/*
 * Copyright 2000-2004 The Apache Software Foundation.
 * 
 * Licensed 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.jetspeed.util;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
/**
 * <code>HashCode</code> generation routines.
 * <p>
 * This class enables a good hashcode to be built for any class. It follows
 * the rules laid out in the book Effective Java, by Joshua Bloch. Writing a 
 * good hashCode is actually quite difficult. This class aims to simplify the 
 * process.
 * <p>
 * All relevant fields from the object should be included in the hashCode. Derived
 * fields may be excluded. In general, any field used in the equals method must be
 * used in the hashCode method. 
 * <p>
 * To use this class write code as follows:
 * <pre>
 * public class Person {
 *   String name;
 *   int age;
 *   boolean isSmoker;
 *   ...
 * 
 *   public int hashCode() {
 *     // you pick a hard-coded, randomly chosen, non-zero, odd number
 *     // ideally different for each class
 *     return new HashCodeBuilder(17, 37).   
 *       append(name).
 *       append(age).
 *       append(smoker).
 *       toHashCode();
 *   }
 * }
 * </pre>
 * <p>
 * Alternatively, there is a method that uses reflection to determine
 * the fields to test. Because these fields are usually private, the method, 
 * <code>reflectionHashCode</code>, uses <code>Field.setAccessible</code> to
 * change the visibility of the fields. This will fail under a security manager, 
 * unless the appropriate permissions are set. It is also slower than testing 
 * explicitly.
 * <p>
 * A typical invocation for this method would look like:
 * <pre>
 * public boolean hashCode(Object o) {
 *   return HashCodeBuilder.reflectionHashCode(this);
 * }
 * </pre>
 * 
 * @author <a href="mailto:scolebourne@joda.org">Stephen Colebourne</a>
 * @version $Id: HashCodeBuilder.java,v 1.2 2004/03/08 00:39:32 jford Exp $
 */
public class HashCodeBuilder
{

    /**
     * Constant to use in building the hashCode
     */
    private final int iConstant;
    /**
     * Running total of the hashCode
     */
    private int iTotal = 0;

    /**
     * Constructor for HashCodeBuilder.
     * This constructor uses two hard coded choices for the constants needed
     * to build a hashCode.
     */
    public HashCodeBuilder()
    {
        super();
        iConstant = 37;
        iTotal = 17;
    }

    /**
     * Constructor for HashCodeBuilder.
     * Two randomly chosen, non-zero, odd numbers must be passed in. Ideally
     * these should be different for each class, however this is not vital.
     * Prime numbers are preferred, especially for the multiplier.
     * 
     * @param initialNonZeroOddNumber  a non-zero, odd number used as the initial value
     * @param multiplierNonZeroOddNumber  a non-zero, odd number used as the multiplier
     * @throws IllegalArgumentException if the number is zero or even
     */
    public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber)
    {
        super();
        if (initialNonZeroOddNumber == 0)
        {
            throw new IllegalArgumentException("HashCodeBuilder requires a non zero initial value");
        }
        if (initialNonZeroOddNumber % 2 == 0)
        {
            throw new IllegalArgumentException("HashCodeBuilder requires an odd initial value");
        }
        if (multiplierNonZeroOddNumber == 0)
        {
            throw new IllegalArgumentException("HashCodeBuilder requires a non zero multiplier");
        }
        if (multiplierNonZeroOddNumber % 2 == 0)
        {
            throw new IllegalArgumentException("HashCodeBuilder requires an odd multiplier");
        }
        iConstant = multiplierNonZeroOddNumber;
        iTotal = initialNonZeroOddNumber;
    }

    //-------------------------------------------------------------------------

    /**
     * This method uses reflection to build a valid hash code. 
     * <p>
     * It uses Field.setAccessible to gain access to private fields. This means
     * that it will throw a security exception if run under a security manger, if
     * the permissions are not set up.
     * It is also not as efficient as testing explicitly. 
     * Transient members will be not be used, as they are likely derived 
     * fields, and not part of the value of the object. 
     * Static fields will not be tested.
     * This constructor uses two hard coded choices for the constants needed
     * to build a hash code.
     * 
     * @param object  the object to create a hash code for
     * @return int hash code
     * @throws IllegalArgumentException if the object is null
     */
    public static int reflectionHashCode(Object object)
    {
        return reflectionHashCode(object, false);
    }

    /**
     * This method uses reflection to build a valid hash code. 
     * <p>
     * It uses Field.setAccessible to gain access to private fields. This means
     * that it will throw a security exception if run under a security manger, if
     * the permissions are not set up.
     * It is also not as efficient as testing explicitly. 
     * If the TestTransients parameter is set to true, transient members will be
     * tested, otherwise they are ignored, as they are likely derived fields, and
     * not part of the value of the object. 
     * Static fields will not be tested.
     * This constructor uses two hard coded choices for the constants needed
     * to build a hash code.
     * 
     * @param object  the object to create a hash code for
     * @param testTransients  whether to include transient fields
     * @return int hash code
     * @throws IllegalArgumentException if the object is null
     */
    public static int reflectionHashCode(Object object, boolean testTransients)
    {
        return reflectionHashCode(17, 37, object, testTransients);
    }

    /**
     * This method uses reflection to build a valid hash code. 
     * <p>
     * It uses Field.setAccessible to gain access to private fields. This means
     * that it will throw a security exception if run under a security manger, if
     * the permissions are not set up.
     * It is also not as efficient as testing explicitly. 
     * Transient members will be not be used, as they are likely derived 
     * fields, and not part of the value of the object. 
     * Static fields will not be tested.
     * <p>
     * Two randomly chosen, non-zero, odd numbers must be passed in. Ideally
     * these should be different for each class, however this is not vital.
     * Prime numbers are preferred, especially for the multiplier.
     * 
     * @param initialNonZeroOddNumber  a non-zero, odd number used as the initial value
     * @param multiplierNonZeroOddNumber  a non-zero, odd number used as the multiplier
     * @param object  the object to create a hash code for
     * @return int hash code
     * @throws IllegalArgumentException if the object is null
     * @throws IllegalArgumentException if the number is zero or even
     */
    public static int reflectionHashCode(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber, Object object)
    {
        return reflectionHashCode(initialNonZeroOddNumber, multiplierNonZeroOddNumber, object, false);
    }

    /**
     * This method uses reflection to build a valid hash code. 
     * <p>
     * It uses Field.setAccessible to gain access to private fields. This means
     * that it will throw a security exception if run under a security manger, if
     * the permissions are not set up.
     * It is also not as efficient as testing explicitly. 
     * If the TestTransients parameter is set to true, transient members will be
     * tested, otherwise they are ignored, as they are likely derived fields, and
     * not part of the value of the object. 
     * Static fields will not be tested.
     * <p>
     * Two randomly chosen, non-zero, odd numbers must be passed in. Ideally
     * these should be different for each class, however this is not vital.
     * Prime numbers are preferred, especially for the multiplier.
     * 
     * @param initialNonZeroOddNumber
     * @param multiplierNonZeroOddNumber
     * @param object  the object to create a hash code for
     * @param testTransients  whether to include transient fields
     * @return int hash code
     * @throws IllegalArgumentException if the object is null
     * @throws IllegalArgumentException if the number is zero or even
     */
    public static int reflectionHashCode(
        int initialNonZeroOddNumber,
        int multiplierNonZeroOddNumber,
        Object object,
        boolean testTransients)
    {

        if (object == null)
        {
            throw new IllegalArgumentException("The object to build a hash code for must not be null");
        }
        HashCodeBuilder hashCodeBuilder = new HashCodeBuilder(initialNonZeroOddNumber, multiplierNonZeroOddNumber);
        Field[] fields = object.getClass().getDeclaredFields();
        Field.setAccessible(fields, true);
        for (int i = 0; i < fields.length; ++i)
        {
            Field f = fields[i];
            if (testTransients || !Modifier.isTransient(f.getModifiers()))
            {
                if (!Modifier.isStatic(f.getModifiers()))
                {
                    try
                    {
                        hashCodeBuilder.append(f.get(object));
                    }
                    catch (IllegalAccessException e)
                    {
                        //this can't happen. Would get a Security exception instead
                        //throw a runtime exception in case the impossible happens.
                        throw new InternalError("Unexpected IllegalAccessException");
                    }
                }
            }
        }
        return hashCodeBuilder.toHashCode();
    }

    //-------------------------------------------------------------------------

    /**
     * Append a hashCode for an Object.
     *
     * @param object  the object to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(Object object)
    {
        if (object == null)
        {
            iTotal = iTotal * iConstant;

        }
        else
        {
            if (object.getClass().isArray() == false)
            {
                //the simple case, not an array, just the element 
                iTotal = iTotal * iConstant + object.hashCode();

            }
            else
            {
                //'Switch' on type of array, to dispatch to the correct handler
                // This handles multi dimensional arrays
                if (object instanceof long[])
                {
                    append((long[]) object);
                }
                else if (object instanceof int[])
                {
                    append((int[]) object);
                }
                else if (object instanceof short[])
                {
                    append((short[]) object);
                }
                else if (object instanceof char[])
                {
                    append((char[]) object);
                }
                else if (object instanceof byte[])
                {
                    append((byte[]) object);
                }
                else if (object instanceof double[])
                {
                    append((double[]) object);
                }
                else if (object instanceof float[])
                {
                    append((float[]) object);
                }
                else if (object instanceof boolean[])
                {
                    append((boolean[]) object);
                }
                else
                {
                    // Not an array of primitives
                    append((Object[]) object);
                }
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a long.
     *
     * @param value  the long to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(long value)
    {
        iTotal = iTotal * iConstant + ((int) (value ^ (value >> 32)));
        return this;
    }

    /**
     * Append a hashCode for an int.
     *
     * @param value  the int to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(int value)
    {
        iTotal = iTotal * iConstant + value;
        return this;
    }

    /**
     * Append a hashCode for a short.
     *
     * @param value  the short to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(short value)
    {
        iTotal = iTotal * iConstant + (int) value;
        return this;
    }

    /**
     * Append a hashCode for a char.
     *
     * @param value  the char to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(char value)
    {
        iTotal = iTotal * iConstant + (int) value;
        return this;
    }

    /**
     * Append a hashCode for a byte.
     *
     * @param value  the byte to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(byte value)
    {
        iTotal = iTotal * iConstant + (int) value;
        return this;
    }

    /**
     * Append a hashCode for a double.
     *
     * @param value  the double to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(double value)
    {
        return append(Double.doubleToLongBits(value));
    }

    /**
     * Append a hashCode for a float.
     *
     * @param value  the float to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(float value)
    {
        iTotal = iTotal * iConstant + Float.floatToIntBits(value);
        return this;
    }

    /**
     * Append a hashCode for a long.
     *
     * @param value  the long to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(boolean value)
    {
        iTotal = iTotal * iConstant + (value ? 0 : 1);
        return this;
    }

    /**
     * Append a hashCode for an Object array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(Object[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a long array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(long[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for an int array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(int[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a short array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(short[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a char array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(char[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a byte array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(byte[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a double array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(double[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a float array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(float[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Append a hashCode for a boolean array.
     *
     * @param array  the array to add to the hashCode
     * @return this
     */
    public HashCodeBuilder append(boolean[] array)
    {
        if (array == null)
        {
            iTotal = iTotal * iConstant;
        }
        else
        {
            for (int i = 0; i < array.length; i++)
            {
                append(array[i]);
            }
        }
        return this;
    }

    /**
     * Return the computed hashCode
     * 
     * @return int hashCode based on the fields appended
     */
    public int toHashCode()
    {
        return iTotal;
    }

}