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 * Licensed to Elasticsearch B.V. under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch B.V. 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.logstash.instrument.metrics;

import java.time.Duration;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.LongSupplier;

/**
 * The {@link SimpleFlowMetric} is an implementation of {@link FlowMetric} that
 * produces only `lifetime` and a naively-calculated `current` rate using the two
 * most-recent {@link FlowCapture}s.
 */
class SimpleFlowMetric extends BaseFlowMetric {

    // useful capture nodes for calculation

    private final AtomicReference<FlowCapture> head;
    private final AtomicReference<FlowCapture> instant = new AtomicReference<>();

    static final String CURRENT_KEY = "current";

    public SimpleFlowMetric(final String name,
                            final Metric<? extends Number> numeratorMetric,
                            final Metric<? extends Number> denominatorMetric) {
        this(System::nanoTime, name, numeratorMetric, denominatorMetric);
    }

    SimpleFlowMetric(final LongSupplier nanoTimeSupplier,
                     final String name,
                     final Metric<? extends Number> numeratorMetric,
                     final Metric<? extends Number> denominatorMetric) {
        super(nanoTimeSupplier, name, numeratorMetric, denominatorMetric);

        this.head = new AtomicReference<>(lifetimeBaseline.orElse(null));
    }

    @Override
    public void capture() {
        final Optional<FlowCapture> possibleCapture = doCapture();
        if (possibleCapture.isEmpty()) { return; }

        final FlowCapture newestHead = possibleCapture.get();
        final FlowCapture previousHead = head.getAndSet(newestHead);
        if (Objects.nonNull(previousHead)) {
            instant.getAndAccumulate(previousHead, (current, given) -> {
                // keep our current value if the given one is less than ~100ms older than our newestHead
                // this is naive and when captures happen too frequently without relief can result in
                // our "current" window growing indefinitely, but we are shipping with a 5s cadence
                // and shouldn't hit this edge-case in practice.
                return (calculateCapturePeriod(newestHead, given).toMillis() > 100) ? given : current;
            });
        }
    }

    /**
     * @return a map containing all available finite rates (see {@link BaseFlowMetric#calculateRate(FlowCapture,FlowCapture)})
     */
    @Override
    public Map<String, Double> getValue() {
        final FlowCapture headCapture = head.get();
        if (Objects.isNull(headCapture)) {
            return Map.of();
        }

        final Map<String, Double> rates = new LinkedHashMap<>();

        calculateRate(headCapture, instant::get).ifPresent((rate) -> rates.put(CURRENT_KEY,  rate));
        injectLifetime(headCapture, rates);

        return Collections.unmodifiableMap(rates);
    }

    private static Duration calculateCapturePeriod(final FlowCapture current, final FlowCapture baseline) {
        return Duration.ofNanos(Math.subtractExact(current.nanoTime(), baseline.nanoTime()));
    }
}
