/*
 * BufferAllocator.java February 2001
 *
 * Copyright (C) 2001, Niall Gallagher <niallg@users.sf.net>
 *
 * Licensed 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.simpleframework.util.buffer;

import java.io.IOException;
import java.io.InputStream;

/**
 * The <code>BufferAllocator</code> object is used to provide a means to
 * allocate buffers using a single underlying buffer. This uses a buffer from a
 * existing allocator to create the region of memory to use to allocate all
 * other buffers. As a result this allows a single buffer to acquire the bytes
 * in a number of associated buffers. This has the advantage of allowing bytes
 * to be read in sequence without joining data from other buffers or allocating
 * multiple regions.
 * 
 * @author Niall Gallagher
 */
public class BufferAllocator extends FilterAllocator implements Buffer {

    /**
     * This is the underlying buffer all other buffers are within.
     */
    private Buffer buffer;

    /**
     * Constructor for the <code>BufferAllocator</code> object. This is used to
     * instantiate the allocator with a default buffer size of half a kilobyte.
     * This ensures that it can be used for general purpose byte storage and for
     * minor I/O tasks.
     * 
     * @param source
     *            this is where the underlying buffer is allocated
     */
    public BufferAllocator(Allocator source) {
        super(source);
    }

    /**
     * Constructor for the <code>BufferAllocator</code> object. This is used to
     * instantiate the allocator with a specified buffer size. This is typically
     * used when a very specific buffer capacity is required, for example a
     * request body with a known length.
     * 
     * @param source
     *            this is where the underlying buffer is allocated
     * @param capacity
     *            the initial capacity of the allocated buffers
     */
    public BufferAllocator(Allocator source, long capacity) {
        super(source, capacity);
    }

    /**
     * Constructor for the <code>BufferAllocator</code> object. This is used to
     * instantiate the allocator with a specified buffer size. This is typically
     * used when a very specific buffer capacity is required, for example a
     * request body with a known length.
     * 
     * @param source
     *            this is where the underlying buffer is allocated
     * @param capacity
     *            the initial capacity of the allocated buffers
     * @param limit
     *            this is the maximum buffer size created by this
     */
    public BufferAllocator(Allocator source, long capacity, long limit) {
        super(source, capacity, limit);
    }

    /**
     * This method is used so that a buffer can be represented as a stream of
     * bytes. This provides a quick means to access the data that has been
     * written to the buffer. It wraps the buffer within an input stream so that
     * it can be read directly.
     * 
     * @return a stream that can be used to read the buffered bytes
     */
    @Override
    public InputStream getInputStream() throws IOException {
        if (this.buffer == null) {
            this.allocate();
        }
        return this.buffer.getInputStream();
    }

    /**
     * This method is used to acquire the buffered bytes as a string. This is
     * useful if the contents need to be manipulated as a string or transferred
     * into another encoding. If the UTF-8 content encoding is not supported the
     * platform default is used, however this is unlikely as UTF-8 should be
     * supported.
     * 
     * @return this returns a UTF-8 encoding of the buffer contents
     */
    @Override
    public String encode() throws IOException {
        if (this.buffer == null) {
            this.allocate();
        }
        return this.buffer.encode();
    }

    /**
     * This method is used to acquire the buffered bytes as a string. This is
     * useful if the contents need to be manipulated as a string or transferred
     * into another encoding. This will convert the bytes using the specified
     * character encoding format.
     * 
     * @return this returns the encoding of the buffer contents
     */
    @Override
    public String encode(String charset) throws IOException {
        if (this.buffer == null) {
            this.allocate();
        }
        return this.buffer.encode(charset);
    }

    /**
     * This method is used to append bytes to the end of the buffer. This will
     * expand the capacity of the buffer if there is not enough space to
     * accommodate the extra bytes.
     * 
     * @param array
     *            this is the byte array to append to this buffer
     * 
     * @return this returns this buffer for another operation
     */
    @Override
    public Buffer append(byte[] array) throws IOException {
        return this.append(array, 0, array.length);
    }

    /**
     * This method is used to append bytes to the end of the buffer. This will
     * expand the capacity of the buffer if there is not enough space to
     * accommodate the extra bytes.
     * 
     * @param array
     *            this is the byte array to append to this buffer
     * @param size
     *            the number of bytes to be read from the array
     * @param off
     *            this is the offset to begin reading the bytes from
     * 
     * @return this returns this buffer for another operation
     */
    @Override
    public Buffer append(byte[] array, int off, int size) throws IOException {
        if (this.buffer == null) {
            this.allocate(size);
        }
        return this.buffer.append(array, off, size);
    }

    /**
     * This will clear all data from the buffer. This simply sets the count to
     * be zero, it will not clear the memory occupied by the instance as the
     * internal buffer will remain. This allows the memory occupied to be reused
     * as many times as is required.
     */
    @Override
    public void clear() throws IOException {
        if (this.buffer != null) {
            this.buffer.clear();
        }
    }

    /**
     * This method is used to ensure the buffer can be closed. Once the buffer
     * is closed it is an immutable collection of bytes and can not longer be
     * modified. This ensures that it can be passed by value without the risk of
     * modification of the bytes.
     */
    @Override
    public void close() throws IOException {
        if (this.buffer == null) {
            this.allocate();
        }
        this.buffer.close();
    }

    /**
     * This method is used to allocate a default buffer. This will allocate a
     * buffer of predetermined size, allowing it to grow to an upper limit to
     * accommodate extra data. If the buffer requested is larger than the limit
     * an exception is thrown.
     * 
     * @return this returns an allocated buffer with a default size
     */
    @Override
    public Buffer allocate() throws IOException {
        return this.allocate(this.capacity);
    }

    /**
     * This method is used to allocate a default buffer. This will allocate a
     * buffer of predetermined size, allowing it to grow to an upper limit to
     * accommodate extra data. If the buffer requested is larger than the limit
     * an exception is thrown.
     * 
     * @param size
     *            the initial capacity of the allocated buffer
     * 
     * @return this returns an allocated buffer with a default size
     */
    @Override
    public Buffer allocate(long size) throws IOException {
        if (size > this.limit)
            throw new BufferException("Specified size %s beyond limit", size);
        if (this.capacity > size) { // lazily create backing buffer
            size = this.capacity;
        }
        if (this.buffer == null) {
            this.buffer = this.source.allocate(size);
        }
        return this.buffer.allocate();
    }
}
