/*
 * Copyright (C) 2003-2011 eXo Platform SAS.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
package org.etk.common.utils;

import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.util.AbstractList;
import java.util.RandomAccess;

/**
 * 
 * <p>A lazy list and uses a {@link org.exoplatform.commons.utils.ListAccess} object to load
 * the elements of the list. The list is read only and any write access to the list will
 * not be permitted.</p>
 *
 * <p>The loading policy is based on a simple batch algorithm that loads the elements by batches.</p>
 *
 * <p>The list also keeps a cache of the retrieved elements. The cache use soft references to provide
 * eviction of the elements if necessary. When a soft reference is cleared and access is made to an
 * evicted element then the elements will be reloaded from the list access object.</p>
 *
 * <p>If the list access fails to load a batch by throwing a checked exception, it will cause the
 * list to throw an {@link IllegalStateException} wrapping the original exception. Any other kind
 * of non checked throwable will be propagated to the caller as it is.</p>
 *
 * <p>The implementation does not perform any kind of versionning check of the underlying data
 * and if the underlying list access changes the state it exposes the lazy list will not be aware
 * of it and may behave in an unexpected manner.</p>
 * 
 * Created by The eXo Platform SAS
 * Author : eXoPlatform
 *          thanhvucong.78@google.com
 * Aug 13, 2011  
 */

public class LazyList<E> extends AbstractList<E> implements RandomAccess {

  /** The batch size. */
  private final int batchSize;

  /** The pages. */
  private Batch[] batches;

  /** The list access. */
  private ListAccess<E> listAccess;

  public LazyList(ListAccess<E> listAccess, int batchSize) {
    if (listAccess == null) {
      throw new IllegalArgumentException("The list access object cannot be null");
    }
    if (batchSize < 1) {
      throw new IllegalArgumentException("No batch size < 1 is accepted");
    }

    //
    this.listAccess = listAccess;
    this.batchSize = batchSize;
  }

  public E get(int index) {
    int size = size();

    //
    if (index < 0) {
      throw new ArrayIndexOutOfBoundsException();
    }
    if (index >= size) {
      throw new ArrayIndexOutOfBoundsException();
    }

    //
    if (batches == null) {
      batches = new Batch[1 + size / batchSize];
    }

    //
    Object[] elements = null;
    int batchIndex = index / batchSize;
    Batch batch = batches[batchIndex];
    if (batch != null) {
      elements = batch.elements.get();
    }

    //
    if (elements == null) {
      try {
        int loadedIndex = batchIndex * batchSize;
        int loadedLength = Math.min(batchSize, size - loadedIndex);
        elements = listAccess.load(loadedIndex, loadedLength);
        batches[batchIndex] = new Batch(elements);
      } catch (Exception e) {
        e.printStackTrace();
        throw new IllegalStateException("Cannot load resource at index " + index, e);
      }
    }

    //
    return (E) elements[index % batchSize];
  }

  public int size() {
    try {
      return listAccess.getSize();
    } catch (Exception e) {
      throw new IllegalStateException(e.getMessage(), e);
    }
  }

  private static class Batch {

    /** . */
    private final Reference<Object[]> elements;

    private Batch(Object[] elements) {
      this.elements = new SoftReference<Object[]>(elements);
    }
  }
}