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/*
* Copyright 2015 Open Networking Laboratory
*
* 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
*
* 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.onlab.util;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.stream.Collectors;
import static com.google.common.base.Preconditions.checkArgument;
/**
* Maintains a sliding window of value counts. The sliding window counter is
* initialized with a number of window slots. Calls to #incrementCount() will
* increment the value in the current window slot. Periodically the window
* slides and the oldest value count is dropped. Calls to #get() will get the
* total count for the last N window slots.
*/
public final class SlidingWindowCounter {
private volatile int headSlot;
private final int windowSlots;
private final List<AtomicLong> counters;
private final ScheduledExecutorService background;
private static final int SLIDE_WINDOW_PERIOD_SECONDS = 1;
/**
* Creates a new sliding window counter with the given total number of
* window slots.
*
* @param windowSlots total number of window slots
*/
public SlidingWindowCounter(int windowSlots) {
checkArgument(windowSlots > 0, "Window size must be a positive integer");
this.windowSlots = windowSlots;
this.headSlot = 0;
// Initialize each item in the list to an AtomicLong of 0
this.counters = Collections.nCopies(windowSlots, 0)
.stream()
.map(AtomicLong::new)
.collect(Collectors.toCollection(ArrayList::new));
background = Executors.newSingleThreadScheduledExecutor();
background.scheduleWithFixedDelay(this::advanceHead, 0,
SLIDE_WINDOW_PERIOD_SECONDS, TimeUnit.SECONDS);
}
/**
* Releases resources used by the SlidingWindowCounter.
*/
public void destroy() {
background.shutdownNow();
}
/**
* Increments the count of the current window slot by 1.
*/
public void incrementCount() {
incrementCount(headSlot, 1);
}
/**
* Increments the count of the current window slot by the given value.
*
* @param value value to increment by
*/
public void incrementCount(long value) {
incrementCount(headSlot, value);
}
private void incrementCount(int slot, long value) {
counters.get(slot).addAndGet(value);
}
/**
* Gets the total count for the last N window slots.
*
* @param slots number of slots to include in the count
* @return total count for last N slots
*/
public long get(int slots) {
checkArgument(slots <= windowSlots,
"Requested window must be less than the total window slots");
long sum = 0;
for (int i = 0; i < slots; i++) {
int currentIndex = headSlot - i;
if (currentIndex < 0) {
currentIndex = counters.size() + currentIndex;
}
sum += counters.get(currentIndex).get();
}
return sum;
}
void advanceHead() {
counters.get(slotAfter(headSlot)).set(0);
headSlot = slotAfter(headSlot);
}
private int slotAfter(int slot) {
return (slot + 1) % windowSlots;
}
}
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