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/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch 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.elasticsearch.common.util;

import com.carrotsearch.hppc.BitMixer;
import org.apache.lucene.util.BytesRef;
import org.elasticsearch.common.lease.Releasable;
import org.elasticsearch.common.lease.Releasables;

/**
 *  Specialized hash table implementation similar to Lucene's BytesRefHash that maps
 *  BytesRef values to ids. Collisions are resolved with open addressing and linear
 *  probing, growth is smooth thanks to {@link BigArrays}, hashes are cached for faster
 *  re-hashing and capacity is always a multiple of 2 for faster identification of buckets.
 *  This class is not thread-safe.
 */
public final class BytesRefHash extends AbstractHash {

    private LongArray startOffsets;
    private ByteArray bytes;
    private IntArray hashes; // we cache hashes for faster re-hashing
    private final BytesRef spare;

    // Constructor with configurable capacity and default maximum load factor.
    public BytesRefHash(long capacity, BigArrays bigArrays) {
        this(capacity, DEFAULT_MAX_LOAD_FACTOR, bigArrays);
    }

    //Constructor with configurable capacity and load factor.
    public BytesRefHash(long capacity, float maxLoadFactor, BigArrays bigArrays) {
        super(capacity, maxLoadFactor, bigArrays);
        startOffsets = bigArrays.newLongArray(capacity + 1, false);
        startOffsets.set(0, 0);
        bytes = bigArrays.newByteArray(capacity * 3, false);
        hashes = bigArrays.newIntArray(capacity, false);
        spare = new BytesRef();
    }

    // BytesRef has a weak hashCode function so we try to improve it by rehashing using Murmur3
    // Feel free to remove rehashing if BytesRef gets a better hash function
    private static int rehash(int hash) {
        return BitMixer.mix32(hash);
    }

    /**
     * Return the key at 0 <= index <= capacity(). The result is undefined if the slot is unused.
     * 

Beware that the content of the {@link BytesRef} may become invalid as soon as {@link #close()} is called

*/ public BytesRef get(long id, BytesRef dest) { final long startOffset = startOffsets.get(id); final int length = (int) (startOffsets.get(id + 1) - startOffset); bytes.get(startOffset, length, dest); return dest; } /** * Get the id associated with key */ public long find(BytesRef key, int code) { final long slot = slot(rehash(code), mask); for (long index = slot; ; index = nextSlot(index, mask)) { final long id = id(index); if (id == -1L || key.bytesEquals(get(id, spare))) { return id; } } } /** Sugar for {@link #find(BytesRef, int) find(key, key.hashCode()} */ public long find(BytesRef key) { return find(key, key.hashCode()); } private long set(BytesRef key, int code, long id) { assert rehash(key.hashCode()) == code; assert size < maxSize; final long slot = slot(code, mask); for (long index = slot; ; index = nextSlot(index, mask)) { final long curId = id(index); if (curId == -1) { // means unset id(index, id); append(id, key, code); ++size; return id; } else if (key.bytesEquals(get(curId, spare))) { return -1 - curId; } } } private void append(long id, BytesRef key, int code) { assert size == id; final long startOffset = startOffsets.get(size); bytes = bigArrays.grow(bytes, startOffset + key.length); bytes.set(startOffset, key.bytes, key.offset, key.length); startOffsets = bigArrays.grow(startOffsets, size + 2); startOffsets.set(size + 1, startOffset + key.length); hashes = bigArrays.grow(hashes, id + 1); hashes.set(id, code); } private boolean assertConsistent(long id, int code) { get(id, spare); return rehash(spare.hashCode()) == code; } private void reset(int code, long id) { assert assertConsistent(id, code); final long slot = slot(code, mask); for (long index = slot; ; index = nextSlot(index, mask)) { final long curId = id(index); if (curId == -1) { // means unset id(index, id); break; } } } /** * Try to add key. Return its newly allocated id if it wasn't in the hash table yet, or -1-id * if it was already present in the hash table. */ public long add(BytesRef key, int code) { if (size >= maxSize) { assert size == maxSize; grow(); } assert size < maxSize; return set(key, rehash(code), size); } /** Sugar to {@link #add(BytesRef, int) add(key, key.hashCode()}. */ public long add(BytesRef key) { return add(key, key.hashCode()); } @Override protected void removeAndAdd(long index) { final long id = id(index, -1); assert id >= 0; final int code = hashes.get(id); reset(code, id); } @Override public void close() { try (Releasable releasable = Releasables.wrap(bytes, hashes, startOffsets)) { super.close(); } } }




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