/*
 * 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;


    /**
     * Constructs a new TimeBucketCounter.
     *
     * @param bucketDuration the bucket duration in seconds
     * @param executorService the executor service for maintenance tasks
     */
    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;
    }


    /**
     * Returns the number of bits used for bucket index calculation.
     *
     * @return the number of shift bits
     */
    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;
    }
}
