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/**
* Copyright 2020 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.core.instrument.step;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Supplier;
import io.micrometer.core.instrument.Clock;
/**
* Tracks 'values' for periods (steps) of time. The previous step's value is
* obtained by calling {@link #poll}.
*
* @author Jon Schneider
* @author Samuel Cox
* @since 1.4.0
*/
public abstract class StepValue {
private final Clock clock;
private final long stepMillis;
private AtomicLong lastInitPos;
private volatile V previous = noValue();
public StepValue(final Clock clock, final long stepMillis) {
this.clock = clock;
this.stepMillis = stepMillis;
lastInitPos = new AtomicLong(clock.wallTime() / stepMillis);
}
protected abstract Supplier valueSupplier();
/**
* @return value that should be returned by {@link #poll} if within the first
* step period or if the previous step didn't record a value.
*/
protected abstract V noValue();
private void rollCount(long now) {
final long stepTime = now / stepMillis;
final long lastInit = lastInitPos.get();
if (lastInit < stepTime && lastInitPos.compareAndSet(lastInit, stepTime)) {
final V v = valueSupplier().get();
// Need to check if there was any activity during the previous step interval. If there was
// then the init position will move forward by 1, otherwise it will be older. No activity
// means the previous interval should be set to the `init` value.
previous = (lastInit == stepTime - 1) ? v : noValue();
}
}
/**
* @return The value for the last completed interval.
*/
public V poll() {
rollCount(clock.wallTime());
return previous;
}
}