/*
    * Copyright (C) 2007 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.checkArgument;
  import static com.google.common.base.Preconditions.checkNotNull;
  import static com.google.common.collect.CollectPreconditions.checkNonnegative;
  import static com.google.common.collect.CollectPreconditions.checkRemove;
  
  import com.google.common.annotations.Beta;
  import com.google.common.annotations.VisibleForTesting;
  import com.google.common.collect.Serialization.FieldSetter;
  import com.google.common.math.IntMath;
  import com.google.common.primitives.Ints;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  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;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import javax.annotation.Nullable;
  
  /**
   * A multiset that supports concurrent modifications and that provides atomic versions of most
   * {@code Multiset} operations (exceptions where noted). Null elements are not supported.
   *
   * <p>See the Guava User Guide article on <a href=
   * "http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Multiset">
   * {@code Multiset}</a>.
   *
   * @author Cliff L. Biffle
   * @author mike nonemacher
   * @since 2.0 (imported from Google Collections Library)
   */
  public final class ConcurrentHashMultiset<E> extends AbstractMultiset<E> implements Serializable {
  
    /*
     * The ConcurrentHashMultiset's atomic operations are implemented primarily in terms of
     * AtomicInteger's atomic operations, with some help from ConcurrentMap's atomic operations on
     * creation and removal (including automatic removal of zeroes). If the modification of an
     * AtomicInteger results in zero, we compareAndSet the value to zero; if that succeeds, we remove
     * the entry from the Map. If another operation sees a zero in the map, it knows that the entry is
     * about to be removed, so this operation may remove it (often by replacing it with a new
     * AtomicInteger).
     */
  
    /** The number of occurrences of each element. */
    private final transient ConcurrentMap<E, AtomicInteger> countMap;
  
    // This constant allows the deserialization code to set a final field. This holder class
    // makes sure it is not initialized unless an instance is deserialized.
    private static class FieldSettersHolder {
      static final FieldSetter<ConcurrentHashMultiset> COUNT_MAP_FIELD_SETTER =
          Serialization.getFieldSetter(ConcurrentHashMultiset.class, "countMap");
    }
  
    /**
     * Creates a new, empty {@code ConcurrentHashMultiset} using the default
     * initial capacity, load factor, and concurrency settings.
     */
    public static <E> ConcurrentHashMultiset<E> create() {
      // TODO(schmoe): provide a way to use this class with other (possibly arbitrary)
      // ConcurrentMap implementors. One possibility is to extract most of this class into
      // an AbstractConcurrentMapMultiset.
      return new ConcurrentHashMultiset<E>(new ConcurrentHashMap<E, AtomicInteger>());
    }
  
    /**
     * Creates a new {@code ConcurrentHashMultiset} containing the specified elements, using
     * the default initial capacity, load factor, and concurrency settings.
     *
     * <p>This implementation is highly efficient when {@code elements} is itself a {@link Multiset}.
     *
     * @param elements the elements that the multiset should contain
     */
    public static <E> ConcurrentHashMultiset<E> create(Iterable<? extends E> elements) {
      ConcurrentHashMultiset<E> multiset = ConcurrentHashMultiset.create();
     Iterables.addAll(multiset, elements);
     return multiset;
   }
 
   /**
    * Creates a new, empty {@code ConcurrentHashMultiset} using {@code mapMaker}
    * to construct the internal backing map.
    *
    * <p>If this {@link MapMaker} is configured to use entry eviction of any kind, this eviction
    * applies to all occurrences of a given element as a single unit. However, most updates to the
    * multiset do not count as map updates at all, since we're usually just mutating the value
    * stored in the map, so {@link MapMaker#expireAfterAccess} makes sense (evict the entry that
    * was queried or updated longest ago), but {@link MapMaker#expireAfterWrite} doesn't, because
    * the eviction time is measured from when we saw the first occurrence of the object.
    *
    * <p>The returned multiset is serializable but any serialization caveats
    * given in {@code MapMaker} apply.
    *
    * <p>Finally, soft/weak values can be used but are not very useful: the values are created
    * internally and not exposed externally, so no one else will have a strong reference to the
    * values. Weak keys on the other hand can be useful in some scenarios.
    *
    * @since 15.0 (source compatible (accepting the since removed {@code GenericMapMaker} class)
    *     since 7.0)
    */
   @Beta
   public static <E> ConcurrentHashMultiset<E> create(MapMaker mapMaker) {
     return new ConcurrentHashMultiset<E>(mapMaker.<E, AtomicInteger>makeMap());
   }
 
   /**
    * 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. It must be empty.
    * @throws IllegalArgumentException if {@code countMap} is not empty
    */
   @VisibleForTesting ConcurrentHashMultiset(ConcurrentMap<E, AtomicInteger> 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) {
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     return (existingCounter == null) ? 0 : existingCounter.get();
   }
 
   /**
    * {@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 (AtomicInteger value : countMap.values()) {
       sum += value.get();
     }
     return Ints.saturatedCast(sum);
   }
 
   /*
    * 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.
    *
    * @param element the element to add
    * @param occurrences the number of occurrences to add
    * @return the previous count of the element before the operation; possibly zero
    * @throws IllegalArgumentException if {@code occurrences} is negative, or if
    *     the resulting amount would exceed {@link Integer#MAX_VALUE}
    */
   @Override public int add(E element, int occurrences) {
     checkNotNull(element);
     if (occurrences == 0) {
       return count(element);
     }
     checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
 
     while (true) {
       AtomicInteger existingCounter = Maps.safeGet(countMap, element);
       if (existingCounter == null) {
         existingCounter = countMap.putIfAbsent(element, new AtomicInteger(occurrences));
         if (existingCounter == null) {
           return 0;
         }
         // existingCounter != null: fall through to operate against the existing AtomicInteger
       }
 
       while (true) {
         int oldValue = existingCounter.get();
         if (oldValue != 0) {
           try {
             int newValue = IntMath.checkedAdd(oldValue, occurrences);
             if (existingCounter.compareAndSet(oldValue, newValue)) {
               // newValue can't == 0, so no need to check & remove
               return oldValue;
             }
           } catch (ArithmeticException overflow) {
             throw new IllegalArgumentException("Overflow adding " + occurrences
                 + " occurrences to a count of " + oldValue);
           }
         } else {
           // In the case of a concurrent remove, we might observe a zero value, which means another
           // thread is about to remove (element, existingCounter) from the map. Rather than wait,
           // we can just do that work here.
           AtomicInteger newCounter = new AtomicInteger(occurrences);
           if ((countMap.putIfAbsent(element, newCounter) == null)
               || countMap.replace(element, existingCounter, newCounter)) {
             return 0;
           }
           break;
         }
       }
 
       // 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 the element to remove
    * @return the count of the element before the operation; possibly zero
    * @throws IllegalArgumentException if {@code occurrences} is negative
    */
   /*
    * TODO(cpovirk): remove and removeExactly currently accept null inputs only
    * if occurrences == 0. This satisfies both NullPointerTester and
    * CollectionRemoveTester.testRemove_nullAllowed, but it's not clear that it's
    * a good policy, especially because, in order for the test to pass, the
    * parameter must be misleadingly annotated as @Nullable. I suspect that
    * we'll want to remove @Nullable, add an eager checkNotNull, and loosen up
    * testRemove_nullAllowed.
    */
   @Override public int remove(@Nullable Object element, int occurrences) {
     if (occurrences == 0) {
       return count(element);
     }
     checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);
 
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     if (existingCounter == null) {
       return 0;
     }
     while (true) {
       int oldValue = existingCounter.get();
       if (oldValue != 0) {
         int newValue = Math.max(0, oldValue - occurrences);
         if (existingCounter.compareAndSet(oldValue, newValue)) {
           if (newValue == 0) {
             // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
             // another thread has already replaced it with a new counter, which is fine.
             countMap.remove(element, existingCounter);
           }
           return oldValue;
         }
       } else {
         return 0;
       }
     }
   }
 
   /**
    * 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);
 
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     if (existingCounter == null) {
       return false;
     }
     while (true) {
       int oldValue = existingCounter.get();
       if (oldValue < occurrences) {
         return false;
       }
       int newValue = oldValue - occurrences;
       if (existingCounter.compareAndSet(oldValue, newValue)) {
         if (newValue == 0) {
           // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
           // another thread has already replaced it with a new counter, which is fine.
           countMap.remove(element, existingCounter);
         }
         return true;
       }
     }
   }
 
   /**
    * 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
    */
   @Override public int setCount(E element, int count) {
     checkNotNull(element);
     checkNonnegative(count, "count");
     while (true) {
       AtomicInteger existingCounter = Maps.safeGet(countMap, element);
       if (existingCounter == null) {
         if (count == 0) {
           return 0;
         } else {
           existingCounter = countMap.putIfAbsent(element, new AtomicInteger(count));
           if (existingCounter == null) {
             return 0;
           }
           // existingCounter != null: fall through
         }
       }
 
       while (true) {
         int oldValue = existingCounter.get();
         if (oldValue == 0) {
           if (count == 0) {
             return 0;
           } else {
             AtomicInteger newCounter = new AtomicInteger(count);
             if ((countMap.putIfAbsent(element, newCounter) == null)
                 || countMap.replace(element, existingCounter, newCounter)) {
               return 0;
             }
           }
           break;
         } else {
           if (existingCounter.compareAndSet(oldValue, count)) {
             if (count == 0) {
               // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
               // another thread has already replaced it with a new counter, which is fine.
               countMap.remove(element, existingCounter);
             }
             return oldValue;
           }
         }
       }
     }
   }
 
   /**
    * Sets the number of occurrences of {@code element} to {@code newCount}, but only if
    * the count is currently {@code expectedOldCount}. If {@code element} does not appear
    * in the multiset exactly {@code expectedOldCount} 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 expectedOldCount == newCount}, the method will return {@code true} if
    *     the condition was met.
    * @throws IllegalArgumentException if {@code expectedOldCount} or {@code newCount} is negative
    */
   @Override public boolean setCount(E element, int expectedOldCount, int newCount) {
     checkNotNull(element);
     checkNonnegative(expectedOldCount, "oldCount");
     checkNonnegative(newCount, "newCount");
 
     AtomicInteger existingCounter = Maps.safeGet(countMap, element);
     if (existingCounter == null) {
       if (expectedOldCount != 0) {
         return false;
       } else if (newCount == 0) {
         return true;
       } else {
         // if our write lost the race, it must have lost to a nonzero value, so we can stop
         return countMap.putIfAbsent(element, new AtomicInteger(newCount)) == null;
       }
     }
     int oldValue = existingCounter.get();
     if (oldValue == expectedOldCount) {
       if (oldValue == 0) {
         if (newCount == 0) {
           // Just observed a 0; try to remove the entry to clean up the map
           countMap.remove(element, existingCounter);
           return true;
         } else {
           AtomicInteger newCounter = new AtomicInteger(newCount);
           return (countMap.putIfAbsent(element, newCounter) == null)
               || countMap.replace(element, existingCounter, newCounter);
         }
       } else {
         if (existingCounter.compareAndSet(oldValue, newCount)) {
           if (newCount == 0) {
             // Just CASed to 0; remove the entry to clean up the map. If the removal fails,
             // another thread has already replaced it with a new counter, which is fine.
             countMap.remove(element, existingCounter);
           }
           return true;
         }
       }
     }
     return false;
   }
 
   // Views
 
   @Override Set<E> createElementSet() {
     final Set<E> delegate = countMap.keySet();
     return new ForwardingSet<E>() {
       @Override protected Set<E> delegate() {
         return delegate;
       }
 
       @Override
       public boolean contains(@Nullable Object object) {
         return object != null && Collections2.safeContains(delegate, object);
       }
 
       @Override
       public boolean containsAll(Collection<?> collection) {
         return standardContainsAll(collection);
       }
 
       @Override public boolean remove(Object object) {
         return object != null && Collections2.safeRemove(delegate, object);
       }
 
       @Override public boolean removeAll(Collection<?> c) {
         return standardRemoveAll(c);
       }
     };
   }
 
   @Override public Set<Multiset.Entry<E>> createEntrySet() {
     return new EntrySet();
   }
 
   @Override int distinctElements() {
     return countMap.size();
   }
 
   @Override public boolean isEmpty() {
     return countMap.isEmpty();
   }
 
   @Override Iterator<Entry<E>> entryIterator() {
     // AbstractIterator makes this fairly clean, but it doesn't support remove(). To support
     // remove(), we create an AbstractIterator, and then use ForwardingIterator to delegate to it.
     final Iterator<Entry<E>> readOnlyIterator =
         new AbstractIterator<Entry<E>>() {
           private Iterator<Map.Entry<E, AtomicInteger>> mapEntries = countMap.entrySet().iterator();
 
           @Override protected Entry<E> computeNext() {
             while (true) {
               if (!mapEntries.hasNext()) {
                 return endOfData();
               }
               Map.Entry<E, AtomicInteger> mapEntry = mapEntries.next();
               int count = mapEntry.getValue().get();
               if (count != 0) {
                 return Multisets.immutableEntry(mapEntry.getKey(), count);
               }
             }
           }
         };
 
     return new ForwardingIterator<Entry<E>>() {
       private Entry<E> last;
 
       @Override protected Iterator<Entry<E>> delegate() {
         return readOnlyIterator;
       }
 
       @Override public Entry<E> next() {
         last = super.next();
         return last;
       }
 
       @Override public void remove() {
         checkRemove(last != null);
         ConcurrentHashMultiset.this.setCount(last.getElement(), 0);
         last = null;
       }
     };
   }
 
   @Override public void clear() {
     countMap.clear();
   }
 
   private class EntrySet extends AbstractMultiset<E>.EntrySet {
     @Override ConcurrentHashMultiset<E> multiset() {
       return ConcurrentHashMultiset.this;
     }
 
     /*
      * 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);
     }
 
     private List<Multiset.Entry<E>> snapshot() {
       List<Multiset.Entry<E>> list = Lists.newArrayListWithExpectedSize(size());
       // Not Iterables.addAll(list, this), because that'll forward right back here.
       Iterators.addAll(list, iterator());
       return list;
     }
   }
 
   /**
    * @serialData the ConcurrentMap of elements and their counts.
    */
   private void writeObject(ObjectOutputStream stream) throws IOException {
     stream.defaultWriteObject();
     stream.writeObject(countMap);
   }
 
   private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
     stream.defaultReadObject();
     @SuppressWarnings("unchecked") // reading data stored by writeObject
     ConcurrentMap<E, Integer> deserializedCountMap =
         (ConcurrentMap<E, Integer>) stream.readObject();
     FieldSettersHolder.COUNT_MAP_FIELD_SETTER.set(this, deserializedCountMap);
   }
 
   private static final long serialVersionUID = 1;
 }