/*
    * 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
    *
    *      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 org.apache.commons.collections4.keyvalue;
  
  import java.io.Serializable;
  import java.lang.reflect.Array;
  import java.util.Arrays;
  import java.util.Objects;
  
  /**
   * A {@code MultiKey} allows multiple map keys to be merged together.
   * <p>
   * The purpose of this class is to avoid the need to write code to handle
   * maps of maps. An example might be the need to look up a file name by
   * key and locale. The typical solution might be nested maps. This class
   * can be used instead by creating an instance passing in the key and locale.
   * </p>
   * <p>
   * Example usage:
   * </p>
   * <pre>
   * // populate map with data mapping key+locale to localizedText
   * Map map = new HashMap();
   * MultiKey multiKey = new MultiKey(key, locale);
   * map.put(multiKey, localizedText);
   *
   * // later retrieve the localized text
   * MultiKey multiKey = new MultiKey(key, locale);
   * String localizedText = (String) map.get(multiKey);
   * </pre>
   *
   * @param <K> the type of keys
   * @since 3.0
   */
  public class MultiKey<K> implements Serializable {
      // This class could implement List, but that would confuse its purpose
  
      /** Serialisation version */
      private static final long serialVersionUID = 4465448607415788805L;
  
      @SuppressWarnings("unchecked")
      private static <T> Class<? extends T> getClass(final T value) {
          return (Class<? extends T>) (value == null ? Object.class : value.getClass());
      }
  
      @SafeVarargs
      private static <T> Class<? extends T> getComponentType(final T... values) {
          @SuppressWarnings("unchecked")
          final Class<? extends T> rootClass = (Class<? extends T>) Object.class;
          if (values == null) {
              return rootClass;
          }
          Class<? extends T> prevClass = values.length > 0 ? getClass(values[0]) : rootClass;
          for (int i = 1; i < values.length; i++) {
              final Class<? extends T> classI = getClass(values[i]);
              if (prevClass != classI) {
                  return rootClass;
              }
              prevClass = classI;
          }
          return prevClass;
      }
  
      private static <T> T[] newArray(final T key1, final T key2) {
          @SuppressWarnings("unchecked")
          final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2), 2);
          array[0] = key1;
          array[1] = key2;
          return array;
      }
  
      private static <T> T[] newArray(final T key1, final T key2, final T key3) {
          @SuppressWarnings("unchecked")
          final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3), 3);
          array[0] = key1;
          array[1] = key2;
          array[2] = key3;
          return array;
      }
  
      private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4) {
          @SuppressWarnings("unchecked")
          final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4), 4);
          array[0] = key1;
          array[1] = key2;
         array[2] = key3;
         array[3] = key4;
         return array;
     }
 
     private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4, final T key5) {
         @SuppressWarnings("unchecked")
         final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4, key5), 5);
         array[0] = key1;
         array[1] = key2;
         array[2] = key3;
         array[3] = key4;
         array[4] = key5;
         return array;
     }
 
     /** The individual keys */
     private final K[] keys;
 
     /** The cached hashCode */
     private transient int hashCode;
 
     /**
      * Constructor taking two keys.
      * <p>
      * The keys should be immutable.
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      *
      * @param key1  the first key
      * @param key2  the second key
      */
     public MultiKey(final K key1, final K key2) {
         this(newArray(key1, key2), false);
     }
 
     /**
      * Constructor taking three keys.
      * <p>
      * The keys should be immutable
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      *
      * @param key1  the first key
      * @param key2  the second key
      * @param key3  the third key
      */
     public MultiKey(final K key1, final K key2, final K key3) {
         this(newArray(key1, key2, key3), false);
     }
 
     /**
      * Constructor taking four keys.
      * <p>
      * The keys should be immutable.
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      *
      * @param key1  the first key
      * @param key2  the second key
      * @param key3  the third key
      * @param key4  the fourth key
      */
     public MultiKey(final K key1, final K key2, final K key3, final K key4) {
         this(newArray(key1, key2, key3, key4), false);
     }
 
     /**
      * Constructor taking five keys.
      * <p>
      * The keys should be immutable.
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      *
      * @param key1  the first key
      * @param key2  the second key
      * @param key3  the third key
      * @param key4  the fourth key
      * @param key5  the fifth key
      */
     public MultiKey(final K key1, final K key2, final K key3, final K key4, final K key5) {
         this(newArray(key1, key2, key3, key4, key5), false);
     }
 
     /**
      * Constructor taking an array of keys which is cloned.
      * <p>
      * The keys should be immutable.
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      * <p>
      * This is equivalent to {@code new MultiKey(keys, true)}.
      * </p>
      *
      * @param keys  the array of keys, not null
      * @throws NullPointerException if the key array is null
      */
     public MultiKey(final K[] keys) {
         this(keys, true);
     }
 
     /**
      * Constructor taking an array of keys, optionally choosing whether to clone.
      * <p>
      * <strong>If the array is not cloned, then it must not be modified.</strong>
      * </p>
      * <p>
      * This method is public for performance reasons only, to avoid a clone.
      * The hash code is calculated once here in this method.
      * Therefore, changing the array passed in would not change the hash code but
      * would change the equals method, which is a bug.
      * </p>
      * <p>
      * This is the only fully safe usage of this constructor, as the object array
      * is never made available in a variable:
      * <pre>
      * new MultiKey(new Object[] {...}, false);
      * </pre>
      * <p>
      * The keys should be immutable.
      * If they are not then they must not be changed after adding to the MultiKey.
      * </p>
      *
      * @param keys  the array of keys, not null
      * @param makeClone  true to clone the array, false to assign it
      * @throws NullPointerException if the key array is null
      * @since 3.1
      */
     public MultiKey(final K[] keys, final boolean makeClone) {
         Objects.requireNonNull(keys, "keys");
         this.keys = makeClone ? keys.clone() : keys;
         calculateHashCode(keys);
     }
 
     /**
      * Calculate the hash code of the instance using the provided keys.
      * @param keys the keys to calculate the hash code for
      */
     private void calculateHashCode(final Object[] keys) {
         int total = 0;
         for (final Object key : keys) {
             if (key != null) {
                 total ^= key.hashCode();
             }
         }
         hashCode = total;
     }
 
     /**
      * Compares this object to another.
      * <p>
      * To be equal, the other object must be a {@code MultiKey} with the
      * same number of keys which are also equal.
      * </p>
      *
      * @param other  the other object to compare to
      * @return true if equal
      */
     @Override
     public boolean equals(final Object other) {
         if (other == this) {
             return true;
         }
         if (other instanceof MultiKey) {
             final MultiKey<?> otherMulti = (MultiKey<?>) other;
             return Arrays.equals(keys, otherMulti.keys);
         }
         return false;
     }
 
     /**
      * Gets the key at the specified index.
      * <p>
      * The key should be immutable.
      * If it is not then it must not be changed.
      * </p>
      *
      * @param index  the index to retrieve
      * @return the key at the index
      * @throws IndexOutOfBoundsException if the index is invalid
      * @since 3.1
      */
     public K getKey(final int index) {
         return keys[index];
     }
 
     /**
      * Gets a clone of the array of keys.
      * <p>
      * The keys should be immutable
      * If they are not then they must not be changed.
      * </p>
      *
      * @return the individual keys
      */
     public K[] getKeys() {
         return keys.clone();
     }
 
     /**
      * Gets the combined hash code that is computed from all the keys.
      * <p>
      * This value is computed once and then cached, so elements should not
      * change their hash codes once created (note that this is the same
      * constraint that would be used if the individual keys elements were
      * themselves {@link java.util.Map Map} keys).
      * </p>
      *
      * @return the hash code
      */
     @Override
     public int hashCode() {
         return hashCode;
     }
 
     /**
      * Recalculate the hash code after deserialization. The hash code of some
      * keys might have change (hash codes based on the system hash code are
      * only stable for the same process).
      * @return the instance with recalculated hash code
      */
     protected Object readResolve() {
         calculateHashCode(keys);
         return this;
     }
 
     /**
      * Gets the size of the list of keys.
      *
      * @return the size of the list of keys
      * @since 3.1
      */
     public int size() {
         return keys.length;
     }
 
     /**
      * Gets a debugging string version of the key.
      *
      * @return a debugging string
      */
     @Override
     public String toString() {
         return "MultiKey" + Arrays.toString(keys);
     }
 }