/*
 * 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.lang;

import static java.util.Arrays.*;

import java.lang.annotation.*;
import java.util.*;

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

/**
 * Utility class for generating integer hash codes.
 *
 * <p>
 * General usage:
 * <p class='bjava'>
 *    <jk>int</jk> <jv>hashCode</jv> = HashCode.<jsm>create</jsm>().add(<js>"foobar"</js>).add(<jv>myobject</jv>).add(123).get();
 * </p>
 *
 */
public class HashCode {

   /**
    * Create a new HashCode object.
    *
    * @return A new HashCode object.
    */
   public static final HashCode create() {
      return new HashCode();
   }

   /**
    * Calculates a hash code over the specified objects.
    *
    * <p>
    * Uses the same algorithm as {@link java.util.Objects#hash(Object...)} (31 * result + element hash).
    *
    * <p>
    * Special handling is provided for:
    * <ul>
    *    <li><b>Annotations:</b> Uses {@link AnnotationUtils#hash(Annotation)} to ensure consistent hashing
    *       according to the {@link java.lang.annotation.Annotation#hashCode()} contract.
    *    <li><b>Arrays:</b> Uses content-based hashing via {@link java.util.Arrays#hashCode(Object[])}
    *       instead of identity-based hashing.
    *    <li><b>Null values:</b> Treated as 0 in the hash calculation.
    * </ul>
    *
    * @param objects The objects to calculate a hashcode over.
    * @return A numerical hashcode value.
    * @see #add(Object)
    * @see AnnotationUtils#hash(Annotation)
    * @see java.util.Objects#hash(Object...)
    */
   public static final int of(Object...objects) {
      HashCode x = create();
      for (var oo : objects)
         x.add(oo);
      return x.get();
   }

   private int hashCode = 1;

   /**
    * Hashes the hashcode into this object.
    *
    * <p>
    * The formula is simply <c>hashCode = 31*hashCode + i;</c>
    *
    * @param i The hashcode to hash into this object's hashcode.
    * @return This object.
    */
   public HashCode add(int i) {
      hashCode = 31 * hashCode + i;
      return this;
   }

   /**
    * Hashes the hashcode of the specified object into this object.
    *
    * <p>
    * The formula is <c>hashCode = 31*hashCode + elementHash;</c>
    *
    * <p>
    * Special handling is provided for:
    * <ul>
    *    <li><b>Null values:</b> Adds 0 to the hash code.
    *    <li><b>Annotations:</b> Uses {@link AnnotationUtils#hash(Annotation)} to ensure consistent hashing
    *       according to the {@link java.lang.annotation.Annotation#hashCode()} contract.
    *    <li><b>Arrays:</b> Uses content-based hashing via {@link java.util.Arrays#hashCode(Object[])}
    *       instead of identity-based hashing. Supports all primitive array types and object arrays.
    *    <li><b>Other objects:</b> Uses the object's {@link Object#hashCode()} method.
    * </ul>
    *
    * @param o The object whose hashcode will be hashed with this object.
    * @return This object.
    * @see AnnotationUtils#hash(Annotation)
    * @see java.util.Arrays#hashCode(Object[])
    */
   public HashCode add(Object o) {
      o = unswap(o);
      if (o == null) {
         add(0);
      } else if (o instanceof Annotation a) {
         add(AnnotationUtils.hash(a));
      } else if (o.getClass().isArray()) {
         // Use content-based hashcode for arrays
         if (o instanceof Object[] o2)
            add(deepHashCode(o2));
         else if (o instanceof int[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof long[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof short[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof byte[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof char[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof boolean[] o2)
            add(Arrays.hashCode(o2));
         else if (o instanceof float[] o2)
            add(java.util.Arrays.hashCode(o2));
         else if (o instanceof double[] o2)
            add(java.util.Arrays.hashCode(o2));
      } else {
         add(o.hashCode());
      }
      return this;
   }

   /**
    * Return the calculated hashcode value.
    *
    * @return The calculated hashcode.
    */
   public int get() {
      return hashCode;
   }

   /**
    * Converts the object to a normalized form before grabbing it's hashcode.
    *
    * <p>
    * Subclasses can override this method to provide specialized handling (e.g. converting numbers to strings so that
    * <c>123</c> and <js>"123"</js> end up creating the same hashcode.)
    *
    * <p>
    * Default implementation does nothing.
    *
    * @param o The object to normalize before getting it's hashcode.
    * @return The normalized object.
    */
   protected Object unswap(Object o) {
      return o;
   }
}