/*
 * Copyright 2017 VMware, Inc.
 *
 * 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
 *
 * https://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.micrometer.prometheus;

import io.micrometer.core.instrument.AbstractMeter;
import io.micrometer.core.instrument.Counter;
import io.micrometer.core.instrument.Meter;
import io.micrometer.core.lang.NonNull;
import io.micrometer.core.lang.Nullable;
import io.prometheus.client.exemplars.CounterExemplarSampler;
import io.prometheus.client.exemplars.Exemplar;

import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.DoubleAdder;

/**
 * {@link Counter} for Prometheus.
 *
 * @author Jon Schneider
 * @author Jonatan Ivanov
 */
public class PrometheusCounter extends AbstractMeter implements Counter {
    private final DoubleAdder count = new DoubleAdder();
    private final AtomicReference<Exemplar> exemplar = new AtomicReference<>();
    @Nullable private final CounterExemplarSampler exemplarSampler;

    PrometheusCounter(Meter.Id id) {
        this(id, null);
    }

    PrometheusCounter(Meter.Id id, @Nullable CounterExemplarSampler exemplarSampler) {
        super(id);
        this.exemplarSampler = exemplarSampler;
    }

    @Override
    public void increment(double amount) {
        if (amount > 0) {
            count.add(amount);
            if (exemplarSampler != null) {
                updateExemplar(amount, exemplarSampler);
            }
        }
    }

    @Override
    public double count() {
        return count.doubleValue();
    }

    @Nullable Exemplar exemplar() {
        return exemplar.get();
    }

    // Similar to exemplar.updateAndGet(...) but it does nothing if the next value is null
    private void updateExemplar(double amount, @NonNull CounterExemplarSampler exemplarSampler) {
        Exemplar prev;
        Exemplar next;
        do {
            prev = exemplar.get();
            next = exemplarSampler.sample(amount, prev);
            if (next == null || next == prev) {
                return;
            }
        }
        while (!exemplar.compareAndSet(prev, next));
    }

}

@@ -2418,111 +2418,57 @@
     
-    if
+%7D%0A        while
  (next 
-=
+!
 = null 
-%7C%7C
+&&
  next 
-=
+!
 = prev
-) %7B%0A                return;%0A            %7D%0A        %7D%0A        while (
+ && 
 !exe

Align PrometheusCounter's updateExemplar with PrometheusHistogram related: gh-3131