Add the Distribution data type for instrumentation

This is one of a series of CLs adding a new metric type, EventMetric, which
is used for tracking numerical distributions.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132103552

java/google/registry/monitoring/metrics/MutableDistribution.java

@@ -0,0 +1,111 @@
+// Copyright 2016 The Domain Registry Authors. All Rights Reserved.
+//
+// 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 google.registry.monitoring.metrics;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+import static google.registry.monitoring.metrics.MetricsUtils.checkDouble;
+
+import com.google.common.collect.ImmutableRangeMap;
+import com.google.common.collect.ImmutableSortedSet;
+import com.google.common.collect.Ordering;
+import com.google.common.collect.Range;
+import com.google.common.collect.TreeRangeMap;
+import com.google.common.primitives.Doubles;
+import java.util.Map;
+import javax.annotation.concurrent.NotThreadSafe;
+
+/**
+ * A mutable {@link Distribution}. Instances of this class <b>should not</b> be used to construct
+ * {@link MetricPoint} instances as {@link MetricPoint} instances are supposed to represent
+ * immutable values.
+ *
+ * @see ImmutableDistribution
+ */
+@NotThreadSafe
+public final class MutableDistribution implements Distribution {
+
+  private final TreeRangeMap<Double, Long> intervalCounts;
+  private final DistributionFitter distributionFitter;
+  private double sumOfSquaredDeviation = 0.0;
+  private double mean = 0.0;
+  private int count = 0;
+
+  /** Constructs an empty Distribution with the specified {@link DistributionFitter}. */
+  public MutableDistribution(DistributionFitter distributionFitter) {
+    this.distributionFitter = checkNotNull(distributionFitter);
+    ImmutableSortedSet<Double> boundaries = distributionFitter.boundaries();
+
+    checkArgument(boundaries.size() > 0);
+    checkArgument(Ordering.natural().isOrdered(boundaries));
+
+    this.intervalCounts = TreeRangeMap.create();
+
+    double[] boundariesArray = Doubles.toArray(distributionFitter.boundaries());
+
+    // Add underflow and overflow intervals
+    this.intervalCounts.put(Range.lessThan(boundariesArray[0]), 0L);
+    this.intervalCounts.put(Range.atLeast(boundariesArray[boundariesArray.length - 1]), 0L);
+
+    // Add finite intervals
+    for (int i = 1; i < boundariesArray.length; i++) {
+      this.intervalCounts.put(Range.closedOpen(boundariesArray[i - 1], boundariesArray[i]), 0L);
+    }
+  }
+
+  public void add(double value) {
+    add(value, 1L);
+  }
+
+  public void add(double value, long numSamples) {
+    checkArgument(numSamples > 0, "numSamples must be greater than 0");
+    checkDouble(value);
+
+    Map.Entry<Range<Double>, Long> entry = intervalCounts.getEntry(value);
+    intervalCounts.put(entry.getKey(), entry.getValue() + numSamples);
+    this.count += numSamples;
+
+    // Update mean and sumOfSquaredDeviation using Welford's method
+    // See Knuth, "The Art of Computer Programming", Vol. 2, page 232, 3rd edition
+    double delta = value - mean;
+    mean += delta * numSamples / count;
+    sumOfSquaredDeviation += delta * (value - mean) * numSamples;
+  }
+
+  @Override
+  public double mean() {
+    return mean;
+  }
+
+  @Override
+  public double sumOfSquaredDeviation() {
+    return sumOfSquaredDeviation;
+  }
+
+  @Override
+  public long count() {
+    return count;
+  }
+
+  @Override
+  public ImmutableRangeMap<Double, Long> intervalCounts() {
+    return ImmutableRangeMap.copyOf(intervalCounts);
+  }
+
+  @Override
+  public DistributionFitter distributionFitter() {
+    return distributionFitter;
+  }
+}