org.archive.util.ms.DefaultBlockFileSystem Maven / Gradle / Ivy
/*
* 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;
}
}