BZip2InputStream.java

/*
 * Copyright (c) 2011 Matthew Francis
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package org.itadaki.bzip2;

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


/**
 * <p>An InputStream wrapper that decompresses BZip2 data</p>
 *
 * <p>A BZip2 stream consists of one or more blocks of compressed data. This decompressor reads a
 * whole block at a time, then progressively returns decompressed output.</p>
 *
 * <p>On encountering any error decoding the compressed stream, an IOException is thrown, and
 * further reads will return {@code -1}</p>
 *
 * <p><b>Note:</b> Each BZip2 compressed block contains a CRC code which is verified after the block
 * has been read completely. If verification fails, an exception is thrown on the final read from
 * the block, <b>potentially after corrupt data has already been returned</b>. The compressed stream
 * also contains a final CRC code which is verified once the end of the stream has been reached.
 * <b>This check may fail even if every individual block in the stream passes CRC verification</b>.
 * If this possibility is of concern, you should read and store the entire decompressed stream
 * before further processing.</p>
 *
 * <p>Instances of this class are not threadsafe.</p>
 */
public class BZip2InputStream extends InputStream {

	/**
	 * The stream from which compressed BZip2 data is read and decoded
	 */
	private InputStream inputStream;

	/**
	 * An InputStream wrapper that provides bit-level reads
	 */
	private BZip2BitInputStream bitInputStream;

	/**
	 * If {@code true}, the caller is assumed to have read away the stream's leading "BZ" identifier
	 * bytes
	 */
	private final boolean headerless;

	/**
	 * (@code true} if the end of the compressed stream has been reached, otherwise {@code false}
	 */
	private boolean streamComplete = false;

	/**
	 * The declared block size of the stream (before final run-length decoding). The final block
	 * will usually be smaller, but no block in the stream has to be exactly this large, and an
	 * encoder could in theory choose to mix blocks of any size up to this value. Its function is
	 * therefore as a hint to the decompressor as to how much working space is sufficient to
	 * decompress blocks in a given stream
	 */
	private int streamBlockSize;

	/**
	 * The merged CRC of all blocks decompressed so far
	 */
	private int streamCRC = 0;

	/**
	 * The decompressor for the current block
	 */
	private BZip2BlockDecompressor blockDecompressor = null;


	/* (non-Javadoc)
	 * @see java.io.InputStream#read()
	 */
	@Override
	public int read() throws IOException {

		int nextByte = -1;
		if (this.blockDecompressor == null) {
			initialiseStream();
		} else {
			nextByte = this.blockDecompressor.read();
		}

		if (nextByte == -1) {
			if (initialiseNextBlock()) {
				nextByte = this.blockDecompressor.read();
			}
		}

		return nextByte;

	}


	/* (non-Javadoc)
	 * @see java.io.InputStream#read(byte[], int, int)
	 */
	@Override
	public int read (final byte[] destination, final int offset, final int length) throws IOException {

		int bytesRead = -1;
		if (this.blockDecompressor == null) {
			initialiseStream();
		} else {
			bytesRead = this.blockDecompressor.read (destination, offset, length);
		}

		if (bytesRead == -1) {
			if (initialiseNextBlock()) {
				bytesRead = this.blockDecompressor.read (destination, offset, length);
			}
		}

		return bytesRead;

	}


	/* (non-Javadoc)
	 * @see java.io.InputStream#close()
	 */
	@Override
	public void close() throws IOException {

		if (this.bitInputStream != null) {
			this.streamComplete = true;
			this.blockDecompressor = null;
			this.bitInputStream = null;

			try {
				this.inputStream.close();
			} finally {
				this.inputStream = null;
			}
		}

	}


	/**
	 * Reads the stream header and checks that the data appears to be a valid BZip2 stream
	 * @throws IOException if the stream header is not valid
	 */
	private void initialiseStream() throws IOException {

		/* If the stream has been explicitly closed, throw an exception */
		if (this.bitInputStream == null) {
			throw new BZip2Exception ("Stream closed");
		}

		/* If we're already at the end of the stream, do nothing */
		if (this.streamComplete) {
			return;
		}

		/* Read the stream header */
		try {
			int marker1 = this.headerless ? 0 : this.bitInputStream.readBits (16);
			int marker2 = this.bitInputStream.readBits (8);
			int blockSize = (this.bitInputStream.readBits (8) - '0');

			if (
					   (!this.headerless && (marker1 != BZip2Constants.STREAM_START_MARKER_1))
					|| (marker2 != BZip2Constants.STREAM_START_MARKER_2)
					|| (blockSize < 1) || (blockSize > 9))
			{
				throw new BZip2Exception ("Invalid BZip2 header");
			}

			this.streamBlockSize = blockSize * 100000;
		} catch (IOException e) {
			// If the stream header was not valid, stop trying to read more data
			this.streamComplete = true;
			throw e;
		}


	}


	/**
	 * Prepares a new block for decompression if any remain in the stream. If a previous block has
	 * completed, its CRC is checked and merged into the stream CRC. If the previous block was the
	 * final block in the stream, the stream CRC is validated
	 * @return {@code true} if a block was successfully initialised, or {@code false} if the end of
	 *                      file marker was encountered
	 * @throws IOException if either the block or stream CRC check failed, if the following data is
	 *                      not a valid block-header or end-of-file marker, or if the following
	 *                      block could not be decoded
	 */
	private boolean initialiseNextBlock() throws IOException {

		/* If we're already at the end of the stream, do nothing */
		if (this.streamComplete) {
			return false;
		}

		/* If a block is complete, check the block CRC and integrate it into the stream CRC */
		if (this.blockDecompressor != null) {
			int blockCRC = this.blockDecompressor.checkCRC();
			this.streamCRC = ((this.streamCRC << 1) | (this.streamCRC >>> 31)) ^ blockCRC;
		}

		/* Read block-header or end-of-stream marker */
		final int marker1 = this.bitInputStream.readBits (24);
		final int marker2 = this.bitInputStream.readBits (24);

		if (marker1 == BZip2Constants.BLOCK_HEADER_MARKER_1 && marker2 == BZip2Constants.BLOCK_HEADER_MARKER_2) {
			// Initialise a new block
			try {
				this.blockDecompressor = new BZip2BlockDecompressor (this.bitInputStream, this.streamBlockSize);
			} catch (IOException e) {
				// If the block could not be decoded, stop trying to read more data
				this.streamComplete = true;
				throw e;
			}
			return true;
		} else if (marker1 == BZip2Constants.STREAM_END_MARKER_1 && marker2 == BZip2Constants.STREAM_END_MARKER_2) {
			// Read and verify the end-of-stream CRC
			this.streamComplete = true;
			final int storedCombinedCRC = this.bitInputStream.readInteger();
			if (storedCombinedCRC != this.streamCRC) {
				throw new BZip2Exception ("BZip2 stream CRC error");
			}
			return false;
		}

		/* If what was read is not a valid block-header or end-of-stream marker, the stream is broken */
		this.streamComplete = true;
		throw new BZip2Exception ("BZip2 stream format error");

	}


	/**
	 * @param inputStream The InputStream to wrap
	 * @param headerless If {@code true}, the caller is assumed to have read away the stream's
	 *                   leading "BZ" identifier bytes
	 */
	public BZip2InputStream (final InputStream inputStream, final boolean headerless) {

		if (inputStream == null) {
			throw new IllegalArgumentException ("Null input stream");
		}

		this.inputStream = inputStream;
		this.bitInputStream = new BZip2BitInputStream (inputStream);
		this.headerless = headerless;

	}

}