//
//  ========================================================================
//  Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//  ------------------------------------------------------------------------
//  All rights reserved. This program and the accompanying materials
//  are made available under the terms of the Eclipse Public License v1.0
//  and Apache License v2.0 which accompanies this distribution.
//
//      The Eclipse Public License is available at
//      http://www.eclipse.org/legal/epl-v10.html
//
//      The Apache License v2.0 is available at
//      http://www.opensource.org/licenses/apache2.0.php
//
//  You may elect to redistribute this code under either of these licenses.
//  ========================================================================
//

package org.eclipse.jetty.util;

import java.util.AbstractList;
import java.util.NoSuchElementException;
import java.util.Queue;

/* ------------------------------------------------------------ */
/**
 * Queue backed by circular array.
 * <p/>
 * This partial Queue implementation (also with {@link #remove()} for stack operation)
 * is backed by a growable circular array.
 *
 * @param <E>
 */
public class ArrayQueue<E> extends AbstractList<E> implements Queue<E>
{
    public static final int DEFAULT_CAPACITY = 64;
    public static final int DEFAULT_GROWTH = 32;

    protected final Object _lock;
    protected final int _growCapacity;
    protected Object[] _elements;
    protected int _nextE;
    protected int _nextSlot;
    protected int _size;

    /* ------------------------------------------------------------ */
    public ArrayQueue()
    {
        this(DEFAULT_CAPACITY, -1);
    }
    
    /* ------------------------------------------------------------ */
    public ArrayQueue(Object lock)
    {
        this(DEFAULT_CAPACITY, -1,lock);
    }

    /* ------------------------------------------------------------ */
    public ArrayQueue(int capacity)
    {
        this(capacity, -1);
    }

    /* ------------------------------------------------------------ */
    public ArrayQueue(int initCapacity, int growBy)
    {
        this(initCapacity, growBy, null);
    }

    /* ------------------------------------------------------------ */
    public ArrayQueue(int initCapacity, int growBy, Object lock)
    {
        _lock = lock == null ? this : lock;
        _growCapacity = growBy;
        _elements = new Object[initCapacity];
    }
    
    /* ------------------------------------------------------------ */
    public Object lock()
    {
        return _lock;
    }
    
    /* ------------------------------------------------------------ */
    public int getCapacity()
    {
        synchronized (_lock)
        {
            return _elements.length;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public boolean add(E e)
    {
        if (!offer(e))
            throw new IllegalStateException("Full");
        return true;
    }

    /* ------------------------------------------------------------ */
    public boolean offer(E e)
    {
        synchronized (_lock)
        {
            return enqueue(e);
        }
    }

    /* ------------------------------------------------------------ */
    private boolean enqueue(E e)
    {
        if (_size == _elements.length && !grow())
            return false;

        _size++;
        _elements[_nextSlot++] = e;
        if (_nextSlot == _elements.length)
            _nextSlot = 0;

        return true;
    }

    /* ------------------------------------------------------------ */
    /**
     * Add without synchronization or bounds checking
     *
     * @param e the element to add
     * @see #add(Object)
     */
    public void addUnsafe(E e)
    {
        if (!enqueue(e))
            throw new IllegalStateException("Full");
    }

    /* ------------------------------------------------------------ */
    public E element()
    {
        synchronized (_lock)
        {
            if (isEmpty())
                throw new NoSuchElementException();
            return at(_nextE);
        }
    }

    /* ------------------------------------------------------------ */
    @SuppressWarnings("unchecked")
    private E at(int index)
    {
        return (E)_elements[index];
    }

    /* ------------------------------------------------------------ */
    public E peek()
    {
        synchronized (_lock)
        {
            if (_size == 0)
                return null;
            return at(_nextE);
        }
    }

    /* ------------------------------------------------------------ */
    public E peekUnsafe()
    {
        if (_size == 0)
            return null;
        return at(_nextE);
    }

    /* ------------------------------------------------------------ */
    public E poll()
    {
        synchronized (_lock)
        {
            if (_size == 0)
                return null;
            return dequeue();
        }
    }
    
    /* ------------------------------------------------------------ */
    public E pollUnsafe()
    {
        if (_size == 0)
            return null;
        return dequeue();
    }

    /* ------------------------------------------------------------ */
    private E dequeue()
    {
        E e = at(_nextE);
        _elements[_nextE] = null;
        _size--;
        if (++_nextE == _elements.length)
            _nextE = 0;
        return e;
    }

    /* ------------------------------------------------------------ */
    public E remove()
    {
        synchronized (_lock)
        {
            if (_size == 0)
                throw new NoSuchElementException();
            return dequeue();
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public void clear()
    {
        synchronized (_lock)
        {
            _size = 0;
            _nextE = 0;
            _nextSlot = 0;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public boolean isEmpty()
    {
        synchronized (_lock)
        {
            return _size == 0;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public int size()
    {
        synchronized (_lock)
        {
            return _size;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public E get(int index)
    {
        synchronized (_lock)
        {
            if (index < 0 || index >= _size)
                throw new IndexOutOfBoundsException("!(" + 0 + "<" + index + "<=" + _size + ")");
            return getUnsafe(index);
        }
    }

    /* ------------------------------------------------------------ */
    /**
     * Get without synchronization or bounds checking.
     *
     * @param  index index of the element to return
     * @return the element at the specified index
     * @see #get(int)
     */
    public E getUnsafe(int index)
    {
        int i = (_nextE + index) % _elements.length;
        return at(i);
    }

    /* ------------------------------------------------------------ */
    @Override
    public E remove(int index)
    {
        synchronized (_lock)
        {
            if (index < 0 || index >= _size)
                throw new IndexOutOfBoundsException("!(" + 0 + "<" + index + "<=" + _size + ")");

            int i = (_nextE + index) % _elements.length;
            E old = at(i);

            if (i < _nextSlot)
            {
                // 0                         _elements.length
                //       _nextE........._nextSlot
                System.arraycopy(_elements, i + 1, _elements, i, _nextSlot - i);
                _nextSlot--;
                _size--;
            }
            else
            {
                // 0                         _elements.length
                // ......_nextSlot   _nextE..........
                System.arraycopy(_elements, i + 1, _elements, i, _elements.length - i - 1);
                if (_nextSlot > 0)
                {
                    _elements[_elements.length - 1] = _elements[0];
                    System.arraycopy(_elements, 1, _elements, 0, _nextSlot - 1);
                    _nextSlot--;
                }
                else
                    _nextSlot = _elements.length - 1;

                _size--;
            }

            return old;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public E set(int index, E element)
    {
        synchronized (_lock)
        {
            if (index < 0 || index >= _size)
                throw new IndexOutOfBoundsException("!(" + 0 + "<" + index + "<=" + _size + ")");

            int i = _nextE + index;
            if (i >= _elements.length)
                i -= _elements.length;
            E old = at(i);
            _elements[i] = element;
            return old;
        }
    }

    /* ------------------------------------------------------------ */
    @Override
    public void add(int index, E element)
    {
        synchronized (_lock)
        {
            if (index < 0 || index > _size)
                throw new IndexOutOfBoundsException("!(" + 0 + "<" + index + "<=" + _size + ")");

            if (_size == _elements.length && !grow())
                throw new IllegalStateException("Full");

            if (index == _size)
            {
                add(element);
            }
            else
            {
                int i = _nextE + index;
                if (i >= _elements.length)
                    i -= _elements.length;

                _size++;
                _nextSlot++;
                if (_nextSlot == _elements.length)
                    _nextSlot = 0;

                if (i < _nextSlot)
                {
                    // 0                         _elements.length
                    //       _nextE.....i..._nextSlot
                    // 0                         _elements.length
                    // ..i..._nextSlot   _nextE..........
                    System.arraycopy(_elements, i, _elements, i + 1, _nextSlot - i);
                    _elements[i] = element;
                }
                else
                {
                    // 0                         _elements.length
                    // ......_nextSlot   _nextE.....i....
                    if (_nextSlot > 0)
                    {
                        System.arraycopy(_elements, 0, _elements, 1, _nextSlot);
                        _elements[0] = _elements[_elements.length - 1];
                    }

                    System.arraycopy(_elements, i, _elements, i + 1, _elements.length - i - 1);
                    _elements[i] = element;
                }
            }
        }
    }

    /* ------------------------------------------------------------ */
    protected boolean grow()
    {
        synchronized (_lock)
        {
            if (_growCapacity <= 0)
                return false;

            Object[] elements = new Object[_elements.length + _growCapacity];

            int split = _elements.length - _nextE;
            if (split > 0)
                System.arraycopy(_elements, _nextE, elements, 0, split);
            if (_nextE != 0)
                System.arraycopy(_elements, 0, elements, split, _nextSlot);

            _elements = elements;
            _nextE = 0;
            _nextSlot = _size;
            return true;
        }
    }
}
