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/*
* (c) Copyright 2024 Palantir Technologies Inc. All rights reserved.
*
* 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 com.palantir.common.concurrent;
import com.google.common.annotations.VisibleForTesting;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
public final class ConcurrentMaps {
private static final float DEFAULT_LOAD_FACTOR = 0.75f;
private static final int INVERSE_CONTENTION_PROBABILITY = 8;
/**
* Returns a new mutable {@link ConcurrentMap} with capacity to
* Increase the initial capacity to reduce contention at startup when concurrently adding elements.
*
* See Caffeine PR 218,
* Caffeine FAQ,
* ConcurrentHashMap comments:
*
* Lock contention probability for two threads accessing distinct elements is roughly 1 / (8 * # of elements)
* under random hashes.
*
* @param expectedEntries number of expected entries
*/
public static ConcurrentMap newWithExpectedEntries(int expectedEntries) {
int concurrencyLevel = Math.max(1, Runtime.getRuntime().availableProcessors());
return new ConcurrentHashMap<>(initialCapacity(expectedEntries), DEFAULT_LOAD_FACTOR, concurrencyLevel);
}
@VisibleForTesting
static int initialCapacity(int expectedEntries) {
return Math.max(0, (int) ((expectedEntries * (INVERSE_CONTENTION_PROBABILITY * DEFAULT_LOAD_FACTOR)) - 1));
}
@VisibleForTesting
static int expectedArraySize(int initialCapacity) {
return Math.max(1, (-1 >>> Integer.numberOfLeadingZeros(initialCapacity - 1)) + 1);
}
private ConcurrentMaps() {}
}