android/guava/src/com/google/common/collect/CompactLinkedHashMap.java

/*
 * Copyright (C) 2012 The Guava Authors
 *
 * 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.google.common.collect;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.Map;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * CompactLinkedHashMap is an implementation of a Map with insertion or LRU iteration order,
 * maintained with a doubly linked list through the entries. All optional operations (put and
 * remove) are supported. Null keys and values are supported.
 *
 * <p>{@code containsKey(k)}, {@code put(k, v)} and {@code remove(k)} are all (expected and
 * amortized) constant time operations. Expected in the hashtable sense (depends on the hash
 * function doing a good job of distributing the elements to the buckets to a distribution not far
 * from uniform), and amortized since some operations can trigger a hash table resize.
 *
 * <p>As compared with {@link java.util.LinkedHashMap}, this structure places significantly reduced
 * load on the garbage collector by only using a constant number of internal objects.
 *
 * <p>This class should not be assumed to be universally superior to {@code
 * java.util.LinkedHashMap}. Generally speaking, this class reduces object allocation and memory
 * consumption at the price of moderately increased constant factors of CPU. Only use this class
 * when there is a specific reason to prioritize memory over CPU.
 *
 * @author Louis Wasserman
 */
@GwtIncompatible // not worth using in GWT for now
class CompactLinkedHashMap<K, V> extends CompactHashMap<K, V> {
  // TODO(lowasser): implement removeEldestEntry so this can be used as a drop-in replacement

  /** Creates an empty {@code CompactLinkedHashMap} instance. */
  public static <K, V> CompactLinkedHashMap<K, V> create() {
    return new CompactLinkedHashMap<>();
  }

  /**
   * Creates a {@code CompactLinkedHashMap} instance, with a high enough "initial capacity" that it
   * <i>should</i> hold {@code expectedSize} elements without rebuilding internal data structures.
   *
   * @param expectedSize the number of elements you expect to add to the returned set
   * @return a new, empty {@code CompactLinkedHashMap} with enough capacity to hold {@code
   *     expectedSize} elements without resizing
   * @throws IllegalArgumentException if {@code expectedSize} is negative
   */
  public static <K, V> CompactLinkedHashMap<K, V> createWithExpectedSize(int expectedSize) {
    return new CompactLinkedHashMap<>(expectedSize);
  }

  private static final int ENDPOINT = -2;

  /**
   * Contains the link pointers corresponding with the entries, in the range of [0, size()). The
   * high 32 bits of each long is the "prev" pointer, whereas the low 32 bits is the "succ" pointer
   * (pointing to the next entry in the linked list). The pointers in [size(), entries.length) are
   * all "null" (UNSET).
   *
   * <p>A node with "prev" pointer equal to {@code ENDPOINT} is the first node in the linked list,
   * and a node with "next" pointer equal to {@code ENDPOINT} is the last node.
   */
  @VisibleForTesting @NullableDecl transient long[] links;

  /** Pointer to the first node in the linked list, or {@code ENDPOINT} if there are no entries. */
  private transient int firstEntry;

  /** Pointer to the last node in the linked list, or {@code ENDPOINT} if there are no entries. */
  private transient int lastEntry;

  private final boolean accessOrder;

  CompactLinkedHashMap() {
    this(CompactHashing.DEFAULT_SIZE);
  }

  CompactLinkedHashMap(int expectedSize) {
    this(expectedSize, false);
  }

  CompactLinkedHashMap(int expectedSize, boolean accessOrder) {
    super(expectedSize);
    this.accessOrder = accessOrder;
  }

  @Override
  void init(int expectedSize) {
    super.init(expectedSize);
    this.firstEntry = ENDPOINT;
    this.lastEntry = ENDPOINT;
  }

  @Override
  int allocArrays() {
    int expectedSize = super.allocArrays();
    this.links = new long[expectedSize];
    return expectedSize;
  }

  @Override
  Map<K, V> createHashFloodingResistantDelegate(int tableSize) {
    return new LinkedHashMap<K, V>(tableSize, 1.0f, accessOrder);
  }

  @Override
  @CanIgnoreReturnValue
  Map<K, V> convertToHashFloodingResistantImplementation() {
    Map<K, V> result = super.convertToHashFloodingResistantImplementation();
    links = null;
    return result;
  }

  private int getPredecessor(int entry) {
    return ((int) (links[entry] >>> 32)) - 1;
  }

  @Override
  int getSuccessor(int entry) {
    return ((int) links[entry]) - 1;
  }

  private void setSuccessor(int entry, int succ) {
    long succMask = (~0L) >>> 32;
    links[entry] = (links[entry] & ~succMask) | ((succ + 1) & succMask);
  }

  private void setPredecessor(int entry, int pred) {
    long predMask = ~0L << 32;
    links[entry] = (links[entry] & ~predMask) | ((long) (pred + 1) << 32);
  }

  private void setSucceeds(int pred, int succ) {
    if (pred == ENDPOINT) {
      firstEntry = succ;
    } else {
      setSuccessor(pred, succ);
    }

    if (succ == ENDPOINT) {
      lastEntry = pred;
    } else {
      setPredecessor(succ, pred);
    }
  }

  @Override
  void insertEntry(int entryIndex, @NullableDecl K key, @NullableDecl V value, int hash, int mask) {
    super.insertEntry(entryIndex, key, value, hash, mask);
    setSucceeds(lastEntry, entryIndex);
    setSucceeds(entryIndex, ENDPOINT);
  }

  @Override
  void accessEntry(int index) {
    if (accessOrder) {
      // delete from previous position...
      setSucceeds(getPredecessor(index), getSuccessor(index));
      // ...and insert at the end.
      setSucceeds(lastEntry, index);
      setSucceeds(index, ENDPOINT);
      incrementModCount();
    }
  }

  @Override
  void moveLastEntry(int dstIndex, int mask) {
    int srcIndex = size() - 1;
    super.moveLastEntry(dstIndex, mask);

    setSucceeds(getPredecessor(dstIndex), getSuccessor(dstIndex));
    if (dstIndex < srcIndex) {
      setSucceeds(getPredecessor(srcIndex), dstIndex);
      setSucceeds(dstIndex, getSuccessor(srcIndex));
    }
    links[srcIndex] = 0;
  }

  @Override
  void resizeEntries(int newCapacity) {
    super.resizeEntries(newCapacity);
    links = Arrays.copyOf(links, newCapacity);
  }

  @Override
  int firstEntryIndex() {
    return firstEntry;
  }

  @Override
  int adjustAfterRemove(int indexBeforeRemove, int indexRemoved) {
    return (indexBeforeRemove >= size()) ? indexRemoved : indexBeforeRemove;
  }

  @Override
  public void clear() {
    if (needsAllocArrays()) {
      return;
    }
    this.firstEntry = ENDPOINT;
    this.lastEntry = ENDPOINT;
    if (links != null) {
      Arrays.fill(links, 0, size(), 0);
    }
    super.clear();
  }
}