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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.

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/*
 * 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.cassandra.index.sai.utils;

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

/**
 * AtomicRatio provides thread safe operations to maintain a {@link Ratio} of a numerator and denominator.
 * The ratio can be updated atomically by multiple threads calling the {@link #update(long, long)} method.
 * The current ratio value can be retrieved via the {@link #get} method.
 * 
 * The class also provides a thread safe {@link #updateCount} that maintains the number of times the {@link Ratio}
 * has been updated. This can be used to determine whether the {@link Ratio} is useful based on the number of updates.
 */
public class AtomicRatio
{
    private final AtomicReference ratio = new AtomicReference<>(new Ratio(0, 0));
    private final AtomicInteger updateCount = new AtomicInteger();

    private static class Ratio
    {
        public final long numerator;
        public final long denominator;

        public Ratio(long numerator, long denominator)
        {
            this.numerator = numerator;
            this.denominator = denominator;
        }
    }

    public void update(long numerator, long denominator)
    {
        ratio.updateAndGet((current) -> new Ratio(current.numerator + numerator, current.denominator + denominator));
        updateCount.incrementAndGet();
    }

    public double get()
    {
        var current = ratio.get();
        return (double) current.numerator / current.denominator;
    }

    public int getUpdateCount()
    {
        return updateCount.get();
    }
}