org.apache.hugegraph.store.HgKvEntry;
import org.apache.hugegraph.store.HgKvIterator;
import org.apache.hugegraph.store.HgKvOrderedIterator;

import lombok.extern.slf4j.Slf4j;

/**
 * Proxy iterator orderly, to switch next one will happen when the current one is empty.
 * <p>
 * created on 2022/03/10
 *
 * @version 0.1.0
 */
@Slf4j
public class SequencedIterator implements HgKvIterator {

    private static final byte[] EMPTY_BYTES = new byte[0];
    private final Queue<HgKvOrderedIterator> queue;
    private final long limit;
    private HgKvOrderedIterator<HgKvEntry> iterator;
    private HgKvEntry entry;
    private int count;
    private byte[] position = EMPTY_BYTES;
    private byte[] position4Seeking = EMPTY_BYTES;

    SequencedIterator(List<HgKvOrderedIterator> iterators, long limit) {
        Collections.sort(iterators);
        this.queue = new LinkedList(iterators);
        this.limit = limit <= 0 ? Integer.MAX_VALUE : limit;
    }

    private HgKvOrderedIterator getIterator() {
        if (this.queue.isEmpty()) {
            return null;
        }
        HgKvOrderedIterator buf;
        while ((buf = this.queue.poll()) != null) {
            buf.seek(this.position4Seeking);
            if (buf.hasNext()) {
                break;
            }
        }
        return buf;
    }

    private void closeIterators() {
        if (this.queue.isEmpty()) {
            return;
        }
        HgKvOrderedIterator buf;
        while ((buf = this.queue.poll()) != null) {
            buf.close();
        }

    }

    @Override
    public byte[] key() {
        if (this.entry != null) {
            return this.entry.key();
        }
        return null;
    }

    @Override
    public byte[] value() {
        if (this.entry != null) {
            return this.entry.value();
        }
        return null;
    }

    @Override
    public byte[] position() {
        return this.position;
    }

    @Override
    public void seek(byte[] pos) {
        if (pos != null) {
            this.position4Seeking = pos;
        }
    }

    @Override
    public boolean hasNext() {
        if (this.count >= this.limit) {
            return false;
        }
        if (this.iterator == null) {
            this.iterator = this.getIterator();
        } else if (!this.iterator.hasNext()) {
            this.iterator.close();
            this.iterator = this.getIterator();
        }
        return this.iterator != null;
    }

    @Override
    public Object next() {
        if (this.iterator == null) {
            hasNext();
        }
        if (this.iterator == null) {
            throw new NoSuchElementException();
        }
        this.entry = this.iterator.next();
        this.position = this.iterator.position();
        if (!this.iterator.hasNext()) {
            this.iterator.close();
            this.iterator = null;
        }
        this.count++;
        return this.entry;
    }

    @Override
    public void close() {
        if (this.iterator != null) {
            this.iterator.close();
        }
        this.closeIterators();
    }
}