/*
 * 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.juneau.commons.collections;

import static org.apache.juneau.commons.utils.AssertionUtils.*;
import static org.apache.juneau.commons.utils.CollectionUtils.*;
import static org.apache.juneau.commons.utils.ThrowableUtils.*;

import java.lang.reflect.*;
import java.util.*;
import java.util.stream.Collectors;

import org.apache.juneau.commons.utils.Utils;

/**
 * An unmodifiable, fixed-size map implementation backed by parallel key and value arrays.
 *
 * <p>
 * This class provides a simple, efficient, immutable map implementation for scenarios where the
 * set of keys and values is known in advance and doesn't need to change. It's particularly useful
 * for small maps (typically less than 10 entries) where the overhead of a {@link HashMap} is not
 * justified.
 *
 * <h5 class='section'>Features:</h5>
 * <ul class='spaced-list'>
 *    <li><b>Unmodifiable:</b> All mutation operations throw {@link UnsupportedOperationException}
 *    <li><b>Fixed Keys and Values:</b> Keys and values are set at construction time
 *    <li><b>Insertion Order:</b> Preserves the order of keys as provided in the constructor
 *    <li><b>Null Support:</b> Supports null keys and values (unlike {@link Map#of()})
 *    <li><b>Array-Backed:</b> Uses simple arrays for storage, avoiding hash computation overhead
 *    <li><b>Memory Efficient:</b> Minimal memory footprint compared to {@link HashMap}
 *    <li><b>Predictable Performance:</b> O(n) lookup time, but faster than {@link HashMap} for small n
 * </ul>
 *
 * <h5 class='section'>Use Cases:</h5>
 * <ul class='spaced-list'>
 *    <li>Immutable configuration maps with a fixed set of known keys
 *    <li>Small lookup tables that should not be modified
 *    <li>Method return values where immutability is desired
 *    <li>Situations where map size is small and keys are known at compile time
 * </ul>
 *
 * <h5 class='section'>Usage:</h5>
 * <p class='bjava'>
 *    <jc>// Create an unmodifiable map with fixed keys and values</jc>
 *    String[] <jv>keys</jv> = {<js>"host"</js>, <js>"port"</js>, <js>"timeout"</js>};
 *    Object[] <jv>values</jv> = {<js>"localhost"</js>, 8080, 30000};
 *
 *    SimpleMap&lt;String,Object&gt; <jv>config</jv> = <jk>new</jk> SimpleMap&lt;&gt;(<jv>keys</jv>, <jv>values</jv>);
 *
 *    <jc>// Get values</jc>
 *    String <jv>host</jv> = (String)<jv>config</jv>.get(<js>"host"</js>);     <jc>// Returns: "localhost"</jc>
 *    Integer <jv>port</jv> = (Integer)<jv>config</jv>.get(<js>"port"</js>);   <jc>// Returns: 8080</jc>
 *
 *    <jc>// Cannot modify</jc>
 *    <jv>config</jv>.put(<js>"port"</js>, 9090);  <jc>// Throws UnsupportedOperationException</jc>
 * </p>
 *
 * <h5 class='section'>Restrictions:</h5>
 * <ul class='spaced-list'>
 *    <li><b>Immutable:</b> Cannot add, remove, or modify entries after construction
 *    <li><b>Fixed Size:</b> Size is determined at construction time
 *    <li><b>Unique Keys:</b> Keys must be unique (no duplicates)
 *    <li><b>Array Length:</b> Keys and values arrays must have the same length
 * </ul>
 *
 * <h5 class='section'>Performance Characteristics:</h5>
 * <ul class='spaced-list'>
 *    <li><b>get(key):</b> O(n) - Linear search through keys array
 *    <li><b>size():</b> O(1) - Direct array length access
 *    <li><b>Memory:</b> Lower overhead than {@link HashMap} for small maps
 * </ul>
 *
 * <h5 class='section'>Thread Safety:</h5>
 * <p>
 * This class is thread-safe since it's immutable after construction. Multiple threads can safely
 * read from a SimpleMap concurrently without synchronization.
 *
 * <h5 class='section'>Example - Configuration Map:</h5>
 * <p class='bjava'>
 *    <jc>// Create immutable configuration</jc>
 *    SimpleMap&lt;String,Object&gt; <jv>defaults</jv> = <jk>new</jk> SimpleMap&lt;&gt;(
 *       <jk>new</jk> String[]{<js>"maxConnections"</js>, <js>"timeout"</js>, <js>"retries"</js>},
 *       <jk>new</jk> Object[]{100, 5000, 3}
 *    );
 *
 *    <jc>// Safe to share across threads</jc>
 *    <jk>return</jk> <jv>defaults</jv>;
 * </p>
 *
 * <h5 class='section'>Comparison with Map.of():</h5>
 * <ul class='spaced-list'>
 *    <li><b>Null Support:</b> SimpleMap supports null keys/values, Map.of() does not
 *    <li><b>Insertion Order:</b> SimpleMap preserves insertion order, Map.of() does not
 *    <li><b>Size Limit:</b> Map.of() limited to 10 entries, SimpleMap has no limit
 * </ul>
 *
 * <h5 class='section'>See Also:</h5>
 * <ul>
 *    <li class='jc'>{@link SimpleMap} - The modifiable version of this class
 *    <li class='link'><a class="doclink" href="../../../../../index.html#juneau-commons">Overview &gt; juneau-commons</a>
 * </ul>
 *
 * @param <K> The key type.
 * @param <V> The value type.
 */
public class SimpleMap<K,V> extends AbstractMap<K,V> {

   /**
    * Inner class representing a single key-value entry in this map.
    * <p>
    * This entry is unmodifiable - calling {@link #setValue(Object)} will throw
    * {@link UnsupportedOperationException}.
    */
   class SimpleUnmodifiableMapEntry implements Map.Entry<K,V> {

      /** The index into the keys/values arrays for this entry. */
      private final int index;

      /**
       * Constructor.
       * @param index The array index for this entry.
       */
      SimpleUnmodifiableMapEntry(int index) {
         this.index = index;
      }

      @Override /* Map.Entry */
      public K getKey() { return keys[index]; }

      @Override /* Map.Entry */
      public V getValue() { return values[index]; }

      @Override /* Map.Entry */
      public V setValue(V val) {
         throw unsupportedOp("Map is unmodifiable");
      }

      @Override
      public String toString() {
         return getKey() + "=" + getValue();
      }

      @Override
      public boolean equals(Object o) {
         return (o instanceof Map.Entry<?,?> o2) && Utils.eq(this, o2, (x,y) -> Utils.eq(x.getKey(), y.getKey()) && Utils.eq(x.getValue(), y.getValue()));
      }

      @Override
      public int hashCode() {
         K key = getKey();
         V value = getValue();
         return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
      }
   }

   /** The array of keys. Keys are immutable after construction. */
   final K[] keys;

   /** The array of values. Values are immutable after construction. */
   final V[] values;

   /** Pre-constructed entries array for {@link #entrySet()}. */
   final SimpleUnmodifiableMapEntry[] entries;

   /**
    * Constructs a new SimpleMap with the specified keys and values.
    *
    * <p>
    * The keys and values arrays are stored by reference (not copied), so external
    * modifications to the arrays after construction will affect the map's behavior.
    * For true immutability, ensure the arrays are not modified after passing them
    * to this constructor.
    *
    * <h5 class='section'>Example:</h5>
    * <p class='bjava'>
    *    String[] <jv>keys</jv> = {<js>"name"</js>, <js>"age"</js>, <js>"city"</js>};
    *    Object[] <jv>values</jv> = {<js>"John"</js>, 30, <js>"NYC"</js>};
    *
    *    SimpleMap&lt;String,Object&gt; <jv>person</jv> = <jk>new</jk> SimpleMap&lt;&gt;(<jv>keys</jv>, <jv>values</jv>);
    * </p>
    *
    * @param keys The map keys. Must not be <jk>null</jk>. Individual keys can be <jk>null</jk>.
    * @param values The map values. Must not be <jk>null</jk> and must have the same length as keys.
    *               Individual values can be <jk>null</jk>.
    * @throws IllegalArgumentException if:
    *         <ul>
    *          <li>The keys array is <jk>null</jk>
    *          <li>The values array is <jk>null</jk>
    *          <li>The keys and values arrays have different lengths
    *          <li>Any key appears more than once (duplicate keys)
    *         </ul>
    */
   @SuppressWarnings("unchecked")
   public SimpleMap(K[] keys, V[] values) {
      assertArgsNotNull("keys", keys, "values", values);
      assertArg(keys.length == values.length, "keys ''{0}'' and values ''{1}'' array lengths differ", keys.length, values.length);

      // Check for duplicate keys
      for (var i = 0; i < keys.length; i++) {
         for (var j = i + 1; j < keys.length; j++) {
            if (Utils.eq(keys[i], keys[j])) {
               throw illegalArg("Duplicate key found: {0}", keys[i]);
            }
         }
      }

      this.keys = keys;
      this.values = values;
      entries = (SimpleUnmodifiableMapEntry[])Array.newInstance(SimpleUnmodifiableMapEntry.class, keys.length);
      for (var i = 0; i < keys.length; i++) {
         entries[i] = new SimpleUnmodifiableMapEntry(i);
      }
   }

   /**
    * Returns a {@link Set} view of the mappings contained in this map.
    *
    * <p>
    * The returned set is unmodifiable and backed by the underlying entries array.
    *
    * <h5 class='section'>Example:</h5>
    * <p class='bjava'>
    *    SimpleMap&lt;String,Integer&gt; <jv>map</jv> = <jk>new</jk> SimpleMap&lt;&gt;(
    *       <jk>new</jk> String[]{<js>"a"</js>, <js>"b"</js>},
    *       <jk>new</jk> Integer[]{1, 2}
    *    );
    *
    *    <jk>for</jk> (Map.Entry&lt;String,Integer&gt; <jv>entry</jv> : <jv>map</jv>.entrySet()) {
    *       System.<jsf>out</jsf>.println(<jv>entry</jv>.getKey() + <js>"="</js> + <jv>entry</jv>.getValue());
    *    }
    * </p>
    *
    * @return An unmodifiable set view of the mappings in this map.
    */
   @Override /* Map */
   public Set<Map.Entry<K,V>> entrySet() {
      return Collections.unmodifiableSet(toSet(entries));
   }

   /**
    * Returns the value associated with the specified key.
    *
    * <p>
    * This method performs a linear search through the keys array, using {@link Object#equals(Object)}
    * for comparison. For small maps (typically less than 10 entries), this is often faster than
    * the hash lookup in {@link HashMap}.
    *
    * <h5 class='section'>Example:</h5>
    * <p class='bjava'>
    *    SimpleMap&lt;String,Integer&gt; <jv>map</jv> = <jk>new</jk> SimpleMap&lt;&gt;(
    *       <jk>new</jk> String[]{<js>"age"</js>, <js>"score"</js>},
    *       <jk>new</jk> Integer[]{25, 100}
    *    );
    *
    *    Integer <jv>age</jv> = <jv>map</jv>.get(<js>"age"</js>);     <jc>// Returns: 25</jc>
    *    Integer <jv>height</jv> = <jv>map</jv>.get(<js>"height"</js>); <jc>// Returns: null (key not found)</jc>
    * </p>
    *
    * @param key The key whose associated value is to be returned.
    * @return The value associated with the specified key, or <jk>null</jk> if the key is not found
    *         (or if the key is mapped to <jk>null</jk>).
    */
   @Override /* Map */
   public V get(Object key) {
      for (var i = 0; i < keys.length; i++)
         if (Utils.eq(keys[i], key))
            return values[i];
      return null;
   }

   /**
    * Returns a {@link Set} view of the keys contained in this map.
    *
    * <p>
    * The returned set is unmodifiable and backed by the underlying keys array.
    *
    * <h5 class='section'>Example:</h5>
    * <p class='bjava'>
    *    SimpleMap&lt;String,Integer&gt; <jv>map</jv> = <jk>new</jk> SimpleMap&lt;&gt;(
    *       <jk>new</jk> String[]{<js>"x"</js>, <js>"y"</js>, <js>"z"</js>},
    *       <jk>new</jk> Integer[]{1, 2, 3}
    *    );
    *
    *    Set&lt;String&gt; <jv>keys</jv> = <jv>map</jv>.keySet();  <jc>// Returns: [x, y, z]</jc>
    * </p>
    *
    * @return An unmodifiable set view of the keys in this map.
    */
   @Override /* Map */
   public Set<K> keySet() {
      return Collections.unmodifiableSet(toSet(keys));
   }

   /**
    * Throws {@link UnsupportedOperationException} as this map is unmodifiable.
    *
    * @param key Ignored.
    * @param value Ignored.
    * @return Never returns normally.
    * @throws UnsupportedOperationException Always thrown, as this map cannot be modified.
    */
   @Override /* Map */
   public V put(K key, V value) {
      throw unsupportedOp("Map is unmodifiable");
   }

   /**
    * Returns a string representation of this map.
    *
    * <p>
    * The format follows the standard Java map convention: <c>"{key1=value1, key2=value2, ...}"</c>
    *
    * @return A string representation of this map.
    */
   @Override
   public String toString() {
      return entrySet().stream().map(Object::toString).collect(Collectors.joining(", ", "{", "}"));
   }

   /**
    * Compares the specified object with this map for equality.
    *
    * <p>
    * Returns <jk>true</jk> if the given object is also a map and the two maps represent the same
    * mappings. More formally, two maps <c>m1</c> and <c>m2</c> represent the same mappings if
    * <c>m1.entrySet().equals(m2.entrySet())</c>.
    *
    * <p>
    * This implementation compares the entry sets of the two maps.
    *
    * @param o Object to be compared for equality with this map.
    * @return <jk>true</jk> if the specified object is equal to this map.
    */
   @Override
   public boolean equals(Object o) {
      return (o instanceof Map o2) && Utils.eq(this, o2, (x, y) -> x.entrySet().equals(y.entrySet()));
   }

   /**
    * Returns the hash code value for this map.
    *
    * <p>
    * The hash code of a map is defined to be the sum of the hash codes of each entry in the map's
    * <c>entrySet()</c> view. This ensures that <c>m1.equals(m2)</c> implies that
    * <c>m1.hashCode()==m2.hashCode()</c> for any two maps <c>m1</c> and <c>m2</c>, as required
    * by the general contract of {@link Object#hashCode()}.
    *
    * <p>
    * This implementation computes the hash code from the entry set.
    *
    * @return The hash code value for this map.
    */
   @Override
   public int hashCode() {
      return entrySet().hashCode();
   }
}