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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 org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.logstash.util.SetOnceReference;

import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.OptionalDouble;
import java.util.function.LongSupplier;
import java.util.function.Supplier;

/**
 * This {@link BaseFlowMetric} is a shared-common base for all internal implementations of {@link FlowMetric}.
 */
abstract class BaseFlowMetric extends AbstractMetric<Map<String, Double>> implements FlowMetric {

    static final Logger LOGGER = LogManager.getLogger(BaseFlowMetric.class);

    // metric sources
    private final Metric<? extends Number> numeratorMetric;
    private final Metric<? extends Number> denominatorMetric;

    protected final SetOnceReference<FlowCapture> lifetimeBaseline = SetOnceReference.unset();

    static final String LIFETIME_KEY = "lifetime";

    final LongSupplier nanoTimeSupplier;

    static final MathContext LIMITED_PRECISION = new MathContext(4, RoundingMode.HALF_UP);
    private static final OptionalDouble NEGATIVE_INFINITY_OPTIONAL_DOUBLE = OptionalDouble.of(Double.NEGATIVE_INFINITY);
    private static final OptionalDouble POSITIVE_INFINITY_OPTIONAL_DOUBLE = OptionalDouble.of(Double.POSITIVE_INFINITY);

    BaseFlowMetric(final LongSupplier nanoTimeSupplier,
                   final String name,
                   final Metric<? extends Number> numeratorMetric,
                   final Metric<? extends Number> denominatorMetric) {
        super(name);
        this.nanoTimeSupplier = nanoTimeSupplier;
        this.numeratorMetric = numeratorMetric;
        this.denominatorMetric = denominatorMetric;

        if (doCapture().isEmpty()) {
            LOGGER.trace("FlowMetric({}) -> DEFERRED", name);
        }
    }

    @Override
    public MetricType getType() {
        return MetricType.FLOW_RATE;
    }

    /**
     * Attempt to perform a capture of our metrics, setting our lifetime baseline if it is not yet set.
     *
     * @return a {@link Optional} that contains a {@link FlowCapture} IFF one can be created.
     */
    protected Optional<FlowCapture> doCapture() {
        final Number numeratorValue = numeratorMetric.getValue();
        if (Objects.isNull(numeratorValue)) {
            LOGGER.trace("FlowMetric({}) numerator metric {} returned null value", name, numeratorMetric.getName());
            return Optional.empty();
        }

        final Number denominatorValue = denominatorMetric.getValue();
        if (Objects.isNull(denominatorValue)) {
            LOGGER.trace("FlowMetric({}) numerator metric {} returned null value", name, denominatorMetric.getName());
            return Optional.empty();
        }

        final FlowCapture currentCapture = new FlowCapture(nanoTimeSupplier.getAsLong(), numeratorValue, denominatorValue);

        // if our lifetime baseline has not been set, set it now.
        if (lifetimeBaseline.offer(currentCapture)) {
            LOGGER.trace("FlowMetric({}) baseline -> {}", name, currentCapture);
        }

        return Optional.of(currentCapture);
    }

    protected void injectLifetime(final FlowCapture currentCapture, final Map<String, Double> rates) {
        calculateRate(currentCapture, lifetimeBaseline::get).ifPresent((rate) -> rates.put(LIFETIME_KEY, rate));
    }

    /**
     * @param current the most-recent {@link FlowCapture}
     * @param baseline a non-null {@link FlowCapture} from which to compare.
     * @return an {@link OptionalDouble} that will be non-empty IFF we have sufficient information
     * to calculate a finite rate of change of the numerator relative to the denominator.
     */
    protected static OptionalDouble calculateRate(final FlowCapture current, final FlowCapture baseline) {
        Objects.requireNonNull(current, "current must not be null");
        Objects.requireNonNull(baseline, "baseline must not be null");
        if (baseline.equals(current)) { return OptionalDouble.empty(); }

        final BigDecimal deltaNumerator = current.numerator().subtract(baseline.numerator());
        final BigDecimal deltaDenominator = current.denominator().subtract(baseline.denominator());

        if (deltaDenominator.signum() == 0) {
            switch (deltaNumerator.signum()) {
                case -1: return NEGATIVE_INFINITY_OPTIONAL_DOUBLE;
                case  0: return OptionalDouble.empty();
                case +1: return POSITIVE_INFINITY_OPTIONAL_DOUBLE;
            }
        }

        final BigDecimal rate = deltaNumerator.divide(deltaDenominator, LIMITED_PRECISION);

        return OptionalDouble.of(rate.doubleValue());
    }

    /**
     * @param current the most-recent {@link FlowCapture}
     * @param possibleBaseline a {@link Supplier}{@code <FlowCapture>} that may return null
     * @return an {@link OptionalDouble} that will be non-empty IFF we have sufficient information
     * to calculate a finite rate of change of the numerator relative to the denominator.
     */
    protected static OptionalDouble calculateRate(final FlowCapture current, final Supplier<FlowCapture> possibleBaseline) {
        return Optional.ofNullable(possibleBaseline.get())
                .map((baseline) -> calculateRate(current, baseline))
                .orElseGet(OptionalDouble::empty);
    }

}
