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The Archive Commons Code Libraries project contains general Java utility libraries, as used by the Heritrix crawler and other projects.

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/*
 *  This file is part of the Heritrix web crawler (crawler.archive.org).
 *
 *  Licensed to the Internet Archive (IA) by one or more individual 
 *  contributors. 
 *
 *  The IA 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.archive.util.ms;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.Map;

import org.archive.io.SeekInputStream;
import org.archive.util.ArchiveUtils;
import org.archive.util.LRU;


/**
 * Default implementation of the Block File System.
 * 
 * 

The overall structure of a BlockFileSystem file (such as a .doc file) is * as follows. The file is divided into blocks, which are of uniform length * (512 bytes). The first block (at file pointer 0) is called the header * block. It's used to look up other blocks in the file. * *

Subfiles contained within the .doc file are organized using a Block * Allocation Table, or BAT. The BAT is basically a linked list; given a * block number, the BAT will tell you the next block number. Note that * the header block has no number; block #0 is the first block after the * header. Thus, to convert a block number to a file pointer: * int filePointer = (blockNumber + 1) * BLOCK_SIZE. * *

The BAT itself is discontinuous, however. To find the blocks that * comprise the BAT, you have to look in the header block. The header block * contains an array of 109 pointers to the blocks that comprise the BAT. * If more than 109 BAT blocks are required (in other words, if the .doc * file is larger than ~6 megabytes), then something called the * XBAT comes into play. * *

XBAT blocks contain pointers to the 110th BAT block and beyond. * The first XBAT block is stored at a file pointer listed in the header. * The other XBAT blocks are always stored in order after the first; the * XBAT table is continuous. One is inclined to wonder why the BAT itself * is not so stored, but oh well. * *

The BAT only tells you the next block for a given block. To find the * first block for a subfile, you have to look up that subfile's directory * entry. Each directory entry is a 128 byte structure in the file, so four * of them fit in a block. The number of the first block of the entry list * is stored in the header. To find subsequent entry blocks, the BAT must * be used. * *

I'm telling you all this so that you understand the caching that this * class provides. * *

First, directory entries are not cached. It's assumed that they will * be looked up at the beginning of a lengthy operation, and then forgotten * about. This is certainly the case for {@link Doc#getText(BlockFileSystem, int)}. * If you need to remember directory entries, you can manually store the Entry * objects in a map or something, as they don't grow stale. * *

This class keeps all 512 bytes of the header block in memory at all * times. This prevents a potentially expensive file pointer repositioning * every time you're trying to figure out what comes next. * *

BAT and XBAT blocks are stored in a least-recently used cache. The * n most recent BAT and XBAT blocks are remembered, where n * is set at construction time. The minimum value of n is 1. For small * files, this can prevent file pointer repositioning for BAT look ups. * *

The BAT/XBAT cache only takes up memory as needed. If the specified * cache size is 100 blocks, but the file only has 4 BAT blocks, then only * 2048 bytes will be used by the cache. * *

Note this class only caches BAT and XBAT blocks. It does not cache the * blocks that actually make up a subfile's contents. It is assumed that those * blocks will only be accessed once per operation (again, this is what * {Doc.getText(BlockFileSystem)} typically requires.) * * @author pjack * @see http://jakarta.apache.org/poi/poifs/fileformat.html */ public class DefaultBlockFileSystem implements BlockFileSystem { /** * Pointers per BAT block. */ final private static int POINTERS_PER_BAT = 128; /** * Size of a BAT pointer in bytes. (In other words, 4). */ final private static int BAT_POINTER_SIZE = BLOCK_SIZE / POINTERS_PER_BAT; /** * The number of BAT pointers in the header block. After this many * BAT blocks, the XBAT blocks must be consulted. */ final private static int HEADER_BAT_LIMIT = 109; /** * The size of an entry record in bytes. */ final private static int ENTRY_SIZE = 128; /** * The number of entries that can fit in a block. */ final private static int ENTRIES_PER_BLOCK = BLOCK_SIZE / ENTRY_SIZE; /** * The .doc file as a stream. */ private SeekInputStream input; /** * The header block. */ private HeaderBlock header; /** * Cache of BAT and XBAT blocks. */ private Map cache; /** * Constructor. * * @param input the file to read from * @param batCacheSize number of BAT and XBAT blocks to cache * @throws IOException if an IO error occurs */ public DefaultBlockFileSystem(SeekInputStream input, int batCacheSize) throws IOException { this.input = input; byte[] temp = new byte[BLOCK_SIZE]; ArchiveUtils.readFully(input, temp); this.header = new HeaderBlock(ByteBuffer.wrap(temp)); this.cache = new LRU(batCacheSize); } public Entry getRoot() throws IOException { // Position to the first block of the entry list. int block = header.getEntriesStart(); input.position((block + 1) * BLOCK_SIZE); // The root entry is always entry #0. return new DefaultEntry(this, input, 0); } /** * Returns the entry with the given number. * * @param entryNumber the number of the entry to return * @return that entry, or null if no such entry exists * @throws IOException if an IO error occurs */ protected Entry getEntry(int entryNumber) throws IOException { // Entry numbers < 0 typically indicate an end-of-stream. if (entryNumber < 0) { return null; } // It's impossible to check against the upper bound, because the // upper bound is not recorded anywhere. // Advance to the block containing the desired entry. int blockCount = entryNumber / ENTRIES_PER_BLOCK; int remainder = entryNumber % ENTRIES_PER_BLOCK; int block = header.getEntriesStart(); for (int i = 0; i < blockCount; i++) { block = getNextBlock(block); } if (block < 0) { // Given entry number exceeded the number of available entries. return null; } int filePos = (block + 1) * BLOCK_SIZE + remainder * ENTRY_SIZE; input.position(filePos); return new DefaultEntry(this, input, entryNumber); } public int getNextBlock(int block) throws IOException { if (block < 0) { return block; } // Index into the header array of BAT blocks. int headerIndex = block / POINTERS_PER_BAT; // Index within that BAT block of the block we're interested in. int batBlockIndex = block % POINTERS_PER_BAT; int batBlockNumber = batLookup(headerIndex); ByteBuffer batBlock = getBATBlock(batBlockNumber); return batBlock.getInt(batBlockIndex * BAT_POINTER_SIZE); } /** * Looks up the block number of a BAT block. * * @param headerIndex * @return * @throws IOException */ private int batLookup(int headerIndex) throws IOException { if (headerIndex < HEADER_BAT_LIMIT + 1) { return header.getBATBlockNumber(headerIndex); } // Find the XBAT block of interest headerIndex -= HEADER_BAT_LIMIT; int xbatBlockNumber = headerIndex / POINTERS_PER_BAT; xbatBlockNumber += header.getExtendedBATStart(); ByteBuffer xbat = getBATBlock(xbatBlockNumber); // Find the bat Block number inside the XBAT block int xbatBlockIndex = headerIndex % POINTERS_PER_BAT; return xbat.getInt(xbatBlockIndex * BAT_POINTER_SIZE); } /** * Returns the BAT block with the given block number. * If the BAT block were previously cached, then the cached version * is returned. Otherwise, the file pointer is repositioned to * the start of the given block, and the 512 bytes are read and * stored in the cache. * * @param block the block number of the BAT block to return * @return the BAT block * @throws IOException */ private ByteBuffer getBATBlock(int block) throws IOException { ByteBuffer r = cache.get(block); if (r != null) { return r; } byte[] buf = new byte[BLOCK_SIZE]; input.position((block + 1) * BLOCK_SIZE); ArchiveUtils.readFully(input, buf); r = ByteBuffer.wrap(buf); r.order(ByteOrder.LITTLE_ENDIAN); cache.put(block, r); return r; } public SeekInputStream getRawInput() { return input; } }





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