/*
    *  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.mina.util;
  
  import java.io.Serializable;
  import java.util.AbstractList;
  import java.util.Arrays;
  import java.util.NoSuchElementException;
  import java.util.Queue;
  
  /**
   * A unbounded circular queue based on array.
   * 
   * @author <a href="http://mina.apache.org">Apache MINA Project</a>
   */
  public class CircularQueue<E> extends AbstractList<E> implements Queue<E>, Serializable {
      /** The serialVersionUID : mandatory for serializable classes */
      private static final long serialVersionUID = 3993421269224511264L;
  
      /** Minimal size of the underlying array */
      private static final int DEFAULT_CAPACITY = 4;
  
      /** The initial capacity of the list */
      private final int initialCapacity;
      
      // XXX: This volatile keyword here is a workaround for SUN Java Compiler bug,
      //      which produces buggy byte code.  I don't event know why adding a volatile
      //      fixes the problem.  Eclipse Java Compiler seems to produce correct byte code.
      private volatile Object[] items;
      private int mask;
      private int first = 0;
      private int last = 0;
      private boolean full;
      private int shrinkThreshold;
  
      /**
       * Construct a new, empty queue.
       */
      public CircularQueue() {
          this(DEFAULT_CAPACITY);
      }
      
      public CircularQueue(int initialCapacity) {
          int actualCapacity = normalizeCapacity(initialCapacity);
          items = new Object[actualCapacity];
          mask = actualCapacity - 1;
          this.initialCapacity = actualCapacity;
          this.shrinkThreshold = 0;
      }
  
      /**
       * The capacity must be a power of 2.
       */
      private static int normalizeCapacity(int initialCapacity) {
          int actualCapacity = 1;
          
          while (actualCapacity < initialCapacity) {
              actualCapacity <<= 1;
              if (actualCapacity < 0) {
                  actualCapacity = 1 << 30;
                  break;
              }
          }
          return actualCapacity;
      }
  
      /**
       * Returns the capacity of this queue.
       */
      public int capacity() {
          return items.length;
      }
  
      @Override
      public void clear() {
          if (!isEmpty()) {
              Arrays.fill(items, null);
              first = 0;
              last = 0;
              full = false;
              shrinkIfNeeded();
          }
     }
 
     @SuppressWarnings("unchecked")
     public E poll() {
         if (isEmpty()) {
             return null;
         }
 
         Object ret = items[first];
         items[first] = null;
         decreaseSize();
         
         if (first == last) {
             first = last = 0;
         }
 
         shrinkIfNeeded();
         return (E) ret;
     }
 
     public boolean offer(E item) {
         if (item == null) {
             throw new IllegalArgumentException("item");
         }
         
         expandIfNeeded();
         items[last] = item;
         increaseSize();
         return true;
     }
 
     @SuppressWarnings("unchecked")
     public E peek() {
         if (isEmpty()) {
             return null;
         }
 
         return (E) items[first];
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public E get(int idx) {
         checkIndex(idx);
         return (E) items[getRealIndex(idx)];
     }
 
     @Override
     public boolean isEmpty() {
         return (first == last) && !full;
     }
 
     @Override
     public int size() {
         if (full) {
             return capacity();
         }
         
         if (last >= first) {
             return last - first;
         }
 
         return last - first + capacity();
     }
     
     @Override
     public String toString() {
         return "first=" + first + ", last=" + last + ", size=" + size()
                 + ", mask = " + mask;
     }
 
     private void checkIndex(int idx) {
         if (idx < 0 || idx >= size()) {
             throw new IndexOutOfBoundsException(String.valueOf(idx));
         }
     }
 
     private int getRealIndex(int idx) {
         return (first + idx) & mask;
     }
 
     private void increaseSize() {
         last = (last + 1) & mask;
         full = first == last;
     }
 
     private void decreaseSize() {
         first = (first + 1) & mask;
         full = false;
     }
 
     private void expandIfNeeded() {
         if (full) {
             // expand queue
             final int oldLen = items.length;
             final int newLen = oldLen << 1;
             Object[] tmp = new Object[newLen];
     
             if (first < last) {
                 System.arraycopy(items, first, tmp, 0, last - first);
             } else {
                 System.arraycopy(items, first, tmp, 0, oldLen - first);
                 System.arraycopy(items, 0, tmp, oldLen - first, last);
             }
     
             first = 0;
             last = oldLen;
             items = tmp;
             mask = tmp.length - 1;
             if (newLen >>> 3 > initialCapacity) {
                 shrinkThreshold = newLen >>> 3;
             }
         }
     }
     
     private void shrinkIfNeeded() {
         int size = size();
         if (size <= shrinkThreshold) {
             // shrink queue
             final int oldLen = items.length;
             int newLen = normalizeCapacity(size);
             if (size == newLen) {
                 newLen <<= 1;
             }
             
             if (newLen >= oldLen) {
                 return;
             }
             
             if (newLen < initialCapacity) {
                 if (oldLen == initialCapacity) {
                     return;
                 }
 
                 newLen = initialCapacity;
             }
             
             Object[] tmp = new Object[newLen];
     
             // Copy only when there's something to copy.
             if (size > 0) {
                 if (first < last) {
                     System.arraycopy(items, first, tmp, 0, last - first);
                 } else {
                     System.arraycopy(items, first, tmp, 0, oldLen - first);
                     System.arraycopy(items, 0, tmp, oldLen - first, last);
                 }
             }
     
             first = 0;
             last = size;
             items = tmp;
             mask = tmp.length - 1;
             shrinkThreshold = 0;
         }
     }
 
     @Override
     public boolean add(E o) {
         return offer(o);
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public E set(int idx, E o) {
         checkIndex(idx);
 
         int realIdx = getRealIndex(idx);
         Object old = items[realIdx];
         items[realIdx] = o;
         return (E) old;
     }
 
     @Override
     public void add(int idx, E o) {
         if (idx == size()) {
             offer(o);
             return;
         }
 
         checkIndex(idx);
         expandIfNeeded();
 
         int realIdx = getRealIndex(idx);
 
         // Make a room for a new element.
         if (first < last) {
             System
                     .arraycopy(items, realIdx, items, realIdx + 1, last
                             - realIdx);
         } else {
             if (realIdx >= first) {
                 System.arraycopy(items, 0, items, 1, last);
                 items[0] = items[items.length - 1];
                 System.arraycopy(items, realIdx, items, realIdx + 1,
                         items.length - realIdx - 1);
             } else {
                 System.arraycopy(items, realIdx, items, realIdx + 1, last
                         - realIdx);
             }
         }
 
         items[realIdx] = o;
         increaseSize();
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public E remove(int idx) {
         if (idx == 0) {
             return poll();
         }
 
         checkIndex(idx);
 
         int realIdx = getRealIndex(idx);
         Object removed = items[realIdx];
 
         // Remove a room for the removed element.
         if (first < last) {
             System.arraycopy(items, first, items, first + 1, realIdx - first);
         } else {
             if (realIdx >= first) {
                 System.arraycopy(items, first, items, first + 1, realIdx
                         - first);
             } else {
                 System.arraycopy(items, 0, items, 1, realIdx);
                 items[0] = items[items.length - 1];
                 System.arraycopy(items, first, items, first + 1, items.length
                         - first - 1);
             }
         }
 
         items[first] = null;
         decreaseSize();
 
         shrinkIfNeeded();
         return (E) removed;
     }
 
     public E remove() {
         if (isEmpty()) {
             throw new NoSuchElementException();
         }
         return poll();
     }
 
     public E element() {
         if (isEmpty()) {
             throw new NoSuchElementException();
         }
         return peek();
     }
 }