io.datarouter.scanner.CollatingScanner Maven / Gradle / Ivy

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/**
 * Copyright © 2009 HotPads ([email protected])
 *
 * Licensed 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 io.datarouter.scanner;

import java.util.Comparator;
import java.util.List;
import java.util.PriorityQueue;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class CollatingScanner extends BaseScanner{
	private static final Logger logger = LoggerFactory.getLogger(CollatingScanner.class);

	private final List> inputs;
	private final Comparator comparator;
	private final PriorityQueue> priorityQueue;
	private boolean opened;
	private boolean closed;

	public CollatingScanner(List> inputs, Comparator comparator){
		this.inputs = inputs;
		this.comparator = comparator;
		this.priorityQueue = new PriorityQueue<>();
		this.opened = false;
		this.closed = false;
	}

	@Override
	public boolean advance(){
		if(closed){
			return false;
		}
		if(!opened){
			Scanner.of(inputs)
					.map(input -> new ComparableScanner<>(input, comparator))
					.include(Scanner::advance)
					.forEach(priorityQueue::add);
			opened = true;
		}
		ComparableScanner firstScanner = priorityQueue.poll();
		if(firstScanner == null){
			current = null;
			return false;
		}
		current = firstScanner.current();
		if(firstScanner.advance()){
			priorityQueue.add(firstScanner);
		}else{
			firstScanner.close();
		}
		return true;
	}

	@Override
	public void close(){
		if(closed){
			return;
		}
		for(Scanner input : priorityQueue){
			try{
				input.close();
			}catch(Exception e){
				logger.warn("scanner exception on input.close", e);
			}
		}
		closed = true;
	}

}