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/*
 * Copyright The OpenTelemetry Authors
 * SPDX-License-Identifier: Apache-2.0
 */

package io.opentelemetry.sdk.metrics.internal.aggregator;

import io.opentelemetry.api.common.Attributes;
import io.opentelemetry.sdk.common.InstrumentationScopeInfo;
import io.opentelemetry.sdk.metrics.data.AggregationTemporality;
import io.opentelemetry.sdk.metrics.data.LongExemplarData;
import io.opentelemetry.sdk.metrics.data.LongPointData;
import io.opentelemetry.sdk.metrics.data.MetricData;
import io.opentelemetry.sdk.metrics.internal.data.ImmutableGaugeData;
import io.opentelemetry.sdk.metrics.internal.data.ImmutableLongPointData;
import io.opentelemetry.sdk.metrics.internal.data.ImmutableMetricData;
import io.opentelemetry.sdk.metrics.internal.data.MutableLongPointData;
import io.opentelemetry.sdk.metrics.internal.descriptor.MetricDescriptor;
import io.opentelemetry.sdk.metrics.internal.exemplar.ExemplarReservoir;
import io.opentelemetry.sdk.metrics.internal.state.Measurement;
import io.opentelemetry.sdk.resources.Resource;
import java.util.Collection;
import java.util.List;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Supplier;
import javax.annotation.Nullable;

/**
 * Aggregator that aggregates recorded values by storing the last recorded value.
 *
 * Limitation: The current implementation does not store a time when the value was recorded, so
 * merging multiple LastValueAggregators will not preserve the ordering of records. This is not a
 * problem because LastValueAggregator is currently only available for Observers which record all
 * values once.
 *
 * This class is internal and is hence not for public use. Its APIs are unstable and can change
 * at any time.
 */
public final class LongLastValueAggregator implements Aggregator {
  private final Supplier> reservoirSupplier;

  public LongLastValueAggregator(Supplier> reservoirSupplier) {
    this.reservoirSupplier = reservoirSupplier;
  }

  @Override
  public AggregatorHandle createHandle() {
    return new Handle(reservoirSupplier.get());
  }

  @Override
  public LongPointData diff(LongPointData previous, LongPointData current) {
    return current;
  }

  @Override
  public void diffInPlace(LongPointData previousReusablePoint, LongPointData currentPoint) {
    ((MutableLongPointData) previousReusablePoint).set(currentPoint);
  }

  @Override
  public LongPointData toPoint(Measurement measurement) {
    return ImmutableLongPointData.create(
        measurement.startEpochNanos(),
        measurement.epochNanos(),
        measurement.attributes(),
        measurement.longValue());
  }

  @Override
  public void toPoint(Measurement measurement, LongPointData reusablePoint) {
    ((MutableLongPointData) reusablePoint)
        .set(
            measurement.startEpochNanos(),
            measurement.epochNanos(),
            measurement.attributes(),
            measurement.longValue());
  }

  @Override
  public LongPointData createReusablePoint() {
    return new MutableLongPointData();
  }

  @Override
  public void copyPoint(LongPointData point, LongPointData toReusablePoint) {
    ((MutableLongPointData) toReusablePoint).set(point);
  }

  @Override
  public MetricData toMetricData(
      Resource resource,
      InstrumentationScopeInfo instrumentationScopeInfo,
      MetricDescriptor descriptor,
      Collection points,
      AggregationTemporality temporality) {
    // Last-Value ignores temporality generally, but we can set a start time on the gauge.
    return ImmutableMetricData.createLongGauge(
        resource,
        instrumentationScopeInfo,
        descriptor.getName(),
        descriptor.getDescription(),
        descriptor.getSourceInstrument().getUnit(),
        ImmutableGaugeData.create(points));
  }

  static final class Handle extends AggregatorHandle {
    @Nullable private static final Long DEFAULT_VALUE = null;
    private final AtomicReference current = new AtomicReference<>(DEFAULT_VALUE);

    Handle(ExemplarReservoir exemplarReservoir) {
      super(exemplarReservoir);
    }

    @Override
    protected LongPointData doAggregateThenMaybeReset(
        long startEpochNanos,
        long epochNanos,
        Attributes attributes,
        List exemplars,
        boolean reset) {
      Long value = reset ? this.current.getAndSet(DEFAULT_VALUE) : this.current.get();
      return ImmutableLongPointData.create(
          startEpochNanos, epochNanos, attributes, Objects.requireNonNull(value), exemplars);
    }

    @Override
    protected void doRecordLong(long value) {
      current.set(value);
    }
  }
}