/*
 * Copyright 2018, OpenCensus Authors
 *
 * 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.opencensus.implcore.metrics;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.util.concurrent.AtomicDouble;
import io.opencensus.common.Clock;
import io.opencensus.implcore.internal.Utils;
import io.opencensus.metrics.DoubleGauge;
import io.opencensus.metrics.LabelKey;
import io.opencensus.metrics.LabelValue;
import io.opencensus.metrics.export.Metric;
import io.opencensus.metrics.export.MetricDescriptor;
import io.opencensus.metrics.export.MetricDescriptor.Type;
import io.opencensus.metrics.export.Point;
import io.opencensus.metrics.export.TimeSeries;
import io.opencensus.metrics.export.Value;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import javax.annotation.Nullable;

/** Implementation of {@link DoubleGauge}. */
public final class DoubleGaugeImpl extends DoubleGauge implements Meter {
  @VisibleForTesting static final LabelValue UNSET_VALUE = LabelValue.create(null);

  private final MetricDescriptor metricDescriptor;
  private volatile Map<List<LabelValue>, PointImpl> registeredPoints =
      Collections.<List<LabelValue>, PointImpl>emptyMap();
  private final int labelKeysSize;
  private final List<LabelValue> defaultLabelValues;
  private final List<LabelValue> constantLabelValues;

  DoubleGaugeImpl(
      String name,
      String description,
      String unit,
      List<LabelKey> labelKeys,
      Map<LabelKey, LabelValue> constantLabels) {
    List<LabelValue> constantLabelValues = new ArrayList<LabelValue>();
    List<LabelKey> allKeys = new ArrayList<>(labelKeys);
    for (Entry<LabelKey, LabelValue> label : constantLabels.entrySet()) {
      // Ensure constant label keys and values are in the same order.
      allKeys.add(label.getKey());
      constantLabelValues.add(label.getValue());
    }
    labelKeysSize = allKeys.size();
    this.metricDescriptor =
        MetricDescriptor.create(name, description, unit, Type.GAUGE_DOUBLE, allKeys);
    this.constantLabelValues = Collections.unmodifiableList(constantLabelValues);

    // initialize defaultLabelValues
    defaultLabelValues = new ArrayList<LabelValue>(labelKeys.size());
    for (int i = 0; i < labelKeys.size(); i++) {
      defaultLabelValues.add(UNSET_VALUE);
    }
    defaultLabelValues.addAll(constantLabelValues);
  }

  @Override
  public DoublePoint getOrCreateTimeSeries(List<LabelValue> labelValues) {
    // lock free point retrieval, if it is present
    PointImpl existingPoint = registeredPoints.get(labelValues);
    if (existingPoint != null) {
      return existingPoint;
    }

    List<LabelValue> labelValuesCopy =
        new ArrayList<LabelValue>(checkNotNull(labelValues, "labelValues"));
    labelValuesCopy.addAll(constantLabelValues);
    return registerTimeSeries(Collections.unmodifiableList(labelValuesCopy));
  }

  @Override
  public DoublePoint getDefaultTimeSeries() {
    // lock free default point retrieval, if it is present
    PointImpl existingPoint = registeredPoints.get(defaultLabelValues);
    if (existingPoint != null) {
      return existingPoint;
    }
    return registerTimeSeries(Collections.unmodifiableList(defaultLabelValues));
  }

  @Override
  public synchronized void removeTimeSeries(List<LabelValue> labelValues) {
    List<LabelValue> labelValuesCopy =
        new ArrayList<LabelValue>(checkNotNull(labelValues, "labelValues"));
    labelValuesCopy.addAll(constantLabelValues);

    Map<List<LabelValue>, PointImpl> registeredPointsCopy =
        new LinkedHashMap<List<LabelValue>, PointImpl>(registeredPoints);
    if (registeredPointsCopy.remove(labelValuesCopy) == null) {
      // The element not present, no need to update the current map of points.
      return;
    }
    registeredPoints = Collections.unmodifiableMap(registeredPointsCopy);
  }

  @Override
  public synchronized void clear() {
    registeredPoints = Collections.<List<LabelValue>, PointImpl>emptyMap();
  }

  @Override
  public MetricDescriptor getMetricDescriptor() {
    return metricDescriptor;
  }

  private synchronized DoublePoint registerTimeSeries(List<LabelValue> labelValues) {
    PointImpl existingPoint = registeredPoints.get(labelValues);
    if (existingPoint != null) {
      // Return a Point that are already registered. This can happen if a multiple threads
      // concurrently try to register the same {@code TimeSeries}.
      return existingPoint;
    }

    checkArgument(
        labelKeysSize == labelValues.size(), "Label Keys and Label Values don't have same size.");
    Utils.checkListElementNotNull(labelValues, "labelValue");

    PointImpl newPoint = new PointImpl(labelValues);
    // Updating the map of points happens under a lock to avoid multiple add operations
    // to happen in the same time.
    Map<List<LabelValue>, PointImpl> registeredPointsCopy =
        new LinkedHashMap<List<LabelValue>, PointImpl>(registeredPoints);
    registeredPointsCopy.put(labelValues, newPoint);
    registeredPoints = Collections.unmodifiableMap(registeredPointsCopy);

    return newPoint;
  }

  @Nullable
  @Override
  public Metric getMetric(Clock clock) {
    Map<List<LabelValue>, PointImpl> currentRegisteredPoints = registeredPoints;
    if (currentRegisteredPoints.isEmpty()) {
      return null;
    }

    if (currentRegisteredPoints.size() == 1) {
      PointImpl point = currentRegisteredPoints.values().iterator().next();
      return Metric.createWithOneTimeSeries(metricDescriptor, point.getTimeSeries(clock));
    }

    List<TimeSeries> timeSeriesList = new ArrayList<TimeSeries>(currentRegisteredPoints.size());
    for (Map.Entry<List<LabelValue>, PointImpl> entry : currentRegisteredPoints.entrySet()) {
      timeSeriesList.add(entry.getValue().getTimeSeries(clock));
    }
    return Metric.create(metricDescriptor, timeSeriesList);
  }

  /** Implementation of {@link DoubleGauge.DoublePoint}. */
  public static final class PointImpl extends DoublePoint {

    // TODO(mayurkale): Consider to use DoubleAdder here, once we upgrade to Java8.
    private final AtomicDouble value = new AtomicDouble(0);
    private final TimeSeries defaultTimeSeries;

    PointImpl(List<LabelValue> labelValues) {
      defaultTimeSeries = TimeSeries.create(labelValues);
    }

    @Override
    public void add(double amt) {
      value.addAndGet(amt);
    }

    @Override
    public void set(double val) {
      value.set(val);
    }

    private TimeSeries getTimeSeries(Clock clock) {
      return defaultTimeSeries.setPoint(Point.create(Value.doubleValue(value.get()), clock.now()));
    }
  }
}
