/*
 * 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.util.io;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.apache.wicket.util.lang.Args;


/**
 * <p>
 * An output stream which will retain data in memory until a specified threshold is reached, and
 * only then commit it to disk. If the stream is closed before the threshold is reached, the data
 * will not be written to disk at all.
 * </p>
 * <p>
 * This class originated in FileUpload processing. In this use case, you do not know in advance the
 * size of the file being uploaded. If the file is small you want to store it in memory (for speed),
 * but if the file is large you want to store it to file (to avoid memory issues).
 * </p>
 * 
 * @author <a href="mailto:martinc@apache.org">Martin Cooper</a>
 */
public class DeferredFileOutputStream extends ThresholdingOutputStream
{
        /**
         * The output stream to which data will be written at any given time. This will always be one of
         * <code>memoryOutputStream</code> or <code>diskOutputStream</code>.
         */
        private OutputStream currentOutputStream;

        /**
         * The output stream to which data will be written prior to the threshold being reached.
         */
        private ByteArrayOutputStream memoryOutputStream;

        /**
         * The file to which output will be directed if the threshold is exceeded.
         */
        private File outputFile;

        private final FileFactory fileFactory;

        /**
         * Constructs an instance of this class which will trigger an event at the specified threshold,
         * and save data to a file beyond that point.
         * 
         * @param threshold
         *            The number of bytes at which to trigger an event.
         * @param outputFile
         *            The file to which data is saved beyond the threshold.
         */
        public DeferredFileOutputStream(final int threshold, final File outputFile)
        {
                super(threshold);

                this.outputFile = Args.notNull(outputFile, "outputFile");
                fileFactory = null;

                memoryOutputStream = new ByteArrayOutputStream();
                currentOutputStream = memoryOutputStream;
        }

        /**
         * Constructs an instance of this class which will trigger an event at the specified threshold,
         * and save data to a file beyond that point.
         * 
         * @param threshold
         *            The number of bytes at which to trigger an event.
         * @param fileFactory
         *            The FileFactory to create the file.
         */
        public DeferredFileOutputStream(final int threshold, final FileFactory fileFactory)
        {
                super(threshold);
                this.fileFactory = Args.notNull(fileFactory, "fileFactory");

                memoryOutputStream = new ByteArrayOutputStream();
                currentOutputStream = memoryOutputStream;
        }

        /**
         * Returns the data for this output stream as an array of bytes, assuming that the data has been
         * retained in memory. If the data was written to disk, this method returns <code>null</code>.
         * 
         * @return The data for this output stream, or <code>null</code> if no such data is available.
         */
        public byte[] getData()
        {
                if (memoryOutputStream != null)
                {
                        return memoryOutputStream.toByteArray();
                }
                return null;
        }

        /**
         * Returns the data for this output stream as a <code>File</code>, assuming that the data was
         * written to disk. If the data was retained in memory, this method returns <code>null</code>.
         * 
         * @return The file for this output stream, or <code>null</code> if no such file exists.
         */
        public File getFile()
        {
                return outputFile;
        }

        /**
         * Determines whether or not the data for this output stream has been retained in memory.
         * 
         * @return <code>true</code> if the data is available in memory; <code>false</code> otherwise.
         */
        public boolean isInMemory()
        {
                return (!isThresholdExceeded());
        }

        /**
         * Returns the current output stream. This may be memory based or disk based, depending on the
         * current state with respect to the threshold.
         * 
         * @return The underlying output stream.
         * @exception IOException
         *                if an error occurs.
         */
        @Override
        protected OutputStream getStream() throws IOException
        {
                return currentOutputStream;
        }

        /**
         * Switches the underlying output stream from a memory based stream to one that is backed by
         * disk. This is the point at which we realize that too much data is being written to keep in
         * memory, so we elect to switch to disk-based storage.
         * 
         * @exception IOException
         *                if an error occurs.
         */
        @Override
        protected void thresholdReached() throws IOException
        {
                byte[] data = memoryOutputStream.toByteArray();
                if (outputFile == null)
                {
                        outputFile = fileFactory.createFile();
                }
                FileOutputStream fos = new FileOutputStream(outputFile);
                fos.write(data);
                currentOutputStream = fos;
                memoryOutputStream = null;
        }

        /**
         * The file factory for this deferred file output stream.
         * 
         * @author jcompagner
         */
        public interface FileFactory
        {
                /**
                 * @return the file to use for disk cache
                 */
                File createFile();
        }
}