/*
    * 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.Serializable;
  import java.util.AbstractSet;
  import java.util.Collection;
  import java.util.Collections;
  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.BoundedMap;
  import org.apache.commons.collections4.KeyValue;
  import org.apache.commons.collections4.OrderedMap;
  import org.apache.commons.collections4.OrderedMapIterator;
  import org.apache.commons.collections4.ResettableIterator;
  import org.apache.commons.collections4.iterators.SingletonIterator;
  import org.apache.commons.collections4.keyvalue.TiedMapEntry;
  
  /**
   * A {@code Map} implementation that holds a single item and is fixed size.
   * <p>
   * The single key/value pair is specified at creation.
   * The map is fixed size so any action that would change the size is disallowed.
   * However, the {@code put} or {@code setValue} methods can <em>change</em>
   * the value associated with the key.
   * </p>
   * <p>
   * If trying to remove or clear the map, an UnsupportedOperationException is thrown.
   * If trying to put a new mapping into the map, an  IllegalArgumentException is thrown.
   * The put method will only succeed if the key specified is the same as the
   * singleton key.
   * </p>
   * <p>
   * The key and value can be obtained by:
   * </p>
   * <ul>
   * <li>normal Map methods and views</li>
   * <li>the {@code MapIterator}, see {@link #mapIterator()}</li>
   * <li>the {@code KeyValue} interface (just cast - no object creation)</li>
   * </ul>
   *
   * @param <K> The type of the keys in this map
   * @param <V> The type of the values in this map
   * @since 3.1
   */
  public class SingletonMap<K, V>
          implements OrderedMap<K, V>, BoundedMap<K, V>, KeyValue<K, V>, Serializable, Cloneable {
  
      /**
       * SingletonMapIterator.
       */
      static class SingletonMapIterator<K, V> implements OrderedMapIterator<K, V>, ResettableIterator<K> {
          private final SingletonMap<K, V> parent;
          private boolean hasNext = true;
          private boolean canGetSet;
  
          SingletonMapIterator(final SingletonMap<K, V> parent) {
              this.parent = parent;
          }
  
          @Override
          public K getKey() {
              if (!canGetSet) {
                  throw new IllegalStateException(AbstractHashedMap.GETKEY_INVALID);
              }
              return parent.getKey();
          }
  
          @Override
          public V getValue() {
              if (!canGetSet) {
                  throw new IllegalStateException(AbstractHashedMap.GETVALUE_INVALID);
              }
              return parent.getValue();
          }
  
          @Override
          public boolean hasNext() {
              return hasNext;
          }
  
         @Override
         public boolean hasPrevious() {
             return !hasNext;
         }
 
         @Override
         public K next() {
             if (!hasNext) {
                 throw new NoSuchElementException(AbstractHashedMap.NO_NEXT_ENTRY);
             }
             hasNext = false;
             canGetSet = true;
             return parent.getKey();
         }
 
         @Override
         public K previous() {
             if (hasNext) {
                 throw new NoSuchElementException(AbstractHashedMap.NO_PREVIOUS_ENTRY);
             }
             hasNext = true;
             return parent.getKey();
         }
 
         /**
          * Always throws {@link UnsupportedOperationException}.
          *
          * @throws UnsupportedOperationException Always thrown.
          */
         @Override
         public void remove() {
             throw new UnsupportedOperationException();
         }
 
         @Override
         public void reset() {
             hasNext = true;
         }
 
         @Override
         public V setValue(final V value) {
             if (!canGetSet) {
                 throw new IllegalStateException(AbstractHashedMap.SETVALUE_INVALID);
             }
             return parent.setValue(value);
         }
 
         @Override
         public String toString() {
             if (hasNext) {
                 return "Iterator[]";
             }
             return "Iterator[" + getKey() + "=" + getValue() + "]";
         }
     }
 
     /**
      * Values implementation for the SingletonMap.
      * This class is needed as values is a view that must update as the map updates.
      *
      * @param <V> The type of the values in this set.
      */
     static class SingletonValues<V> extends AbstractSet<V> implements Serializable {
         private static final long serialVersionUID = -3689524741863047872L;
         private final SingletonMap<?, V> parent;
 
         SingletonValues(final SingletonMap<?, V> parent) {
             this.parent = parent;
         }
 
         /**
          * Always throws {@link UnsupportedOperationException}.
          *
          * @throws UnsupportedOperationException Always thrown.
          */
         @Override
         public void clear() {
             throw new UnsupportedOperationException();
         }
         @Override
         public boolean contains(final Object object) {
             return parent.containsValue(object);
         }
         @Override
         public boolean isEmpty() {
             return false;
         }
         @Override
         public Iterator<V> iterator() {
             return new SingletonIterator<>(parent.getValue(), false);
         }
         @Override
         public int size() {
             return 1;
         }
     }
 
     /** Serialization version */
     private static final long serialVersionUID = -8931271118676803261L;
 
     /** Singleton key */
     private final K key;
 
     /** Singleton value */
     private V value;
 
     /**
      * Constructor that creates a map of {@code null} to {@code null}.
      */
     public SingletonMap() {
         this.key = null;
     }
 
     /**
      * Constructor specifying the key and value.
      *
      * @param key  The key to use
      * @param value  The value to use
      */
     public SingletonMap(final K key, final V value) {
         this.key = key;
         this.value = value;
     }
 
     /**
      * Constructor specifying the key and value as a {@code KeyValue}.
      *
      * @param keyValue  The key value pair to use
      */
     public SingletonMap(final KeyValue<K, V> keyValue) {
         this.key = keyValue.getKey();
         this.value = keyValue.getValue();
     }
 
     /**
      * Constructor specifying the key and value as a {@code MapEntry}.
      *
      * @param mapEntry  The mapEntry to use
      */
     public SingletonMap(final Map.Entry<? extends K, ? extends V> mapEntry) {
         this.key = mapEntry.getKey();
         this.value = mapEntry.getValue();
     }
 
     /**
      * Constructor copying elements from another map.
      *
      * @param map  The map to copy, must be size 1
      * @throws NullPointerException if the map is null
      * @throws IllegalArgumentException if the size is not 1
      */
     public SingletonMap(final Map<? extends K, ? extends V> map) {
         if (map.size() != 1) {
             throw new IllegalArgumentException("The map size must be 1");
         }
         final Map.Entry<? extends K, ? extends V> entry = map.entrySet().iterator().next();
         this.key = entry.getKey();
         this.value = entry.getValue();
     }
 
     /**
      * Always throws {@link UnsupportedOperationException}.
      *
      * @throws UnsupportedOperationException Always thrown.
      */
     @Override
     public void clear() {
         throw new UnsupportedOperationException();
     }
 
     /**
      * Clones the map without cloning the key or value.
      *
      * @return A shallow clone
      */
     @Override
     @SuppressWarnings("unchecked")
     public SingletonMap<K, V> clone() {
         try {
             return (SingletonMap<K, V>) super.clone();
         } 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) {
         return isEqualKey(key);
     }
 
     /**
      * 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) {
         return isEqualValue(value);
     }
 
     /**
      * Gets the entrySet view of the map.
      * Changes made via {@code setValue} affect this map.
      * To simply iterate through the entries, use {@link #mapIterator()}.
      *
      * @return The entrySet view
      */
     @Override
     public Set<Map.Entry<K, V>> entrySet() {
         final Map.Entry<K, V> entry = new TiedMapEntry<>(this, getKey());
         return Collections.singleton(entry);
     }
 
     /**
      * 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 (!(obj instanceof Map)) {
             return false;
         }
         final Map<?, ?> other = (Map<?, ?>) obj;
         if (other.size() != 1) {
             return false;
         }
         final Map.Entry<?, ?> entry = other.entrySet().iterator().next();
         return isEqualKey(entry.getKey()) && isEqualValue(entry.getValue());
     }
 
     /**
      * Gets the first (and only) key in the map.
      *
      * @return The key
      */
     @Override
     public K firstKey() {
         return getKey();
     }
 
     /**
      * 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 (isEqualKey(key)) {
             return value;
         }
         return null;
     }
 
     /**
      * Gets the key.
      *
      * @return The key
      */
     @Override
     public K getKey() {
         return key;
     }
 
     /**
      * Gets the value.
      *
      * @return The value
      */
     @Override
     public V getValue() {
         return value;
     }
 
     /**
      * Gets the standard Map hashCode.
      *
      * @return The hash code defined in the Map interface
      */
     @Override
     public int hashCode() {
         return (getKey() == null ? 0 : getKey().hashCode()) ^
                (getValue() == null ? 0 : getValue().hashCode());
     }
 
     /**
      * Checks whether the map is currently empty, which it never is.
      *
      * @return false always
      */
     @Override
     public boolean isEmpty() {
         return false;
     }
 
     /**
      * Compares the specified key to the stored key.
      *
      * @param key  The key to compare
      * @return true if equal
      */
     protected boolean isEqualKey(final Object key) {
         return Objects.equals(key, getKey());
     }
 
     /**
      * Compares the specified value to the stored value.
      *
      * @param value  The value to compare
      * @return true if equal
      */
     protected boolean isEqualValue(final Object value) {
         return Objects.equals(value, getValue());
     }
 
     /**
      * Is the map currently full, always true.
      *
      * @return true always
      */
     @Override
     public boolean isFull() {
         return true;
     }
 
     /**
      * Gets the unmodifiable 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() {
         return Collections.singleton(key);
     }
 
     /**
      * Gets the last (and only) key in the map.
      *
      * @return The key
      */
     @Override
     public K lastKey() {
         return getKey();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public OrderedMapIterator<K, V> mapIterator() {
         return new SingletonMapIterator<>(this);
     }
 
     /**
      * Gets the maximum size of the map, always 1.
      *
      * @return 1 always
      */
     @Override
     public int maxSize() {
         return 1;
     }
 
     /**
      * Gets the next key after the key specified, always null.
      *
      * @param key  The next key
      * @return null always
      */
     @Override
     public K nextKey(final K key) {
         return null;
     }
 
     /**
      * Gets the previous key before the key specified, always null.
      *
      * @param key  The next key
      * @return null always
      */
     @Override
     public K previousKey(final K key) {
         return null;
     }
 
     /**
      * Puts a key-value mapping into this map where the key must match the existing key.
      * <p>
      * An IllegalArgumentException is thrown if the key does not match as the map
      * is fixed size.
      * </p>
      *
      * @param key  The key to set, must be the key of the map
      * @param value  The value to set
      * @return The value previously mapped to this key, null if none
      * @throws IllegalArgumentException if the key does not match
      */
     @Override
     public V put(final K key, final V value) {
         if (isEqualKey(key)) {
             return setValue(value);
         }
         throw new IllegalArgumentException("Cannot put new key/value pair - Map is fixed size singleton");
     }
 
     /**
      * Puts the values from the specified map into this map.
      * <p>
      * The map must be of size 0 or size 1.
      * If it is size 1, the key must match the key of this map otherwise an
      * IllegalArgumentException is thrown.
      * </p>
      *
      * @param map  The map to add, must be size 0 or 1, and the key must match
      * @throws NullPointerException if the map is null
      * @throws IllegalArgumentException if the key does not match
      */
     @Override
     public void putAll(final Map<? extends K, ? extends V> map) {
         switch (map.size()) {
         case 0:
             return;
 
         case 1:
             final Map.Entry<? extends K, ? extends V> entry = map.entrySet().iterator().next();
             put(entry.getKey(), entry.getValue());
             return;
 
         default:
             throw new IllegalArgumentException("The map size must be 0 or 1");
         }
     }
 
     /**
      * Always throws {@link UnsupportedOperationException}.
      *
      * @param key Ignored.
      * @throws UnsupportedOperationException Always thrown.
      */
     @Override
     public V remove(final Object key) {
         throw new UnsupportedOperationException();
     }
 
     /**
      * Sets the value.
      *
      * @param value  The new value to set
      * @return The old value
      */
     public V setValue(final V value) {
         final V old = this.value;
         this.value = value;
         return old;
     }
 
     /**
      * Gets the size of the map, always 1.
      *
      * @return The size of 1
      */
     @Override
     public int size() {
         return 1;
     }
 
     /**
      * Gets the map as a String.
      *
      * @return A string version of the map
      */
     @Override
     public String toString() {
         return new StringBuilder(128)
             .append('{')
             .append(getKey() == this ? "(this Map)" : getKey())
             .append('=')
             .append(getValue() == this ? "(this Map)" : getValue())
             .append('}')
             .toString();
     }
 
     /**
      * Gets the unmodifiable 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() {
         return new SingletonValues<>(this);
     }
 
 }