/*
 * Copyright (C) 2007 Google Inc.
 *
 * 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.base.Nullable;
import static com.google.common.base.Preconditions.checkArgument;

import java.io.Serializable;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * A multiset that supports concurrent modifications and that provides atomic
 * versions of most {@code Multiset} operations (exceptions where noted).
 * 
 * <p>This implementation is backed by a {@link ConcurrentMap}, and several of
 * its properties will depend on the behavior of this backing map: for example,
 * it can contain only values that are legal keys for the backing map. Thus,
 * when using the default {@link ConcurrentHashMap}, this implementation cannot
 * contain {@code null}.
 * 
 * <p>An instance of this class is serializable, as long as all of its elements
 * are serializable and the instance wasn't created by passing a
 * non-serializable map to {@link #ConcurrentMultiset(ConcurrentMap)}.  
 *
 * @author Cliff L. Biffle
 */
public final class ConcurrentMultiset<E> extends AbstractMultiset<E>
    implements Serializable {
  /*
   * The ConcurrentMultiset's atomic operations are implemented in terms of
   * ConcurrentMap's atomic operations. Many of them, such as add(E, int), are
   * read-modify-write sequences, and so are implemented as loops that wrap
   * ConcurrentMap's compare-and-set operations (like putIfAbsent).
   */
  
  /** The number of occurrences of each element. */
  private final ConcurrentMap<E, Integer> countMap;

  /**
   * Creates an empty instance using a {@link ConcurrentHashMap} to store
   * elements and their counts.
   * 
   * <p>The {@code ConcurrentHashMap} will have default capacity, load factor,
   * and concurrency settings. For more control, use
   * {@link #ConcurrentMultiset(ConcurrentMap)}.
   */
  public ConcurrentMultiset() {
    this(new ConcurrentHashMap<E, Integer>());
  }

  /**
   * Creates an instance using a {@link ConcurrentHashMap} to store elements
   * and their counts, and initially containing all the elements from a given
   * collection.
   *
   * <p>The {@code ConcurrentHashMap} will have default capacity, load factor,
   * and concurrency settings. For more control, use
   * {@link #ConcurrentMultiset(ConcurrentMap)}.
   */
  public ConcurrentMultiset(Collection<? extends E> collection) {
    this(new ConcurrentHashMap<E, Integer>());
    addAll(collection);
  }

  /**
   * Creates an instance using {@code countMap} to store elements and their
   * counts.
   * 
   * <p>This instance will assume ownership of {@code countMap}, and other code
   * should not maintain references to the map or modify it in any way.
   * 
   * @param countMap backing map for storing the elements in the multiset and
   *     their counts. Must be empty.
   * @throws IllegalArgumentException if {@code countMap} is not empty
   */
  public ConcurrentMultiset(ConcurrentMap<E, Integer> countMap) {
    checkArgument(countMap.isEmpty());
    this.countMap = countMap;
  }

  // Query Operations

  /**
   * Returns the number of occurrences of {@code element} in this multiset.
   *
   * @param element the element to look for
   * @return the nonnegative number of occurrences of the element
   */
  @Override public int count(@Nullable Object element) {
    return unbox(countMap.get(element));
  }

  /*
   * Note: the superclass toArray() methods assume that size() gives a correct
   * answer, which ours does not.
   */

  @Override public Object[] toArray() {
    return snapshot().toArray();
  }

  @Override public <T> T[] toArray(T[] array) {
    return snapshot().toArray(array);
  }

  /*
   * We'd love to use 'new ArrayList(this)' or 'list.addAll(this)', but
   * either of these would recurse back to us again!
   */
  private List<E> snapshot() {
    List<E> list = Lists.newArrayListWithExpectedSize(size());
    for (Multiset.Entry<E> entry : entrySet()) {
      E element = entry.getElement();
      for (int i = entry.getCount(); i > 0; i--) {
        list.add(element);
      }
    }
    return list;
  }

  // Modification Operations

  /**
   * Adds a number of occurrences of the specified element to this multiset.
   *
   * <p>To succeed, {@code element} must be a legal key in the backing map.  In
   * the case of the default {@link ConcurrentHashMap}, this means it must not
   * be {@code null}, but other {@code Map} implementations may allow {@code
   * null}.
   *
   * @param element the element to add
   * @param occurrences the number of occurrences to add
   * @return {@code true} if the collection changed as a result (this should
   *     always be the case unless {@code occurrences} is zero)
   * @throws IllegalArgumentException if {@code occurrences} is negative, or if
   *     the resulting amount would exceed {@link Integer#MAX_VALUE}
   */
  @Override public boolean add(E element, int occurrences) {
    if (occurrences == 0) {
      return false;
    }
    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);

    while (true) {
      int current = count(element);
      if (current == 0) {
        if (countMap.putIfAbsent(element, occurrences) == null) {
          return true;
        }
      } else {
        checkArgument(occurrences <= Integer.MAX_VALUE - current,
            "Overflow adding %s occurrences to a count of %s",
            occurrences, current);
        int next = current + occurrences;
        if (countMap.replace(element, current, next)) {
          return true;
        }
      }
      // If we're still here, there was a race, so just try again.
    }
  }

  /**
   * Removes a number of occurrences of the specified element from this
   * multiset. If the multiset contains fewer than this number of occurrences to
   * begin with, all occurrences will be removed.
   *
   * @param element the element whose occurrences should be removed
   * @param occurrences the number of occurrences of this element to remove
   * @return the number of occurrences that were successfully removed (zero if
   *     the element was not present)
   * @throws IllegalArgumentException if {@code occurrences} is negative
   */
  @Override public int remove(@Nullable Object element, int occurrences) {
    if (occurrences == 0) {
      return 0;
    }
    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);

    while (true) {
      int current = count(element);
      if (current == 0) {
        return 0;
      }
      if (occurrences >= current) {
        if (countMap.remove(element, current)) {
          return current;
        }
      } else {
        // We know it's an "E" because it already exists in the map.
        @SuppressWarnings("unchecked")
        E casted = (E) element;

        if (countMap.replace(casted, current, current - occurrences)) {
          return occurrences;
        }
      }
      // If we're still here, there was a race, so just try again.
    }
  }

  /**
   * Removes <b>all</b> occurrences of the specified element from this multiset.
   * This method complements {@link Multiset#remove(Object)}, which removes only
   * one occurrence at a time.
   *
   * @param element the element whose occurrences should all be removed
   * @return the number of occurrences successfully removed, possibly zero
   */
  @Override public int removeAllOccurrences(@Nullable Object element) {
    return unbox(countMap.remove(element));
  }

  // TODO: move this above isEmpty() where it belongs
  /**
   * {@inheritDoc}
   *
   * <p>If the data in the multiset is modified by any other threads during this
   * method, it is undefined which (if any) of these modifications will be
   * reflected in the result.
   */
  @Override public int size() {
    long sum = 0L;
    for (Integer value : countMap.values()) {
      sum += value;
    }
    return (int) Math.min(sum, Integer.MAX_VALUE);
  }


  /**
   * Adds or removes occurrences of {@code element} such that the {@link #count}
   * of the element becomes {@code count}.
   *
   * @return the count of {@code element} in the multiset before this call
   * @throws IllegalArgumentException if {@code count} is negative
   */
  public int setCount(E element, int count) {
    checkArgument(count >= 0, "Invalid count: %s", count);
    return (count == 0)
        ? removeAllOccurrences(element)
        : unbox(countMap.put(element, count));
  }

  /**
   * Sets the number of occurrences of {@code element} to {@code newCount}, but
   * only if the count is currently {@code oldCount}. If {@code element} does
   * not appear in the multiset exactly {@code oldCount} times, no changes will
   * be made.
   * 
   * @return {@code true} if the change was successful. This usually indicates
   *         that the multiset has been modified, but not always: in the case
   *         that {@code oldCount == newCount}, the method will return
   *         {@code true} if the condition was met.
   * @throws IllegalArgumentException if {@code oldCount} or {@code newCount} is
   *         negative
   */
  public boolean setCount(E element, int oldCount, int newCount) {
    checkArgument(oldCount >= 0, "Invalid oldCount: %s", oldCount);
    checkArgument(newCount >= 0, "Invalid newCount: %s", newCount);
    if (newCount == 0) {
      if (oldCount == 0) {
        // No change to make, but must return true if the element is not present
        return !countMap.containsKey(element);
      } else {
        return countMap.remove(element, box(oldCount));
      }
    }
    if (oldCount == 0) {
      return countMap.putIfAbsent(element, newCount) == null;
    }
    return countMap.replace(element, oldCount, newCount);
  }

  // TODO: move this above removeAllOccurrences() where it belongs
  /**
   * Removes exactly the specified number of occurrences of {@code element}, or
   * makes no change if this is not possible.
   *
   * <p>This method, in contrast to {@link #remove(Object, int)}, has no effect
   * when the element count is smaller than {@code occurrences}.
   *
   * @param element the element to remove
   * @param occurrences the number of occurrences of {@code element} to remove
   * @return {@code true} if the removal was possible (including if {@code
   *     occurrences} is zero)
   */
  public boolean removeExactly(@Nullable Object element, int occurrences) {
    if (occurrences == 0) {
      return true;
    }
    checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);

    while (true) {
      int current = count(element);
      if (occurrences > current) {
        return false;
      }
      if (occurrences == current) {
        if (countMap.remove(element, occurrences)) {
          return true;
        }
      } else {
        @SuppressWarnings("unchecked") // it's in the map, must be an "E"
        E casted = (E) element;
        if (countMap.replace(casted, current, current - occurrences)) {
          return true;
        }
      }
      // If we're still here, there was a race, so just try again.
    }
  }

  // Views

  @Override public Set<E> elementSet() {
    return countMap.keySet();
  }

  private transient EntrySet entrySet;

  @Override public Set<Multiset.Entry<E>> entrySet() {
    EntrySet es = entrySet;
    return (es == null) ? (entrySet = new EntrySet()) : es;
  }
  
  private class EntrySet extends AbstractSet<Multiset.Entry<E>> {
    @Override public int size() {
      return countMap.size();
    }

    @Override public boolean isEmpty() {
      return countMap.isEmpty();
    }

    @Override public boolean contains(Object object) {
      if (object instanceof Multiset.Entry<?>) {
        Multiset.Entry<?> entry = (Multiset.Entry<?>) object;
        Object element = entry.getElement();
        int entryCount = entry.getCount();
        return entryCount > 0 && count(element) == entryCount;
      }
      return false;
    }

    @Override public Iterator<Multiset.Entry<E>> iterator() {
      final Iterator<Map.Entry<E, Integer>> backingIterator
          = countMap.entrySet().iterator();
      return new Iterator<Multiset.Entry<E>>() {
        public boolean hasNext() {
          return backingIterator.hasNext();
        }

        public Multiset.Entry<E> next() {
          Map.Entry<E, Integer> backingEntry = backingIterator.next();
          return Multisets.immutableEntry(
              backingEntry.getKey(), backingEntry.getValue());
        }

        public void remove() {
          backingIterator.remove();
        }
      };
    }

    /*
     * Note: the superclass toArray() methods assume that size() gives a correct
     * answer, which ours does not.
     */

    @Override public Object[] toArray() {
      return snapshot().toArray();
    }

    @Override public <T> T[] toArray(T[] array) {
      return snapshot().toArray(array);
    }

    /*
     * We'd love to use 'new ArrayList(this)' or 'list.addAll(this)', but
     * either of these would recurse back to us again!
     */
    private List<Multiset.Entry<E>> snapshot() {
      List<Multiset.Entry<E>> list = Lists.newArrayListWithExpectedSize(size());
      for (Multiset.Entry<E> entry : this) {
        list.add(entry);
      }
      return list;
    }

    @Override public boolean remove(Object object) {
      if (object instanceof Multiset.Entry<?>) {
        Multiset.Entry<?> entry = (Multiset.Entry<?>) object;
        Object element = entry.getElement();
        return countMap.remove(element, box(entry.getCount()));
      }
      return false;
    }

    @Override public void clear() {
      countMap.clear();
    }
  }

  /*
   * We use a special form of boxing and unboxing that treats null as zero
   * and zero as null.
   */

  private static Integer box(int i) {
    return (i == 0) ? null : i;
    }

  private static int unbox(Integer i) {
    return (i == null) ? 0 : i;
  }

  /*
   * This class can't use MultisetSerializedForm for serialization because it
   * can't generate the arbitrary empty ConcurrentMap passed to the
   * ConcurrentMultiset(ConcurrentMap) constructor. 
   */
  private static final long serialVersionUID = 0L;
}