/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF 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.apache.catalina.util;
import java.util.concurrent.ScheduledExecutorService;
/**
* A fast counter that optimizes efficiency at the expense of approximate bucket indexing.
*/
public class TimeBucketCounter extends TimeBucketCounterBase {
// Milliseconds bucket size as a Power of 2 for bit shift math, e.g. 16 for 65_536ms which is about 1:05 minute.
private final int numBits;
// Ratio of actual duration to config duration
private final double ratio;
public TimeBucketCounter(int bucketDuration, ScheduledExecutorService executorService) {
super(getActualDuration(bucketDuration), executorService);
this.numBits = determineShiftBitsOfDuration(bucketDuration);
this.ratio = ratioToPowerOf2(bucketDuration * 1000);
}
/**
* {@inheritDoc}
* <p>
* Calculates the current time bucket index by shifting bits for fast division, e.g. shift 16 bits is the same as
* dividing by 65,536 which is about 1:05m.
*/
@Override
public long getBucketIndex(long timestamp) {
return timestamp >> this.numBits;
}
public int getNumBits() {
return numBits;
}
/**
* The actual duration may differ from the configured duration because it is set to the next power of 2 value in
* order to perform very fast bit shift arithmetic.
*
* @return the actual bucket duration in milliseconds
*/
public int getActualDuration() {
return (int) Math.pow(2, getNumBits());
}
/**
* Determines the bits of shift for the specific bucket duration in seconds, which used to figure out the correct
* bucket index.
*
* @param duration bucket duration in seconds
*
* @return bits to be shifted
*/
protected static int determineShiftBitsOfDuration(int duration) {
int bits = 0;
int pof2 = nextPowerOf2(duration * 1000);
int bitCheck = pof2;
while (bitCheck > 1) {
bitCheck = pof2 >> ++bits;
}
return bits;
}
/**
* The actual duration may differ from the configured duration because it is set to the next power of 2 value in
* order to perform very fast bit shift arithmetic.
*
* @param duration in seconds
*
* @return the actual bucket duration in seconds
*
* @see TimeBucketCounter#determineShiftBitsOfDuration(int)
*/
private static int getActualDuration(int duration) {
return (int) (1L << determineShiftBitsOfDuration(duration)) / 1000;
}
/**
* Returns the ratio between the configured duration param and the actual duration which will be set to the next
* power of 2. We then multiply the configured requests param by the same ratio in order to compensate for the added
* time, if any.
*
* @return the ratio, e.g. 1.092 if the actual duration is 65_536 for the configured duration of 60_000
*/
@Override
public double getRatio() {
return ratio;
}
/**
* Returns the ratio to the next power of 2 so that we can adjust the value.
*
* @param value of target duration in seconds
*
* @return the ratio to the next power of 2 so that we can adjust the value
*/
static double ratioToPowerOf2(int value) {
double nextPO2 = nextPowerOf2(value);
return Math.round((1000 * nextPO2 / value)) / 1000d;
}
/**
* Returns the next power of 2 given a value, e.g. 256 for 250, or 1024, for 1000.
*/
static int nextPowerOf2(int value) {
int valueOfHighestBit = Integer.highestOneBit(value);
if (valueOfHighestBit == value) {
return value;
}
return valueOfHighestBit << 1;
}
@Override
public long getMillisUntilNextBucket() {
long millis = System.currentTimeMillis();
long nextTimeBucketMillis = ((millis + (long) Math.pow(2, numBits)) >> numBits) << numBits;
return nextTimeBucketMillis - millis;
}
}