/*
 * Copyright (C) 2011 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 static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Objects;
import com.google.common.collect.Multisets.ImmutableEntry;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.concurrent.LazyInit;
import java.util.Arrays;
import java.util.Collection;
import org.checkerframework.checker.nullness.qual.Nullable;

Implementation of ImmutableMultiset with zero or more elements. 
Author:
Jared Levy, Louis Wasserman/**
 * Implementation of {@link ImmutableMultiset} with zero or more elements.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 */
@GwtCompatible(emulated = true, serializable = true)
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
class RegularImmutableMultiset<E> extends ImmutableMultiset<E> {
  static final ImmutableMultiset<Object> EMPTY = create(ImmutableList.<Entry<Object>>of());

  static <E> ImmutableMultiset<E> create(Collection<? extends Entry<? extends E>> entries) {
    int distinct = entries.size();
    @SuppressWarnings("unchecked")
    Multisets.ImmutableEntry<E>[] entryArray = new Multisets.ImmutableEntry[distinct];
    if (distinct == 0) {
      return new RegularImmutableMultiset<>(entryArray, null, 0, 0, ImmutableSet.of());
    }
    int tableSize = Hashing.closedTableSize(distinct, MAX_LOAD_FACTOR);
    int mask = tableSize - 1;
    @SuppressWarnings("unchecked")
    Multisets.ImmutableEntry<E>[] hashTable = new Multisets.ImmutableEntry[tableSize];

    int index = 0;
    int hashCode = 0;
    long size = 0;
    for (Entry<? extends E> entry : entries) {
      E element = checkNotNull(entry.getElement());
      int count = entry.getCount();
      int hash = element.hashCode();
      int bucket = Hashing.smear(hash) & mask;
      Multisets.ImmutableEntry<E> bucketHead = hashTable[bucket];
      Multisets.ImmutableEntry<E> newEntry;
      if (bucketHead == null) {
        boolean canReuseEntry =
            entry instanceof Multisets.ImmutableEntry && !(entry instanceof NonTerminalEntry);
        newEntry =
            canReuseEntry
                ? (Multisets.ImmutableEntry<E>) entry
                : new Multisets.ImmutableEntry<E>(element, count);
      } else {
        newEntry = new NonTerminalEntry<E>(element, count, bucketHead);
      }
      hashCode += hash ^ count;
      entryArray[index++] = newEntry;
      hashTable[bucket] = newEntry;
      size += count;
    }

    return hashFloodingDetected(hashTable)
        ? JdkBackedImmutableMultiset.create(ImmutableList.asImmutableList(entryArray))
        : new RegularImmutableMultiset<E>(
            entryArray, hashTable, Ints.saturatedCast(size), hashCode, null);
  }

  private static boolean hashFloodingDetected(Multisets.ImmutableEntry<?>[] hashTable) {
    for (int i = 0; i < hashTable.length; i++) {
      int bucketLength = 0;
      for (Multisets.ImmutableEntry<?> entry = hashTable[i];
          entry != null;
          entry = entry.nextInBucket()) {
        bucketLength++;
        if (bucketLength > MAX_HASH_BUCKET_LENGTH) {
          return true;
        }
      }
    }
    return false;
  }

  
Closed addressing tends to perform well even with high load factors. Being conservative here
ensures that the table is still likely to be relatively sparse (hence it misses fast) while
saving space.
/**
   * Closed addressing tends to perform well even with high load factors. Being conservative here
   * ensures that the table is still likely to be relatively sparse (hence it misses fast) while
   * saving space.
   */
  @VisibleForTesting static final double MAX_LOAD_FACTOR = 1.0;

  
Maximum allowed false positive probability of detecting a hash flooding attack given random
input.
/**
   * Maximum allowed false positive probability of detecting a hash flooding attack given random
   * input.
   */
  @VisibleForTesting static final double HASH_FLOODING_FPP = 0.001;

  
Maximum allowed length of a hash table bucket before falling back to a j.u.HashMap based
implementation. Experimentally determined.
/**
   * Maximum allowed length of a hash table bucket before falling back to a j.u.HashMap based
   * implementation. Experimentally determined.
   */
  @VisibleForTesting static final int MAX_HASH_BUCKET_LENGTH = 9;

  private final transient Multisets.ImmutableEntry<E>[] entries;
  private final transient Multisets.ImmutableEntry<E> @Nullable [] hashTable;
  private final transient int size;
  private final transient int hashCode;

  @LazyInit private transient ImmutableSet<E> elementSet;

  private RegularImmutableMultiset(
      ImmutableEntry<E>[] entries,
      ImmutableEntry<E>[] hashTable,
      int size,
      int hashCode,
      ImmutableSet<E> elementSet) {
    this.entries = entries;
    this.hashTable = hashTable;
    this.size = size;
    this.hashCode = hashCode;
    this.elementSet = elementSet;
  }

  private static final class NonTerminalEntry<E> extends Multisets.ImmutableEntry<E> {
    private final Multisets.ImmutableEntry<E> nextInBucket;

    NonTerminalEntry(E element, int count, ImmutableEntry<E> nextInBucket) {
      super(element, count);
      this.nextInBucket = nextInBucket;
    }

    @Override
    public ImmutableEntry<E> nextInBucket() {
      return nextInBucket;
    }
  }

  @Override
  boolean isPartialView() {
    return false;
  }

  @Override
  public int count(@Nullable Object element) {
    Multisets.ImmutableEntry<E>[] hashTable = this.hashTable;
    if (element == null || hashTable == null) {
      return 0;
    }
    int hash = Hashing.smearedHash(element);
    int mask = hashTable.length - 1;
    for (Multisets.ImmutableEntry<E> entry = hashTable[hash & mask];
        entry != null;
        entry = entry.nextInBucket()) {
      if (Objects.equal(element, entry.getElement())) {
        return entry.getCount();
      }
    }
    return 0;
  }

  @Override
  public int size() {
    return size;
  }

  @Override
  public ImmutableSet<E> elementSet() {
    ImmutableSet<E> result = elementSet;
    return (result == null) ? elementSet = new ElementSet<E>(Arrays.asList(entries), this) : result;
  }

  @Override
  Entry<E> getEntry(int index) {
    return entries[index];
  }

  @Override
  public int hashCode() {
    return hashCode;
  }
}