edu.harvard.hul.ois.jhove.module.pdf.CrossRefStream Maven / Gradle / Ivy
/**********************************************************************
* Jhove - JSTOR/Harvard Object Validation Environment
* Copyright 2005 by JSTOR and the President and Fellows of Harvard College
**********************************************************************/
package edu.harvard.hul.ois.jhove.module.pdf;
import java.io.*;
import java.util.*;
/**
* This class implements the Cross-Reference Stream, an alternative
* to the Cross-Reference Table starting in PDF 1.4.
*
* A cross-reference stream is identified by a startxref keyword,
* as opposed to the xref keyword which identifies the old-style
* cross-reference table.
*
* JHOVE supports only FlateDecode as a filter for cross-reference
* streams. This is consistent with the implementation limitation
* described in Appendix H of the PDF manual for Acrobat 6 and earlier.
*
*
* @author Gary McGath
*
*/
public class CrossRefStream {
private PdfStream _xstrm; // The underlying Stream object.
private PdfDictionary _dict;
private int _size;
private IndexRange[] _index;
private int[] _fieldSizes;
private int _freeCount;
private Filter[] _filters;
private int _readRange;
private int _readIndex;
private int _bytesPerEntry;
private long _prevXref; // byte offset to previous xref stream, if any
/* Per-object variables. */
private int _objType;
private int _objNum;
private int _objField1;
private int _objField2;
/**
* Range elements of the _index array:
* Starting object and number of objects.
*/
private class IndexRange {
public int start;
public int len;
}
/**
* Constructor.
*
* @param xstrm PdfStream object which contains a presumed
* cross-reference stream.
*/
public CrossRefStream(PdfStream xstrm) {
_xstrm = xstrm;
_dict = xstrm.getDict ();
_freeCount = 0;
}
/**
* Returns true if the PdfStream object meets
* the requirements of a cross-reference stream. Also extracts
* information from the dictionary for subsequent processing.
*/
public boolean isValid () {
try {
PdfObject typeObj = _dict.get ("Type");
String typeStr = null;
if (typeObj instanceof PdfSimpleObject) {
typeStr = ((PdfSimpleObject) typeObj).getStringValue ();
if (!("XRef".equals (typeStr))) {
return false;
}
}
if (typeStr == null) {
return false;
}
PdfObject sizeObj = _dict.get ("Size");
if (sizeObj instanceof PdfSimpleObject) {
_size = ((PdfSimpleObject) sizeObj).getIntValue();
}
else {
return false;
}
// The Index entry is optional, but must have the right
// format if present: [start1, length1, start2, length2, ...]
PdfObject indexObj = _dict.get ("Index");
if (indexObj instanceof PdfArray) {
Vector indexObjs = ((PdfArray) indexObj).getContent();
int indexObjCount = indexObjs.size();
// Must contain an even number of objects
if (indexObjCount % 2 != 0) return false;
_index = new IndexRange[indexObjCount / 2];
for (int i = 0; i < _index.length; i++) {
_index[i] = new IndexRange();
_index[i].start = ((PdfSimpleObject)indexObjs.get(i * 2)).getIntValue();
_index[i].len = ((PdfSimpleObject)indexObjs.get(i * 2 + 1)).getIntValue();
}
}
else {
// Set up default index.
_index = new IndexRange[1];
_index[0] = new IndexRange();
_index[0].start = 0;
_index[0].len = _size;
}
// Get the field sizes.
PdfObject wObj = _dict.get ("W");
if (wObj instanceof PdfArray) {
Vector vec = ((PdfArray) wObj).getContent ();
int len = vec.size ();
_fieldSizes = new int[len];
for (int i = 0; i < len; i++) {
PdfSimpleObject ob = (PdfSimpleObject) vec.get (i);
_fieldSizes[i] = ob.getIntValue ();
}
}
// Get the offset to the previous xref stream, if any.
PdfObject prevObj = _dict.get ("Prev");
if (prevObj instanceof PdfSimpleObject) {
_prevXref = ((PdfSimpleObject) prevObj).getIntValue();
}
else {
_prevXref = -1;
}
// Get the filter, for subsequent decompression.
// We're guaranteed by the spec that this won't be a decryption
// filter.
_filters = _xstrm.getFilters();
// Why isn't this being used?
// passed all tests
return true;
}
catch (Exception e) {
return false;
}
}
/**
* Prepares for reading the Stream.
* If the filter list includes one which we don't support, throws a
* PdfException.
*/
public void initRead (RandomAccessFile raf)
throws IOException, PdfException
{
Stream strm = _xstrm.getStream ();
strm.setFilters (_xstrm.getFilters ());
strm.initRead (raf);
_readRange = 0;
_readIndex = 0;
/* Calculate the total bytes per entry. This may have
* some utility. */
_bytesPerEntry = 0;
for (int i = 0; i < _fieldSizes.length; i++) {
_bytesPerEntry += _fieldSizes[i];
}
}
/**
* Reads the next object in the stream.
*
* After calling readObject, it is possible to
* call accessors to get information about the object.
* For the moment, we punt on the question of how to deal with
* Object Streams.
*
* Free objects are skipped over while being counted. After
* readNextObject() returns false, the caller
* may call getFreeCount() to determine the
* number of free objects.
*
* @return true if there is an object, false
* if no more objects are available.
*/
public boolean readNextObject () throws IOException
{
/* Get the field type. */
int wid;
Stream is = _xstrm.getStream ();
int i;
int b;
for (;;) {
/* Loop till we find an actual object; we just count
* type 0's, which are free entries. */
wid = _fieldSizes[0];
if (_readIndex++ >= _index[_readRange].len) {
_readIndex = 1;
if (_readRange++ >= _index.length) {
return false; // Read full complement
}
}
if (wid != 0) {
/* "Fields requiring more than one byte are stored
* with the high-order byte first." */
_objType = 0;
for (i = 0; i < wid; i++) {
b = is.read ();
if (b < 0) {
return false;
}
_objType = _objType * 256 + b;
}
}
else {
_objType = 1; // Default if field width is 0
}
wid = _fieldSizes[1];
_objField1 = 0;
for (i = 0; i < wid; i++) {
b = is.read ();
if (b < 0) {
return false;
}
_objField1 = _objField1 * 256 + b;
}
wid = _fieldSizes[2];
_objField2 = 0;
for (i = 0; i < wid; i++) {
b = is.read ();
if (b < 0) {
return false;
}
_objField2 = _objField2 * 256 + b;
}
if (_objType != 0) {
_objNum = _index[_readRange].start + _readIndex - 1;
return true;
}
++_freeCount;
}
}
/**
* Returns number of the last object read by
* readNextObject ().
* Do not call if readNextObject ()
* returns false.
*/
public int getObjNum ()
{
return _objNum;
}
/**
* Returns true if the last object read by
* readNextObject () is a compressed object.
* Do not call if readNextObject ()
* returns false.
*/
public boolean isObjCompressed ()
{
return (_objType == 2);
}
/**
* Returns the number of free objects detected. This may
* be called after readNextObject returns
* false, signifying that all the objects
* have been read and all the free objects counted.
*/
public int getFreeCount ()
{
return _freeCount;
}
/**
* Returns the total number of objects in the document's
* cross-reference table at the time this stream was written.
*/
public int getCrossRefTableSize()
{
return _size;
}
/**
* Returns the offset of the last object read.
* This is meaningful only if the last object read
* was type 1 (uncompressed).
*/
public int getOffset ()
{
return _objField1;
}
/**
* Returns the object number of the content stream in
* which this object is stored.
* This is meaningful only if the last object read
* was type 2 (compressed in content stream).
*/
public int getContentStreamObjNum ()
{
return _objField1;
}
/**
* Returns the offset of the previous cross-reference stream,
* or -1 if none is specified.
*/
public long getPrevXref ()
{
return _prevXref;
}
/**
* Returns the content stream index of the last object read.
* This is meaningful only if the last object read
* was type 2 (compressed in content stream).
*/
public int getContentStreamIndex ()
{
return _objField2;
}
}
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