com.hazelcast.internal.util.hashslot.HashSlotArray Maven / Gradle / Ivy
/*
* Copyright (c) 2008-2016, Hazelcast, 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.hazelcast.internal.util.hashslot;
import com.hazelcast.internal.memory.MemoryAccessor;
import com.hazelcast.internal.util.hashslot.impl.HashSlotArray16byteKeyNoValue;
import com.hazelcast.internal.util.hashslot.impl.HashSlotArray8byteKeyNoValue;
import com.hazelcast.nio.Disposable;
/**
* A Flyweight object that manages the backbone array of an off-heap open-addressed hashtable.
* The backbone consists of slots, where each slot has a key part and an optional value part.
* The size of the key part varies by subtype ({@link HashSlotArray8byteKey}, {@link HashSlotArray16byteKey}).
* The size of the value part can be specified on instantiation and there are subtypes specialized for
* zero-size values ({@link HashSlotArray8byteKeyNoValue}, {@link HashSlotArray16byteKeyNoValue}).
* At this level of abstraction the slot-accessing methods are not defined because they are specific
* to a given key size. These methods are defined in the key size-specific subtypes. The notes presented
* here, however, apply to all cases.
*
* The update operations only ensure that a slot for a given key exists/doesn't exist and it is
* up to the caller to manage the contents of the value part. The caller will be provided
* with the raw address of the value, suitable for accessing with the associated
* {@link MemoryAccessor MemoryAccessor}.
* The returned address is valid only up to the next map update operation.
*
* Since this is a Flyweight-style object, the same instance can be used to manage many
* hashtables, one at a time. The {@link #gotoAddress(long)} method resets the instance to work
* with the hashtable at the provided address and {@link #gotoNew()} allocates a new hashtable.
* It is the caller's duty to ensure against memory leaks by keeping track of all existing hashtables'
* base addresses. Since an update operation may trigger a new allocation, freeing the old block,
* it is the caller's duty to save the new address before moving on to another base address.
*
* Memory layout
*
* The base address, returned by {@link #address()}, is the addres of the first slot in the array.
* It is preceded by the header of size {@value com.hazelcast.internal.util.hashslot.impl.HashSlotArrayBase#HEADER_SIZE}
* which holds the metadata
* that pertains to the structure as a whole ({@code capacity}, {@code size}, and {@code expandAt}).
* A slot consists of the key part and the value part, in that order, and the size of a slot is exactly the sum of the
* sizes of the key and value parts. The size of the memory block that backs the entire structure is equal to
* {@value com.hazelcast.internal.util.hashslot.impl.HashSlotArrayBase#HEADER_SIZE} + ({@link #capacity()} * slot size), and
* its base address is {@link #address()} - {@value com.hazelcast.internal.util.hashslot.impl.HashSlotArrayBase#HEADER_SIZE}.
*
* A special value must be reserved to mark an unassigned slot. The offset of the marker
* in the slot can be customized, as well as the choice of the special value.
*
*/
public interface HashSlotArray extends Disposable {
/**
* @return current base address of this flyweight
*/
long address();
/**
* Position this flyweight to the supplied base address.
*/
void gotoAddress(long address);
/**
* Allocate a new array and position this flyweight to its base address.
* @return the base address of the new array.
*/
long gotoNew();
/**
* @return the number of assigned slots in the current hash slot array
*/
long size();
/**
* @return the total number of slots in the current hash slot array
*/
long capacity();
/** Array expansion and rehashing will be triggered when {@code size} reaches this value. */
long expansionThreshold();
/**
* After this method returns, all slots in the current hash slot array are unassigned.
*/
void clear();
/** Compact the array by reducing its capacity, if allowed by the current size and load factor.
* @return true if the array was compacted; false if no action was taken. */
boolean trimToSize();
}