/*
 * 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.wicket.markup.repeater;

import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.wicket.markup.html.navigation.paging.IPageableItems;
import org.apache.wicket.model.IModel;


/**
 * An abstract repeater view that provides paging functionality to its subclasses.
 * <p>
 * The view is populated by overriding the <code>getItemModels(int offset, int count)</code> method
 * and providing an iterator that returns models for items in the current page. The
 * AbstractPageableView builds the items that will be rendered by looping over the models and
 * calling the <code>newItem(String id, int index, IModel model)</code> to generate the child item
 * container followed by <code>populateItem(Component item)</code> to let the user populate the
 * newly created item container with with custom components.
 * </p>
 * 
 * @see org.apache.wicket.markup.repeater.RefreshingView
 * @see org.apache.wicket.markup.html.navigation.paging.IPageable
 * 
 * @author Igor Vaynberg (ivaynberg)
 * 
 * @param <T>
 *            type of elements contained in the model's list
 */
public abstract class AbstractPageableView<T> extends RefreshingView<T> implements IPageableItems
{
        /** */
        private static final long serialVersionUID = 1L;

        /**
         * Keeps track of the number of items we show per page. The default is Integer.MAX_VALUE which
         * effectively disables paging.
         */
        private long itemsPerPage = Long.MAX_VALUE;

        /**
         * Keeps track of the current page number.
         */
        private long currentPage;

        /**
         * <code>cachedItemCount</code> is used to cache the call to <code>internalGetItemCount()</code>
         * for the duration of the request because that call can potentially be expensive ( a select
         * count query ) and so we do not want to execute it multiple times.
         */
        private transient Long cachedItemCount;

        /**
         * Constructor
         * 
         * @param id
         * @param model
         * @see org.apache.wicket.Component#Component(String, IModel)
         */
        public AbstractPageableView(String id, IModel<? extends Collection<? extends T>> model)
        {
                super(id, model);
        }

        /** @see org.apache.wicket.Component#Component(String) */
        public AbstractPageableView(String id)
        {
                super(id);
        }

        /**
         * This method retrieves the subset of models for items in the current page and allows
         * RefreshingView to generate items.
         * 
         * @return iterator over models for items in the current page
         */
        @Override
        protected Iterator<IModel<T>> getItemModels()
        {
                long offset = getFirstItemOffset();
                long size = getViewSize();

                Iterator<IModel<T>> models = getItemModels(offset, size);

                models = new CappedIteratorAdapter<T>(models, size);

                return models;
        }

        /**
         * @see org.apache.wicket.markup.repeater.AbstractRepeater#onBeforeRender()
         */
        @Override
        protected void onBeforeRender()
        {
                clearCachedItemCount();
                super.onBeforeRender();
        }

        /**
         * Returns an iterator over models for items in the current page
         * 
         * @param offset
         *            index of first item in this page
         * @param size
         *            number of items that will be shown in the current page
         * @return an iterator over models for items in the current page
         */
        protected abstract Iterator<IModel<T>> getItemModels(long offset, long size);

        // /////////////////////////////////////////////////////////////////////////
        // ITEM COUNT CACHE
        // /////////////////////////////////////////////////////////////////////////

        private void clearCachedItemCount()
        {
                cachedItemCount = null;
        }

        // /////////////////////////////////////////////////////////////////////////
        // PAGING
        // /////////////////////////////////////////////////////////////////////////

        /**
         * @return maximum number of items that will be shown per page
         */
        @Override
        public long getItemsPerPage()
        {
                return itemsPerPage;
        }

        /**
         * Sets the maximum number of items to show per page. The current page will also be set to zero
         * 
         * @param items
         */
        @Override
        public final void setItemsPerPage(long items)
        {
                if (items < 1)
                {
                        throw new IllegalArgumentException("Argument [itemsPerPage] cannot be less than 1");
                }

                if (itemsPerPage != items)
                {
                        if (isVersioned())
                        {
                                addStateChange();
                        }
                }

                itemsPerPage = items;

                // because items per page can effect the total number of pages we always
                // reset the current page back to zero
                setCurrentPage(0);
        }

        /**
         * @return total item count
         */
        protected abstract long internalGetItemCount();

        /**
         * Get the row count.
         * 
         * @see #getItemCount()
         * 
         * @return total item count, but 0 if not visible in the hierarchy
         */
        public final long getRowCount()
        {
                if (!isVisibleInHierarchy())
                {
                        return 0;
                }

                return getItemCount();
        }

        /**
         * Get the item count. Since dataprovider.size() could potentially be expensive, the item count
         * is cached.
         * 
         * @see #getRowCount()
         * 
         * @return the item count
         */
        @Override
        public final long getItemCount()
        {
                if (cachedItemCount != null)
                {
                        return cachedItemCount;
                }

                long count = internalGetItemCount();

                cachedItemCount = count;
                return count;
        }

        /**
         * @see org.apache.wicket.markup.html.navigation.paging.IPageable#getCurrentPage()
         */
        @Override
        public final long getCurrentPage()
        {
                long page = currentPage;

                /*
                 * trim current page if its out of bounds this can happen if items are added/deleted between
                 * requests
                 */

                if (page > 0 && page >= getPageCount())
                {
                        page = Math.max(getPageCount() - 1, 0);
                        currentPage = page;
                        return page;
                }

                return page;
        }

        /**
         * @see org.apache.wicket.markup.html.navigation.paging.IPageable#setCurrentPage(long)
         */
        @Override
        public final void setCurrentPage(long page)
        {
                if (currentPage != page)
                {
                        if (isVersioned())
                        {
                                addStateChange();

                        }
                }
                currentPage = page;
        }

        /**
         * @see org.apache.wicket.markup.html.navigation.paging.IPageable#getPageCount()
         */
        @Override
        public long getPageCount()
        {
                long total = getRowCount();
                long itemsPerPage = getItemsPerPage();
                long count = total / itemsPerPage;

                if (itemsPerPage * count < total)
                {
                        count++;
                }

                return count;

        }

        /**
         * @return the index of the first visible item in the view
         */
        public long getFirstItemOffset()
        {
                return getCurrentPage() * getItemsPerPage();
        }


        /**
         * @return the number of items visible
         */
        public long getViewSize()
        {
                return Math.min(getItemsPerPage(), getRowCount() - getFirstItemOffset());
        }

        // /////////////////////////////////////////////////////////////////////////
        // HELPER CLASSES
        // /////////////////////////////////////////////////////////////////////////

        /**
         * Iterator adapter that makes sure only the specified max number of items can be accessed from
         * its delegate.
         * 
         * @param <T>
         *            Model object type
         */
        private static class CappedIteratorAdapter<T> implements Iterator<IModel<T>>
        {
                private final long max;
                private long index;
                private final Iterator<IModel<T>> delegate;

                /**
                 * Constructor
                 * 
                 * @param delegate
                 *            delegate iterator
                 * @param max
                 *            maximum number of items that can be accessed.
                 */
                public CappedIteratorAdapter(Iterator<IModel<T>> delegate, long max)
                {
                        this.delegate = delegate;
                        this.max = max;
                }

                /**
                 * @see java.util.Iterator#remove()
                 */
                @Override
                public void remove()
                {
                        throw new UnsupportedOperationException();
                }

                /**
                 * @see java.util.Iterator#hasNext()
                 */
                @Override
                public boolean hasNext()
                {
                        return (index < max) && delegate.hasNext();
                }

                /**
                 * @see java.util.Iterator#next()
                 */
                @Override
                public IModel<T> next()
                {
                        if (index >= max)
                        {
                                throw new NoSuchElementException();
                        }
                        index++;
                        return delegate.next();
                }

        }

        /**
         * @see org.apache.wicket.Component#onDetach()
         */
        @Override
        protected void onDetach()
        {
                clearCachedItemCount();
                super.onDetach();
        }
}