/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You 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
    *
    *      https://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 org.apache.commons.collections4.map;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  import java.util.AbstractCollection;
  import java.util.AbstractSet;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Objects;
  import java.util.Set;
  
  import org.apache.commons.collections4.CollectionUtils;
  import org.apache.commons.collections4.IterableMap;
  import org.apache.commons.collections4.MapIterator;
  import org.apache.commons.collections4.ResettableIterator;
  import org.apache.commons.collections4.iterators.EmptyIterator;
  import org.apache.commons.collections4.iterators.EmptyMapIterator;
  
  /**
   * A {@code Map} implementation that stores data in simple fields until
   * the size is greater than 3.
   * <p>
   * This map is designed for performance and can outstrip HashMap.
   * It also has good garbage collection characteristics.
   * </p>
   * <ul>
   * <li>Optimized for operation at size 3 or less.</li>
   * <li>Still works well once size 3 exceeded.</li>
   * <li>Gets at size 3 or less are about 0-10% faster than HashMap,</li>
   * <li>Puts at size 3 or less are over 4 times faster than HashMap.</li>
   * <li>Performance 5% slower than HashMap once size 3 exceeded once.</li>
   * </ul>
   * <p>
   * The design uses two distinct modes of operation - flat and delegate.
   * While the map is size 3 or less, operations map straight onto fields using
   * switch statements. Once size 4 is reached, the map switches to delegate mode
   * and only switches back when cleared. In delegate mode, all operations are
   * forwarded straight to a HashMap resulting in the 5% performance loss.
   * </p>
   * <p>
   * The performance gains on puts are due to not needing to create a Map Entry
   * object. This is a large saving not only in performance but in garbage collection.
   * </p>
   * <p>
   * Whilst in flat mode this map is also easy for the garbage collector to dispatch.
   * This is because it contains no complex objects or arrays which slow the progress.
   * </p>
   * <p>
   * Do not use {@code Flat3Map} if the size is likely to grow beyond 3.
   * </p>
   * <p>
   * <strong>Note that Flat3Map is not synchronized and is not thread-safe.</strong>
   * If you wish to use this map from multiple threads concurrently, you must use
   * appropriate synchronization. The simplest approach is to wrap this map
   * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
   * exceptions when accessed by concurrent threads without synchronization.
   * </p>
   *
   * @param <K> The type of the keys in this map
   * @param <V> The type of the values in this map
   * @since 3.0
   */
  public class Flat3Map<K, V> implements IterableMap<K, V>, Serializable, Cloneable {
  
      abstract static class EntryIterator<K, V> {
          private final Flat3Map<K, V> parent;
          private int nextIndex;
          private FlatMapEntry<K, V> currentEntry;
  
          /**
           * Create a new Flat3Map.EntryIterator.
           */
          EntryIterator(final Flat3Map<K, V> parent) {
              this.parent = parent;
          }
  
          public boolean hasNext() {
              return nextIndex < parent.size;
          }
 
         public Map.Entry<K, V> nextEntry() {
             if (!hasNext()) {
                 throw new NoSuchElementException(AbstractHashedMap.NO_NEXT_ENTRY);
             }
             currentEntry = new FlatMapEntry<>(parent, ++nextIndex);
             return currentEntry;
         }
 
         public void remove() {
             if (currentEntry == null) {
                 throw new IllegalStateException(AbstractHashedMap.REMOVE_INVALID);
             }
             parent.remove(currentEntry.getKey());
             currentEntry.setRemoved(true);
             nextIndex--;
             currentEntry = null;
         }
 
     }
 
     /**
      * EntrySet
      */
     static class EntrySet<K, V> extends AbstractSet<Map.Entry<K, V>> {
         private final Flat3Map<K, V> parent;
 
         EntrySet(final Flat3Map<K, V> parent) {
             this.parent = parent;
         }
 
         @Override
         public void clear() {
             parent.clear();
         }
 
         @Override
         public Iterator<Map.Entry<K, V>> iterator() {
             if (parent.delegateMap != null) {
                 return parent.delegateMap.entrySet().iterator();
             }
             if (parent.isEmpty()) {
                 return EmptyIterator.<Map.Entry<K, V>>emptyIterator();
             }
             return new EntrySetIterator<>(parent);
         }
 
         @Override
         public boolean remove(final Object obj) {
             if (!(obj instanceof Map.Entry)) {
                 return false;
             }
             if (!contains(obj)) {
                 return false;
             }
             final Map.Entry<?, ?> entry = (Map.Entry<?, ?>) obj;
             parent.remove(entry.getKey());
             return true;
         }
 
         @Override
         public int size() {
             return parent.size();
         }
     }
 
     /**
      * EntrySetIterator and MapEntry
      */
     static class EntrySetIterator<K, V> extends EntryIterator<K, V> implements Iterator<Map.Entry<K, V>> {
         EntrySetIterator(final Flat3Map<K, V> parent) {
             super(parent);
         }
 
         @Override
         public Map.Entry<K, V> next() {
             return nextEntry();
         }
     }
     static class FlatMapEntry<K, V> implements Map.Entry<K, V> {
         private final Flat3Map<K, V> parent;
         private final int index;
         private volatile boolean removed;
 
         FlatMapEntry(final Flat3Map<K, V> parent, final int index) {
             this.parent = parent;
             this.index = index;
             this.removed = false;
         }
 
         @Override
         public boolean equals(final Object obj) {
             if (removed) {
                 return false;
             }
             if (!(obj instanceof Map.Entry)) {
                 return false;
             }
             final Map.Entry<?, ?> other = (Map.Entry<?, ?>) obj;
             return Objects.equals(getKey(), other.getKey()) &&
                    Objects.equals(getValue(), other.getValue());
         }
 
         @Override
         public K getKey() {
             if (removed) {
                 throw new IllegalStateException(AbstractHashedMap.GETKEY_INVALID);
             }
             switch (index) {
             case 3:
                 return parent.key3;
             case 2:
                 return parent.key2;
             case 1:
                 return parent.key1;
             }
             throw new IllegalStateException("Invalid map index: " + index);
         }
 
         @Override
         public V getValue() {
             if (removed) {
                 throw new IllegalStateException(AbstractHashedMap.GETVALUE_INVALID);
             }
             switch (index) {
             case 3:
                 return parent.value3;
             case 2:
                 return parent.value2;
             case 1:
                 return parent.value1;
             }
             throw new IllegalStateException("Invalid map index: " + index);
         }
 
         @Override
         public int hashCode() {
             if (removed) {
                 return 0;
             }
             final Object key = getKey();
             final Object value = getValue();
             return (key == null ? 0 : key.hashCode()) ^
                    (value == null ? 0 : value.hashCode());
         }
 
         /**
          * Used by the iterator that created this entry to indicate that
          * {@link java.util.Iterator#remove()} has been called.
          * <p>
          * As a consequence, all subsequent call to {@link #getKey()},
          * {@link #setValue(Object)} and {@link #getValue()} will fail.
          *
          * @param removed The new value of the removed flag
          */
         void setRemoved(final boolean removed) {
             this.removed = removed;
         }
 
         @Override
         public V setValue(final V value) {
             if (removed) {
                 throw new IllegalStateException(AbstractHashedMap.SETVALUE_INVALID);
             }
             final V old = getValue();
             switch (index) {
             case 3:
                 parent.value3 = value;
                 break;
             case 2:
                 parent.value2 = value;
                 break;
             case 1:
                 parent.value1 = value;
                 break;
             default:
                 throw new IllegalStateException("Invalid map index: " + index);
             }
             return old;
         }
 
         @Override
         public String toString() {
             if (!removed) {
                 return getKey() + "=" + getValue();
             }
             return "";
         }
 
     }
 
     /**
      * FlatMapIterator
      */
     static class FlatMapIterator<K, V> implements MapIterator<K, V>, ResettableIterator<K> {
         private final Flat3Map<K, V> parent;
         private int nextIndex;
         private boolean canRemove;
 
         FlatMapIterator(final Flat3Map<K, V> parent) {
             this.parent = parent;
         }
 
         @Override
         public K getKey() {
             if (!canRemove) {
                 throw new IllegalStateException(AbstractHashedMap.GETKEY_INVALID);
             }
             switch (nextIndex) {
             case 3:
                 return parent.key3;
             case 2:
                 return parent.key2;
             case 1:
                 return parent.key1;
             }
             throw new IllegalStateException("Invalid map index: " + nextIndex);
         }
 
         @Override
         public V getValue() {
             if (!canRemove) {
                 throw new IllegalStateException(AbstractHashedMap.GETVALUE_INVALID);
             }
             switch (nextIndex) {
             case 3:
                 return parent.value3;
             case 2:
                 return parent.value2;
             case 1:
                 return parent.value1;
             }
             throw new IllegalStateException("Invalid map index: " + nextIndex);
         }
 
         @Override
         public boolean hasNext() {
             return nextIndex < parent.size;
         }
 
         @Override
         public K next() {
             if (!hasNext()) {
                 throw new NoSuchElementException(AbstractHashedMap.NO_NEXT_ENTRY);
             }
             canRemove = true;
             nextIndex++;
             return getKey();
         }
 
         @Override
         public void remove() {
             if (!canRemove) {
                 throw new IllegalStateException(AbstractHashedMap.REMOVE_INVALID);
             }
             parent.remove(getKey());
             nextIndex--;
             canRemove = false;
         }
 
         @Override
         public void reset() {
             nextIndex = 0;
             canRemove = false;
         }
 
         @Override
         public V setValue(final V value) {
             if (!canRemove) {
                 throw new IllegalStateException(AbstractHashedMap.SETVALUE_INVALID);
             }
             final V old = getValue();
             switch (nextIndex) {
             case 3:
                 parent.value3 = value;
                 break;
             case 2:
                 parent.value2 = value;
                 break;
             case 1:
                 parent.value1 = value;
                 break;
             default:
                 throw new IllegalStateException("Invalid map index: " + nextIndex);
             }
             return old;
         }
 
         @Override
         public String toString() {
             if (canRemove) {
                 return "Iterator[" + getKey() + "=" + getValue() + "]";
             }
             return "Iterator[]";
         }
     }
 
     /**
      * KeySet
      */
     static class KeySet<K> extends AbstractSet<K> {
 
         private final Flat3Map<K, ?> parent;
 
         KeySet(final Flat3Map<K, ?> parent) {
             this.parent = parent;
         }
 
         @Override
         public void clear() {
             parent.clear();
         }
 
         @Override
         public boolean contains(final Object key) {
             return parent.containsKey(key);
         }
 
         @Override
         public Iterator<K> iterator() {
             if (parent.delegateMap != null) {
                 return parent.delegateMap.keySet().iterator();
             }
             if (parent.isEmpty()) {
                 return EmptyIterator.<K>emptyIterator();
             }
             return new KeySetIterator<>(parent);
         }
 
         @Override
         public boolean remove(final Object key) {
             final boolean result = parent.containsKey(key);
             parent.remove(key);
             return result;
         }
 
         @Override
         public int size() {
             return parent.size();
         }
     }
 
     /**
      * KeySetIterator
      */
     static class KeySetIterator<K> extends EntryIterator<K, Object> implements Iterator<K> {
 
         @SuppressWarnings("unchecked")
         KeySetIterator(final Flat3Map<K, ?> parent) {
             super((Flat3Map<K, Object>) parent);
         }
 
         @Override
         public K next() {
             return nextEntry().getKey();
         }
     }
 
     /**
      * Values
      */
     static class Values<V> extends AbstractCollection<V> {
 
         private final Flat3Map<?, V> parent;
 
         Values(final Flat3Map<?, V> parent) {
             this.parent = parent;
         }
 
         @Override
         public void clear() {
             parent.clear();
         }
 
         @Override
         public boolean contains(final Object value) {
             return parent.containsValue(value);
         }
 
         @Override
         public Iterator<V> iterator() {
             if (parent.delegateMap != null) {
                 return parent.delegateMap.values().iterator();
             }
             if (parent.isEmpty()) {
                 return EmptyIterator.<V>emptyIterator();
             }
             return new ValuesIterator<>(parent);
         }
 
         @Override
         public int size() {
             return parent.size();
         }
     }
 
     /**
      * ValuesIterator
      */
     static class ValuesIterator<V> extends EntryIterator<Object, V> implements Iterator<V> {
 
         @SuppressWarnings("unchecked")
         ValuesIterator(final Flat3Map<?, V> parent) {
             super((Flat3Map<Object, V>) parent);
         }
 
         @Override
         public V next() {
             return nextEntry().getValue();
         }
     }
 
     /** Serialization version */
     private static final long serialVersionUID = -6701087419741928296L;
 
     /** The size of the map, used while in flat mode */
     private transient int size;
 
     /** Hash, used while in flat mode */
     private transient int hash1;
 
     /** Hash, used while in flat mode */
     private transient int hash2;
 
     /** Hash, used while in flat mode */
     private transient int hash3;
 
     /** Key, used while in flat mode */
     private transient K key1;
 
     /** Key, used while in flat mode */
     private transient K key2;
 
     /** Key, used while in flat mode */
     private transient K key3;
 
     /** Value, used while in flat mode */
     private transient V value1;
 
     /** Value, used while in flat mode */
     private transient V value2;
 
     /** Value, used while in flat mode */
     private transient V value3;
 
     /** Map, used while in delegate mode */
     private transient AbstractHashedMap<K, V> delegateMap;
 
     /**
      * Constructs a new instance.
      */
     public Flat3Map() {
     }
 
     /**
      * Constructor copying elements from another map.
      *
      * @param map  The map to copy
      * @throws NullPointerException if the map is null
      */
     public Flat3Map(final Map<? extends K, ? extends V> map) {
         putAll(map);
     }
 
     /**
      * Clears the map, resetting the size to zero and nullifying references
      * to avoid garbage collection issues.
      */
     @Override
     public void clear() {
         if (delegateMap != null) {
             delegateMap.clear();  // should aid gc
             delegateMap = null;  // switch back to flat mode
         } else {
             size = 0;
             hash1 = hash2 = hash3 = 0;
             key1 = key2 = key3 = null;
             value1 = value2 = value3 = null;
         }
     }
 
     /**
      * Clones the map without cloning the keys or values.
      *
      * @return A shallow clone
      * @since 3.1
      */
     @Override
     @SuppressWarnings("unchecked")
     public Flat3Map<K, V> clone() {
         try {
             final Flat3Map<K, V> cloned = (Flat3Map<K, V>) super.clone();
             if (cloned.delegateMap != null) {
                 cloned.delegateMap = cloned.delegateMap.clone();
             }
             return cloned;
         } catch (final CloneNotSupportedException ex) {
             throw new UnsupportedOperationException(ex);
         }
     }
 
     /**
      * Checks whether the map contains the specified key.
      *
      * @param key  The key to search for
      * @return true if the map contains the key
      */
     @Override
     public boolean containsKey(final Object key) {
         if (delegateMap != null) {
             return delegateMap.containsKey(key);
         }
         if (key == null) {
             switch (size) {  // drop through
             case 3:
                 if (key3 == null) {
                     return true;
                 }
             case 2:
                 if (key2 == null) {
                     return true;
                 }
             case 1:
                 if (key1 == null) {
                     return true;
                 }
             }
         } else if (size > 0) {
             final int hashCode = key.hashCode();
             switch (size) {  // drop through
             case 3:
                 if (hash3 == hashCode && key.equals(key3)) {
                     return true;
                 }
             case 2:
                 if (hash2 == hashCode && key.equals(key2)) {
                     return true;
                 }
             case 1:
                 if (hash1 == hashCode && key.equals(key1)) {
                     return true;
                 }
             }
         }
         return false;
     }
 
     /**
      * Checks whether the map contains the specified value.
      *
      * @param value  The value to search for
      * @return true if the map contains the key
      */
     @Override
     public boolean containsValue(final Object value) {
         if (delegateMap != null) {
             return delegateMap.containsValue(value);
         }
         if (value == null) {  // drop through
             switch (size) {
             case 3:
                 if (value3 == null) {
                     return true;
                 }
             case 2:
                 if (value2 == null) {
                     return true;
                 }
             case 1:
                 if (value1 == null) {
                     return true;
                 }
             }
         } else {
             switch (size) {  // drop through
             case 3:
                 if (value.equals(value3)) {
                     return true;
                 }
             case 2:
                 if (value.equals(value2)) {
                     return true;
                 }
             case 1:
                 if (value.equals(value1)) {
                     return true;
                 }
             }
         }
         return false;
     }
 
     /**
      * Converts the flat map data to a map.
      */
     private void convertToMap() {
         delegateMap = createDelegateMap();
         switch (size) {  // drop through
         case 3:
             delegateMap.put(key3, value3);
         case 2:
             delegateMap.put(key2, value2);
         case 1:
             delegateMap.put(key1, value1);
         case 0:
             break;
         default:
             throw new IllegalStateException("Invalid map index: " + size);
         }
 
         size = 0;
         hash1 = hash2 = hash3 = 0;
         key1 = key2 = key3 = null;
         value1 = value2 = value3 = null;
     }
 
     /**
      * Create an instance of the map used for storage when in delegation mode.
      * <p>
      * This can be overridden by subclasses to provide a different map implementation.
      * Not every AbstractHashedMap is suitable, identity and reference based maps
      * would be poor choices.
      * </p>
      *
      * @return A new AbstractHashedMap or subclass
      * @since 3.1
      */
     protected AbstractHashedMap<K, V> createDelegateMap() {
         return new HashedMap<>();
     }
 
     /**
      * Gets the entrySet view of the map.
      * Changes made to the view affect this map.
      * <p>
      * NOTE: from 4.0, the returned Map Entry will be an independent object and will
      * not change anymore as the iterator progresses. To avoid this additional object
      * creation and simply iterate through the entries, use {@link #mapIterator()}.
      * </p>
      *
      * @return The entrySet view
      */
     @Override
     public Set<Map.Entry<K, V>> entrySet() {
         if (delegateMap != null) {
             return delegateMap.entrySet();
         }
         return new EntrySet<>(this);
     }
 
     /**
      * Compares this map with another.
      *
      * @param obj  The object to compare to
      * @return true if equal
      */
     @Override
     public boolean equals(final Object obj) {
         if (obj == this) {
             return true;
         }
         if (delegateMap != null) {
             return delegateMap.equals(obj);
         }
         if (!(obj instanceof Map)) {
             return false;
         }
         final Map<?, ?> other = (Map<?, ?>) obj;
         if (size != other.size()) {
             return false;
         }
         if (size > 0) {
             Object otherValue = null;
             switch (size) {  // drop through
             case 3:
                 if (!other.containsKey(key3)) {
                     return false;
                 }
                 otherValue = other.get(key3);
                 if (!Objects.equals(value3, otherValue)) {
                     return false;
                 }
             case 2:
                 if (!other.containsKey(key2)) {
                     return false;
                 }
                 otherValue = other.get(key2);
                 if (!Objects.equals(value2, otherValue)) {
                     return false;
                 }
             case 1:
                 if (!other.containsKey(key1)) {
                     return false;
                 }
                 otherValue = other.get(key1);
                 if (!Objects.equals(value1, otherValue)) {
                     return false;
                 }
             }
         }
         return true;
     }
 
     /**
      * Gets the value mapped to the key specified.
      *
      * @param key  The key
      * @return The mapped value, null if no match
      */
     @Override
     public V get(final Object key) {
         if (delegateMap != null) {
             return delegateMap.get(key);
         }
         if (key == null) {
             switch (size) {
             // drop through
             case 3:
                 if (key3 == null) {
                     return value3;
                 }
             case 2:
                 if (key2 == null) {
                     return value2;
                 }
             case 1:
                 if (key1 == null) {
                     return value1;
                 }
             }
         } else if (size > 0) {
             final int hashCode = key.hashCode();
             switch (size) {
             // drop through
             case 3:
                 if (hash3 == hashCode && key.equals(key3)) {
                     return value3;
                 }
             case 2:
                 if (hash2 == hashCode && key.equals(key2)) {
                     return value2;
                 }
             case 1:
                 if (hash1 == hashCode && key.equals(key1)) {
                     return value1;
                 }
             }
         }
         return null;
     }
 
     /**
      * Gets the standard Map hashCode.
      *
      * @return The hash code defined in the Map interface
      */
     @Override
     public int hashCode() {
         if (delegateMap != null) {
             return delegateMap.hashCode();
         }
         int total = 0;
         switch (size) {  // drop through
         case 3:
             total += hash3 ^ (value3 == null ? 0 : value3.hashCode());
         case 2:
             total += hash2 ^ (value2 == null ? 0 : value2.hashCode());
         case 1:
             total += hash1 ^ (value1 == null ? 0 : value1.hashCode());
         case 0:
             break;
         default:
             throw new IllegalStateException("Invalid map index: " + size);
         }
         return total;
     }
 
     /**
      * Checks whether the map is currently empty.
      *
      * @return true if the map is currently size zero
      */
     @Override
     public boolean isEmpty() {
         return size() == 0;
     }
 
     /**
      * Gets the keySet view of the map.
      * Changes made to the view affect this map.
      * To simply iterate through the keys, use {@link #mapIterator()}.
      *
      * @return The keySet view
      */
     @Override
     public Set<K> keySet() {
         if (delegateMap != null) {
             return delegateMap.keySet();
         }
         return new KeySet<>(this);
     }
 
     /**
      * Gets an iterator over the map.
      * Changes made to the iterator affect this map.
      * <p>
      * A MapIterator returns the keys in the map. It also provides convenient
      * methods to get the key and value, and set the value.
      * It avoids the need to create an entrySet/keySet/values object.
      * It also avoids creating the Map Entry object.
      * </p>
      *
      * @return The map iterator
      */
     @Override
     public MapIterator<K, V> mapIterator() {
         if (delegateMap != null) {
             return delegateMap.mapIterator();
         }
         if (size == 0) {
             return EmptyMapIterator.<K, V>emptyMapIterator();
         }
         return new FlatMapIterator<>(this);
     }
 
     /**
      * Puts a key-value mapping into this map.
      *
      * @param key  The key to add
      * @param value  The value to add
      * @return The value previously mapped to this key, null if none
      */
     @Override
     public V put(final K key, final V value) {
         if (delegateMap != null) {
             return delegateMap.put(key, value);
         }
         // change existing mapping
         if (key == null) {
             switch (size) {  // drop through
             case 3:
                 if (key3 == null) {
                     final V old = value3;
                     value3 = value;
                     return old;
                 }
             case 2:
                 if (key2 == null) {
                     final V old = value2;
                     value2 = value;
                     return old;
                 }
             case 1:
                 if (key1 == null) {
                     final V old = value1;
                     value1 = value;
                     return old;
                 }
             }
         } else if (size > 0) {
             final int hashCode = key.hashCode();
             switch (size) {  // drop through
             case 3:
                 if (hash3 == hashCode && key.equals(key3)) {
                     final V old = value3;
                     value3 = value;
                     return old;
                 }
             case 2:
                 if (hash2 == hashCode && key.equals(key2)) {
                     final V old = value2;
                     value2 = value;
                     return old;
                 }
             case 1:
                 if (hash1 == hashCode && key.equals(key1)) {
                     final V old = value1;
                     value1 = value;
                     return old;
                 }
             }
         }
 
         // add new mapping
         switch (size) {
         case 2:
             hash3 = key == null ? 0 : key.hashCode();
             key3 = key;
             value3 = value;
             break;
         case 1:
             hash2 = key == null ? 0 : key.hashCode();
             key2 = key;
             value2 = value;
             break;
         case 0:
             hash1 = key == null ? 0 : key.hashCode();
             key1 = key;
             value1 = value;
             break;
         default:
             convertToMap();
             delegateMap.put(key, value);
             return null;
         }
         size++;
         return null;
     }
 
     /**
      * Puts all the values from the specified map into this map.
      *
      * @param map  The map to add
      * @throws NullPointerException if the map is null
      */
     @Override
     public void putAll(final Map<? extends K, ? extends V> map) {
         final int size = map.size();
         if (size == 0) {
             return;
         }
         if (delegateMap != null) {
             delegateMap.putAll(map);
             return;
         }
         if (size < 4) {
             for (final Map.Entry<? extends K, ? extends V> entry : map.entrySet()) {
                 put(entry.getKey(), entry.getValue());
             }
         } else {
             convertToMap();
             delegateMap.putAll(map);
         }
     }
 
     /**
      * Deserializes the map in using a custom routine.
      *
      * @param in The input stream
      * @throws IOException Thrown if an error occurs while reading from the stream
      * @throws ClassNotFoundException if an object read from the stream cannot be loaded
      */
     @SuppressWarnings("unchecked")
     private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
         in.defaultReadObject();
         final int count = in.readInt();
         if (count > 3) {
             delegateMap = createDelegateMap();
         }
         for (int i = count; i > 0; i--) {
             put((K) in.readObject(), (V) in.readObject());
         }
     }
 
     /**
      * Removes the specified mapping from this map.
      *
      * @param key  The mapping to remove
      * @return The value mapped to the removed key, null if key not in map
      */
     @Override
     public V remove(final Object key) {
         if (delegateMap != null) {
             return delegateMap.remove(key);
         }
         if (size == 0) {
             return null;
         }
         if (key == null) {
             switch (size) {  // drop through
             case 3:
                 if (key3 == null) {
                     final V old = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 if (key2 == null) {
                     final V old = value2;
                     hash2 = hash3;
                     key2 = key3;
                     value2 = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 if (key1 == null) {
                     final V old = value1;
                     hash1 = hash3;
                     key1 = key3;
                     value1 = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 return null;
             case 2:
                 if (key2 == null) {
                     final V old = value2;
                     hash2 = 0;
                     key2 = null;
                     value2 = null;
                     size = 1;
                     return old;
                 }
                 if (key1 == null) {
                     final V old = value1;
                     hash1 = hash2;
                     key1 = key2;
                     value1 = value2;
                     hash2 = 0;
                     key2 = null;
                     value2 = null;
                     size = 1;
                     return old;
                 }
                 return null;
             case 1:
                 if (key1 == null) {
                     final V old = value1;
                     hash1 = 0;
                     key1 = null;
                     value1 = null;
                     size = 0;
                     return old;
                 }
             }
         } else if (size > 0) {
             final int hashCode = key.hashCode();
             switch (size) {  // drop through
             case 3:
                 if (hash3 == hashCode && key.equals(key3)) {
                     final V old = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 if (hash2 == hashCode && key.equals(key2)) {
                     final V old = value2;
                     hash2 = hash3;
                     key2 = key3;
                     value2 = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 if (hash1 == hashCode && key.equals(key1)) {
                     final V old = value1;
                     hash1 = hash3;
                     key1 = key3;
                     value1 = value3;
                     hash3 = 0;
                     key3 = null;
                     value3 = null;
                     size = 2;
                     return old;
                 }
                 return null;
             case 2:
                 if (hash2 == hashCode && key.equals(key2)) {
                     final V old = value2;
                     hash2 = 0;
                     key2 = null;
                     value2 = null;
                     size = 1;
                     return old;
                 }
                 if (hash1 == hashCode && key.equals(key1)) {
                     final V old = value1;
                     hash1 = hash2;
                     key1 = key2;
                     value1 = value2;
                     hash2 = 0;
                     key2 = null;
                     value2 = null;
                     size = 1;
                     return old;
                 }
                 return null;
             case 1:
                 if (hash1 == hashCode && key.equals(key1)) {
                     final V old = value1;
                     hash1 = 0;
                     key1 = null;
                     value1 = null;
                     size = 0;
                     return old;
                 }
             }
         }
         return null;
     }
 
     /**
      * Gets the size of the map.
      *
      * @return The size
      */
     @Override
     public int size() {
         if (delegateMap != null) {
             return delegateMap.size();
         }
         return size;
     }
 
     /**
      * Gets the map as a String.
      *
      * @return A string version of the map
      */
     @Override
     public String toString() {
         if (delegateMap != null) {
             return delegateMap.toString();
         }
         if (size == 0) {
             return "{}";
         }
         final StringBuilder buf = new StringBuilder(128);
         buf.append('{');
         switch (size) {  // drop through
         case 3:
             buf.append(key3 == this ? "(this Map)" : key3);
             buf.append('=');
             buf.append(value3 == this ? "(this Map)" : value3);
             buf.append(CollectionUtils.COMMA);
         case 2:
             buf.append(key2 == this ? "(this Map)" : key2);
             buf.append('=');
             buf.append(value2 == this ? "(this Map)" : value2);
             buf.append(CollectionUtils.COMMA);
         case 1:
             buf.append(key1 == this ? "(this Map)" : key1);
             buf.append('=');
             buf.append(value1 == this ? "(this Map)" : value1);
             break;
         // case 0: has already been dealt with
         default:
             throw new IllegalStateException("Invalid map index: " + size);
         }
         buf.append('}');
         return buf.toString();
     }
 
     /**
      * Gets the values view of the map.
      * Changes made to the view affect this map.
      * To simply iterate through the values, use {@link #mapIterator()}.
      *
      * @return The values view
      */
     @Override
     public Collection<V> values() {
         if (delegateMap != null) {
             return delegateMap.values();
         }
         return new Values<>(this);
     }
 
     /**
      * Serializes this object to an ObjectOutputStream.
      *
      * @param out The target ObjectOutputStream.
      * @throws IOException thrown when an I/O errors occur writing to the target stream.
      */
     private void writeObject(final ObjectOutputStream out) throws IOException {
         out.defaultWriteObject();
         out.writeInt(size());
         for (final MapIterator<?, ?> it = mapIterator(); it.hasNext();) {
             out.writeObject(it.next());  // key
             out.writeObject(it.getValue());  // value
         }
     }
 
 }