jena-arq/src/main/java/org/apache/jena/riot/lang/BlankNodeAllocatorHash.java

/**
 * 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.jena.riot.lang;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.UUID;
import java.util.concurrent.Callable;

import org.apache.jena.atlas.lib.Bytes;
import org.apache.jena.atlas.lib.Cache;
import org.apache.jena.atlas.lib.CacheFactory;
import org.apache.jena.atlas.lib.InternalErrorException;
import org.apache.jena.graph.Node;
import org.apache.jena.graph.NodeFactory;

/**
 * Allocate bnode labels using a per-run seed and the label presented.
 * <p>
 * This is the most scalable, always legal allocator.
 * <p>
 * New allocators must be created per parser run, or .reset() called. These are
 * fed to a digest to give a bit string, (currently MD5, to get a 128bit bit
 * string) that is used to form a bNode AnonId of hex digits.
 * <p>
 * In addition, there is a cache of label{@literal ->}node allocations, using the natural
 * tendency to locality in a database dump. (subject bNodes, bNodes in lists
 * and other data values structures like unit values).
 * <p>
 * Not thread safe.
 */

public class BlankNodeAllocatorHash implements BlankNodeAllocator {
    private static String       DigestAlgorithm = "MD5";
    private static int          CacheSize       = 1000;
    private MessageDigest       mDigest;
    private byte[]              seedBytes;
    // long+2 bytes to distinguish from UTF-8 bytes.
    private byte[]              counterBytes    = new byte[10]; 
    private Cache<String, Node> cache;
    private long                counter         = 0;

    public BlankNodeAllocatorHash() {
        reset();
        try {
            mDigest = MessageDigest.getInstance(DigestAlgorithm);
        } catch (NoSuchAlgorithmException e) {
            throw new InternalErrorException("failed to create message digest", e);
        }
        cache = CacheFactory.createCache(CacheSize);
    }
    
    /**
     * Gets a fresh seed value
     * <p>
     * Note that this is called almost immediately by the constructor
     * and on this initial call you will not yet have access to any 
     * implementation specific information used to select the seed.
     * </p>
     * <p>
     * Implementations <strong>must</strong> return a non-null value
     * so if you can't decide a seed prior to seeing your derived
     * implementations constructor inputs you should return a temporary
     * fake value initially.  You can then call {@link #reset()} in your 
     * own constructor after you've taken the necessary steps that allow
     * you to decide how to generate your own seed. 
     * </p>
     * @return Seed value
     */
    protected UUID freshSeed() {
        return UUID.randomUUID();
    }

    @Override
    public void reset() {
        UUID seed = this.freshSeed();
        seedBytes = new byte[128 / 8];
        Bytes.setLong(seed.getMostSignificantBits(), seedBytes, 0);
        Bytes.setLong(seed.getLeastSignificantBits(), seedBytes, 8);
        if ( cache != null )
            cache.clear();
    }

    @Override
    public Node alloc(final String label) {
        Callable<Node> getter = new Callable<Node>() {
            @Override
            public Node call() {
                return alloc(Bytes.string2bytes(label));
            }
        };
        Node n = cache.getOrFill(label, getter);
        return n;
    }

    @Override
    public Node create() {
        counter++;
        // Make illegal string bytes so can't clash with alloc(String).
        // It is different because it has zeros in it.
        counterBytes[0] = 0;
        counterBytes[1] = 0;
        Bytes.setLong(counter, counterBytes, 2);
        return alloc(counterBytes);
    }

    private Node alloc(byte[] labelBytes) {
        // UUID.nameUUIDFromBytes(seedBytes+labelBytes) uses MD5 but creates the digester
        // each time. It also stamps in the UUID version/variant bits.
        mDigest.update(seedBytes);
        mDigest.update(labelBytes);
        byte[] bytes = mDigest.digest(); // resets
        String hexString = Bytes.asHexLC(bytes);
        return NodeFactory.createBlankNode(hexString);
    }
}