compressedstreamstore.java

来自「EXCEL read and write」· Java 代码 · 共 93 行

/* ====================================================================
   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.poi.hdgf.streams;

import java.io.ByteArrayInputStream;
import java.io.IOException;

import org.apache.poi.hdgf.HDGFLZW;

/**
 * A StreamStore where the data on-disk is compressed,
 *  using the crazy Visio LZW
 */
public class CompressedStreamStore extends StreamStore {
	/** The raw, compressed contents */
	private byte[] compressedContents;
	/** 
	 * We're not sure what this is, but it comes before the
	 *  real contents in the de-compressed data
	 */
	private byte[] blockHeader = new byte[4];
	private boolean blockHeaderInContents = false;
	
	protected byte[] _getCompressedContents() { return compressedContents; }
	protected byte[] _getBlockHeader() { return blockHeader; }
	
	/**
	 * Creates a new compressed StreamStore, which will handle
	 *  the decompression.
	 */
	protected CompressedStreamStore(byte[] data, int offset, int length) throws IOException {
		this(decompress(data,offset,length));
		
		compressedContents = new byte[length];
		System.arraycopy(data, offset, compressedContents, 0, length);
	}
	/**
	 * Handles passing the de-compressed data onto our superclass.
	 */
	private CompressedStreamStore(byte[][] decompressedData) {
		super(decompressedData[1], 0, decompressedData[1].length);
		blockHeader = decompressedData[0];
	}
	
	/**
	 * Some kinds of streams expect their 4 byte header to be
	 *  on the front of the contents.
	 * They can call this to have it sorted.
	 */
	protected void copyBlockHeaderToContents() {
		if(blockHeaderInContents) return;
		
		prependContentsWith(blockHeader);
		blockHeaderInContents = true;
	}
	

	/**
	 * Decompresses the given data, returning it as header + contents
	 */
	public static byte[][] decompress(byte[] data, int offset, int length) throws IOException {
		ByteArrayInputStream bais = new ByteArrayInputStream(data, offset, length);
		
		// Decompress
		HDGFLZW lzw = new HDGFLZW();
		byte[] decompressed = lzw.decode(bais);
		
		// Split into header and contents
		byte[][] ret = new byte[2][];
		ret[0] = new byte[4];
		ret[1] = new byte[decompressed.length - 4];
		
		System.arraycopy(decompressed, 0, ret[0], 0, 4);
		System.arraycopy(decompressed, 4, ret[1], 0, ret[1].length);
		
		// All done
		return ret;
	}
}