/*
 * 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.flink.streaming.runtime.metrics;

import org.apache.flink.metrics.Gauge;

/**
 * A {@link Gauge} for exposing the minimum watermark of a {@link WatermarkGauge} pair.
 */
public class MinWatermarkGauge implements Gauge<Long> {

	private WatermarkGauge watermarkGauge1;
	private WatermarkGauge watermarkGauge2;

	public MinWatermarkGauge(WatermarkGauge watermarkGauge1, WatermarkGauge watermarkGauge2) {
		this.watermarkGauge1 = watermarkGauge1;
		this.watermarkGauge2 = watermarkGauge2;
	}

	@Override
	public Long getValue() {
		return Math.min(watermarkGauge1.getValue(), watermarkGauge2.getValue());
	}
}

/*
 * 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.flink.streaming.runtime.metrics;

import org.apache.flink.metrics.Gauge;

import java.util.Arrays;

import static org.apache.flink.util.Preconditions.checkArgument;

/**
 * A {@link Gauge} for exposing the minimum watermark of chosen {@link WatermarkGauge}s.
 */
public class MinWatermarkGauge implements Gauge<Long> {
	private final WatermarkGauge[] watermarkGauges;

	public MinWatermarkGauge(WatermarkGauge ...watermarkGauges) {
		checkArgument(watermarkGauges.length > 0);
		this.watermarkGauges = watermarkGauges;
	}

	@Override
	public Long getValue() {
		return Arrays.stream(watermarkGauges)
			.mapToLong(WatermarkGauge::getValue)
			.min()
			.orElse(0);
	}
}

Generalize MinWatermarkGauge to accept an aribtrary number of input gauges