[FLINK-2626] [core] Add an average accumulator

This closes #1096

flink-core/src/main/java/org/apache/flink/api/common/accumulators/AverageAccumulator.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.flink.api.common.accumulators;
+
+/**
+ * An accumulator that get the average values.
+ * Input can be {@code long}, {@code integer}, {@code double} as the result is {@code double}.
+ */
+public class AverageAccumulator implements SimpleAccumulator <Double> {
+
+	private double localValue = 0;
+	private long count = 0;
+
+	@Override
+	public void add(Double value) {
+		this.count++;
+		this.localValue += value;
+	}
+
+	public void add(double value) {
+		this.count++;
+		this.localValue += value;
+	}
+
+	public void add(long value) {
+		this.count++;
+		this.localValue += value;
+	}
+
+	public void add(int value) {
+		this.count++;
+		this.localValue += value;
+	}
+
+	@Override
+	public Double getLocalValue() {
+		if (this.count == 0) {
+			return 0.0;
+		}
+		return this.localValue / (double)this.count;
+	}
+
+	@Override
+	public void resetLocal() {
+		this.count = 0;
+		this.localValue = 0;
+	}
+
+	@Override
+	public void merge(Accumulator<Double, Double> other) {
+		if (other instanceof AverageAccumulator) {
+			AverageAccumulator temp = (AverageAccumulator)other;
+			this.count += temp.count;
+			this.localValue += other.getLocalValue();
+		} else {
+			throw new RuntimeException("The merged accumulator must be AverageAccumulator.");
+		}
+	}
+
+	@Override
+	public AverageAccumulator clone() {
+		AverageAccumulator average = new AverageAccumulator();
+		average.localValue = this.localValue;
+		average.count = this.count;
+		return average;
+	}
+
+	@Override
+	public String toString() {
+		return "AverageAccumulator " + this.localValue + " count " + this.count;
+	}
+}

flink-core/src/test/java/org/apache/flink/api/common/accumulators/AverageAccumulatorTest.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.flink.api.common.accumulators;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+public class AverageAccumulatorTest {
+
+	@Test
+	public void testGet() {
+		AverageAccumulator average = new AverageAccumulator();
+		assertEquals(Double.valueOf(0), average.getLocalValue());
+	}
+
+	@Test
+	public void testAdd() {
+		AverageAccumulator average = new AverageAccumulator();
+		int i1;
+		for (i1 = 0; i1 < 10; i1++) {
+			average.add(i1);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		Integer i2;
+		for (i2 = 0; i2 < 10; i2++) {
+			average.add(i2);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		long i3;
+		for (i3 = 0; i3 < 10; i3++) {
+			average.add(i3);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		Long i4;
+		for (i4 = 0L; i4 < 10; i4++) {
+			average.add(i4);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		double i5;
+		for (i5 = 0; i5 < 10; i5++) {
+			average.add(i5);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		Double i6;
+		for (i6 = 0.0; i6 < 10; i6++) {
+			average.add(i6);
+		}
+		assertEquals(Double.valueOf(4.5), average.getLocalValue());
+		average.resetLocal();
+
+		assertEquals(Double.valueOf(0), average.getLocalValue());
+	}
+
+	@Test
+	public void testMergeSuccess() {
+		AverageAccumulator average = new AverageAccumulator();
+		AverageAccumulator averageNew = new AverageAccumulator();
+		average.add(1);
+		averageNew.add(2);
+		average.merge(averageNew);
+		assertEquals(Double.valueOf(1.5), average.getLocalValue());
+	}
+
+	@Test
+	public void testMergeFailed() {
+		AverageAccumulator average = new AverageAccumulator();
+		Accumulator<Double, Double> averageNew = null;
+		average.add(1);
+		try {
+			average.merge(averageNew);
+		} catch (Exception e) {
+			assertTrue(e.toString().indexOf("The merged accumulator must be AverageAccumulator.") != -1);
+			return;
+		}
+		fail();
+	}
+
+	@Test
+	public void testClone() {
+		AverageAccumulator average = new AverageAccumulator();
+		average.add(1);
+		AverageAccumulator averageNew = average.clone();
+		assertEquals(Double.valueOf(1), averageNew.getLocalValue());
+	}
+}