[dbs-leipzig#1571] add degree variance operator

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+[#1559] add unit-tests
+
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.
+#
+# Date:      Tue Aug 2 11:06:54 2022 +0200
+#
+# interactive rebase in progress; onto 5f1b5d5a004
+# Last commands done (3 commands done):
+#    reword dc25f24377f add help functions
+#    reword 22c9104636f add unit-tests
+# No commands remaining.
+# You are currently editing a commit while rebasing branch '#1559_min_max_avg_degree_evolution' on '5f1b5d5a004'.
+#
+# Changes to be committed:
+#	new file:   gradoop-temporal/src/test/java/org/gradoop/temporal/model/impl/operators/metric/AvgDegreeEvolutionTest.java
+#	new file:   gradoop-temporal/src/test/java/org/gradoop/temporal/model/impl/operators/metric/MaxDegreeEvolutionTest.java
+#	new file:   gradoop-temporal/src/test/java/org/gradoop/temporal/model/impl/operators/metric/MinDegreeEvolutionTest.java
+#

gradoop-temporal/src/main/java/org/gradoop/temporal/model/impl/operators/metric/DegreeVarianceEvolution.java

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+/*
+ * Copyright © 2014 - 2021 Leipzig University (Database Research Group)
+ *
+ * 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 org.gradoop.temporal.model.impl.operators.metric;
+
+import org.apache.flink.api.java.DataSet;
+import org.apache.flink.api.java.tuple.Tuple1;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.gradoop.common.model.impl.id.GradoopId;
+import org.gradoop.flink.model.api.operators.UnaryBaseGraphToValueOperator;
+import org.gradoop.flink.model.impl.operators.sampling.functions.VertexDegree;
+import org.gradoop.temporal.model.api.TimeDimension;
+import org.gradoop.temporal.model.impl.TemporalGraph;
+import org.gradoop.temporal.model.impl.operators.metric.functions.*;
+import org.gradoop.temporal.model.impl.operators.metric.functions.ExtractAllTimePointsReduce;
+import org.gradoop.temporal.model.impl.operators.metric.functions.GroupDegreeTreesToVariance;
+
+import java.util.Objects;
+import java.util.TreeMap;
+
+/**
+ * Operator that calculates the degree variance evolution of a temporal graph for the
+ * whole lifetime of the graph.
+ */
+public class DegreeVarianceEvolution implements UnaryBaseGraphToValueOperator<TemporalGraph, DataSet<Tuple2<Long, Double>>> {
+    /**
+     * The time dimension that will be considered.
+     */
+    private final TimeDimension dimension;
+
+    /**
+     * The degree type (IN, OUT, BOTH);
+     */
+    private final VertexDegree degreeType;
+
+    /**
+     * Creates an instance of this average degree evolution operator.
+     *
+     * @param degreeType the degree type to use (IN, OUT, BOTH).
+     * @param dimension the time dimension to use (VALID_TIME, TRANSACTION_TIME).
+     */
+    public DegreeVarianceEvolution(VertexDegree degreeType, TimeDimension dimension) {
+        this.degreeType = Objects.requireNonNull(degreeType);
+        this.dimension = Objects.requireNonNull(dimension);
+    }
+
+    @Override
+    public DataSet<Tuple2<Long, Double>> execute(TemporalGraph graph) {
+        DataSet<Tuple2<GradoopId, TreeMap<Long, Integer>>> absoluteDegreeTrees = graph.getEdges()
+                // 1) Extract vertex id(s) and corresponding time intervals
+                .flatMap(new FlatMapVertexIdEdgeInterval(dimension, degreeType))
+                // 2) Group them by the vertex id
+                .groupBy(0)
+                // 3) For each vertex id, build a degree tree data structure
+                .reduceGroup(new BuildTemporalDegreeTree())
+                // 4) Transform each tree to aggregated evolution
+                .map(new TransformDeltaToAbsoluteDegreeTree());
+
+        DataSet<Tuple1<Long>> timePoints = absoluteDegreeTrees
+                // 5) extract all timestamps where degree of any vertex changes
+                .reduceGroup(new ExtractAllTimePointsReduce())
+                .distinct();
+
+        return absoluteDegreeTrees
+                // join with interval degree mappings
+                // 6) Merge trees together and calculate aggregation
+                .reduceGroup(new GroupDegreeTreesToVariance(timePoints));
+    }
+}

gradoop-temporal/src/main/java/org/gradoop/temporal/model/impl/operators/metric/functions/ExtractAllTimePointsReduce.java

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+/*
+ * Copyright © 2014 - 2021 Leipzig University (Database Research Group)
+ *
+ * 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 org.gradoop.temporal.model.impl.operators.metric.functions;
+
+import org.apache.flink.api.common.functions.GroupReduceFunction;
+import org.apache.flink.api.java.tuple.Tuple1;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.util.Collector;
+import org.gradoop.common.model.impl.id.GradoopId;
+
+import java.util.SortedSet;
+import java.util.TreeMap;
+import java.util.TreeSet;
+
+/**
+ * Reduce function to extract all timestamps where the degree of a vertex changes.
+ */
+public class ExtractAllTimePointsReduce implements GroupReduceFunction<Tuple2<GradoopId, TreeMap<Long, Integer>>, Tuple1<Long>> {
+
+    public ExtractAllTimePointsReduce() {
+    }
+
+    @Override
+    public void reduce(Iterable<Tuple2<GradoopId, TreeMap<Long, Integer>>> iterable, Collector<Tuple1<Long>> collector) throws Exception {
+        SortedSet<Long> timePoints = new TreeSet<>();
+
+        for (Tuple2<GradoopId, TreeMap<Long, Integer>> tuple : iterable) {
+            timePoints.addAll(tuple.f1.keySet());
+        }
+
+        for (Long timePoint: timePoints) {
+            collector.collect(new Tuple1<>(timePoint));
+        }
+
+    }
+}

gradoop-temporal/src/main/java/org/gradoop/temporal/model/impl/operators/metric/functions/GroupDegreeTreesToVariance.java

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+/*
+ * Copyright © 2014 - 2021 Leipzig University (Database Research Group)
+ *
+ * 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 org.gradoop.temporal.model.impl.operators.metric.functions;
+
+import org.apache.flink.api.common.functions.GroupReduceFunction;
+import org.apache.flink.api.java.DataSet;
+import org.apache.flink.api.java.tuple.Tuple1;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.util.Collector;
+import org.gradoop.common.model.impl.id.GradoopId;
+
+import java.util.*;
+import java.util.stream.Stream;
+
+/**
+ * A group reduce function that merges all Tuples (vId, degreeTree) to a dataset of tuples (time, aggDegree)
+ * that represents the aggregated degree value for the whole graph at the given time.
+ */
+public class GroupDegreeTreesToVariance
+        implements GroupReduceFunction<Tuple2<GradoopId, TreeMap<Long, Integer>>, Tuple2<Long, Double>> {
+
+    /**
+     * The timestamps where at least one vertex degree changes.
+     */
+    private final SortedSet<Long> timePoints;
+
+    /**
+     * Creates an instance of this group reduce function.
+     *
+     * @param timePoints the timestamps where vertex degree changes.
+     */
+    public GroupDegreeTreesToVariance(DataSet<Tuple1<Long>> timePoints) {
+
+        List<Tuple1<Long>> tuples;
+        try {
+            tuples = timePoints.collect();
+            this.timePoints = new TreeSet<>();
+
+            for (int i = 0; i < timePoints.count(); i = i + 1) {
+                this.timePoints.add(tuples.get(i).getField(0));
+            }
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+
+    }
+
+    @Override
+    public void reduce(Iterable<Tuple2<GradoopId, TreeMap<Long, Integer>>> iterable,
+                       Collector<Tuple2<Long, Double>> collector) throws Exception {
+
+        // init necessary maps and set
+        HashMap<GradoopId, TreeMap<Long, Integer>> degreeTrees = new HashMap<>();
+        HashMap<GradoopId, Integer> vertexDegrees = new HashMap<>();
+
+        // convert the iterables to a hashmap and remember all possible timestamps
+        for (Tuple2<GradoopId, TreeMap<Long, Integer>> tuple : iterable) {
+            degreeTrees.put(tuple.f0, tuple.f1);
+        }
+
+        int numberOfVertices = degreeTrees.size();
+
+        // Add default times
+        timePoints.add(Long.MIN_VALUE);
+
+        for (Long timePoint : timePoints) {
+            // skip last default time
+            if (Long.MAX_VALUE == timePoint) {
+                continue;
+            }
+            // Iterate over all vertices
+            for (Map.Entry<GradoopId, TreeMap<Long, Integer>> entry : degreeTrees.entrySet()) {
+                // Make sure the vertex is registered in the current vertexDegrees capture
+                if (!vertexDegrees.containsKey(entry.getKey())) {
+                    vertexDegrees.put(entry.getKey(), 0);
+                }
+
+                // Check if timestamp is in tree, if not, take the lower key
+                if (entry.getValue().containsKey(timePoint)) {
+                    vertexDegrees.put(entry.getKey(), entry.getValue().get(timePoint));
+                } else {
+                    Long lowerKey = entry.getValue().lowerKey(timePoint);
+                    if (lowerKey != null) {
+                        vertexDegrees.put(entry.getKey(), entry.getValue().get(lowerKey));
+                    }
+                }
+            }
+
+            Optional<Integer> opt = vertexDegrees.values().stream().reduce(Math::addExact);
+            Optional<Double> opt2 = Optional.empty();
+
+            double mean;
+
+            if (opt.isPresent()) {
+                mean = (double) opt.get() / (double) numberOfVertices;
+                opt2 = Optional.of(vertexDegrees.values().stream().mapToDouble(val -> (val - mean) * (val - mean)).sum());
+            }
+
+            opt2.ifPresent(val -> collector.collect(
+                    new Tuple2<>(timePoint, val / (double) numberOfVertices)));
+        }
+    }
+}

gradoop-temporal/src/main/java/org/gradoop/temporal/model/impl/operators/metric/functions/TransformDeltaToAbsoluteDegreeTree.java

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+/*
+ * Copyright © 2014 - 2021 Leipzig University (Database Research Group)
+ *
+ * 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 org.gradoop.temporal.model.impl.operators.metric.functions;
+
+import org.apache.flink.api.common.functions.MapFunction;
+import org.apache.flink.api.java.functions.FunctionAnnotation;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.gradoop.common.model.impl.id.GradoopId;
+
+import java.util.Map;
+import java.util.TreeMap;
+
+/**
+ * Replaces the degree tree, that just stores the degree changes for each time, with a degree tree that
+ * stores the actual degree of the vertex at that time.
+ */
+@FunctionAnnotation.ForwardedFields("f0")
+public class TransformDeltaToAbsoluteDegreeTree
+        implements MapFunction<Tuple2<GradoopId, TreeMap<Long, Integer>>,
+        Tuple2<GradoopId, TreeMap<Long, Integer>>> {
+
+    /**
+     * To reduce object instantiations.
+     */
+    private TreeMap<Long, Integer> absoluteDegreeTree;
+
+    @Override
+    public Tuple2<GradoopId, TreeMap<Long, Integer>> map(
+            Tuple2<GradoopId, TreeMap<Long, Integer>> vIdTreeMapTuple) throws Exception {
+        // init the degree and the temporal tree
+        int degree = 0;
+        absoluteDegreeTree = new TreeMap<>();
+
+        // aggregate the degrees
+        for (Map.Entry<Long, Integer> entry : vIdTreeMapTuple.f1.entrySet()) {
+            degree += entry.getValue();
+            absoluteDegreeTree.put(entry.getKey(), degree);
+        }
+        vIdTreeMapTuple.f1 = absoluteDegreeTree;
+        return vIdTreeMapTuple;
+    }
+}