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Concourse is a self-tuning database that is designed for both ad hoc analytics and high volume transactions at scale. Developers use Concourse to quickly build mission critical software while also benefiting from real time insight into their most important data. With Concourse, end-to-end data management requires no extra infrastructure, no prior configuration and no additional coding–all of which greatly reduce costs and allow developers to focus on core business problems.

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/*
 * Copyright (c) 2013-2019 Cinchapi Inc.
 *
 * 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.cinchapi.concourse.cache;

import javax.annotation.Nullable;

import com.google.common.base.Preconditions;
import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;
import com.google.common.hash.HashCode;
import com.google.common.hash.Hashing;

/**
 * 
 * A concurrent cache that holds references to objects so as to prevent
 * unnecessary constructor calls. The cache does not have a maximum capacity or
 * expulsion policy, but it uses a SoftReference for each stored object so that
 * the garbage collector will clear the cache only before an OOM occurs.
 * 
 * Example:
 * 
 *  *  private class Foo {
 *    
 *    int id;
 *    String label;
 *    Bar bar;
 *    
 *    private static final ReferenceCache{@literal } cache = new ReferenceCache{@literal }();
 *    
 *    public static Foo newInstance(int id, String label, Bar bar){
 *    	Foo foo = cache.get(id, label, bar); // use the combination of id, label and bar as a cacheKey
 *    	if(foo == null){
 *    	    foo = new Foo(id, label, bar);
 *    		cache.put(foo, id, label, bar);
 *    	}
 *    	return foo;
 *    }
 *    
 *    private Foo(int id, String label, Bar bar){
 *    	this.id = id;
 *    	this.label = label;
 *    	this.bar = bar;
 *    }
 * }
 * 
 * 
 * @author Jeff Nelson
 * @param  - the cached object type.
 */
public class ReferenceCache {

    private static final int INITIAL_CAPACITY = 500000;
    private static final int CONCURRENCY_LEVEL = 16;

    private final Cache cache = CacheBuilder.newBuilder()
            .initialCapacity(INITIAL_CAPACITY)
            .concurrencyLevel(CONCURRENCY_LEVEL).softValues().build();

    /**
     * Return the cache value associated with the group of {@code args} or
     * {@code null} if not value is found.
     * 
     * @param args
     * @return the cached value.
     */
    @Nullable
    public T get(Object... args) {
        HashCode id = getCacheKey(args);
        return cache.getIfPresent(id);
    }

    /**
     * Cache {@code value} and associated it with the group of {@code args}.
     * Each arg should be a value that is used to construct
     * the object.
     * 
     * @param value
     * @param args
     */
    public void put(T value, Object... args) {
        Preconditions.checkNotNull(value);
        Preconditions.checkNotNull(args);
        Preconditions.checkArgument(args.length > 0,
                "You must specify at least one key");
        HashCode id = getCacheKey(args);
        cache.put(id, value);
    }

    /**
     * Remove the value associated with the group of {@code args} from the
     * cache.
     * 
     * @param args
     */
    public void remove(Object... args) {
        cache.invalidate(getCacheKey(args));
    }

    /**
     * Return a unique identifier for a group of {@code args}.
     * 
     * @param args
     * @return the identifier.
     */
    @SuppressWarnings("deprecation")
    private HashCode getCacheKey(Object... args) {
        StringBuilder key = new StringBuilder();
        for (Object o : args) {
            key.append(o.hashCode());
            key.append(o.getClass().getName());
        }
        return Hashing.md5().hashUnencodedChars(key.toString());
    }

}