Class: HexaPDF::Type::XRefStream

Inherits:
Stream show all
Defined in:
lib/hexapdf/type/xref_stream.rb

Overview

Represents PDF type XRef, cross-reference streams.

A cross-reference stream is used as a more compact representation for an cross-reference section and trailer dictionary. The trailer dictionary is incorporated into the stream dictionary and the cross-reference section entries are stored in the stream itself, compressed to save space.

How are Cross-reference Streams Used?

Cross-reference stream objects are only used when parsing or writing a PDF document.

When a file is read and a cross-reference stream is found, it is loaded and its information is stored in a HexaPDF::Revision object. So from a user’s perspective nothing changes when a cross-reference stream instead of a cross-reference section and trailer is encountered.

This also means that all information stored in a cross-reference stream between parsing and writing is discarded when the PDF document gets written!

Upon writing a revision it is checked whether that revision contains a cross-reference stream object. If it does the cross-reference stream object is updated with the cross-reference section and trailer information and then written. Otherwise a normal cross-reference section plus trailer are written.

See: PDF2.0 s7.5.8

Constant Summary

Constants included from DictionaryFields

DictionaryFields::Boolean, DictionaryFields::PDFByteString, DictionaryFields::PDFDate

Instance Attribute Summary

Attributes inherited from Object

#data, #document, #must_be_indirect

Instance Method Summary collapse

Methods inherited from Stream

#must_be_indirect?, #raw_stream, #set_filter, #stream, #stream=, #stream_decoder, #stream_encoder, #stream_source

Methods inherited from Dictionary

#[], #[]=, define_field, define_type, #delete, #each, each_field, #empty?, field, #key?, #to_hash, type, #type

Methods inherited from Object

#<=>, #==, #cache, #cached?, #clear_cache, deep_copy, #deep_copy, #document?, #eql?, field, #gen, #gen=, #hash, #indirect?, #initialize, #inspect, make_direct, #must_be_indirect?, #null?, #oid, #oid=, #type, #validate, #value, #value=

Constructor Details

This class inherits a constructor from HexaPDF::Object

Instance Method Details

#trailer ⇒ Object

Returns a hash with the entries that represent the file trailer part of the cross-reference stream’s dictionary.

See: Type::Trailer



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# File 'lib/hexapdf/type/xref_stream.rb', line 93

def trailer
  trailer = {Type: :XRef}
  Trailer.each_field.with_object(trailer) do |(name, _data), hash|
    hash[name] = value[name] if key?(name)
  end
end

#update_with_xref_section_and_trailer(xref_section, trailer) ⇒ Object

Makes this cross-reference stream represent the data in the given HexaPDF::XRefSection and Type::Trailer.

The xref_section needs to contain an entry for this cross-reference stream and it is necessary that this entry is the one with the highest byte position (for calculating the correct /W entry).

The given cross-reference section is not stored but only used to rewrite the associated stream to reflect the cross-reference section. The dictionary is updated with the information from the trailer and the needed entries for the cross-reference section.

If there are changes to the cross-reference section or trailer, this method has to be invoked again.



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# File 'lib/hexapdf/type/xref_stream.rb', line 113

def update_with_xref_section_and_trailer(xref_section, trailer)
  value.replace(trailer)
  value[:Type] = :XRef
  write_xref_section_to_stream(xref_section)
  set_filter(:FlateDecode, Columns: value[:W].inject(:+), Predictor: 12)
end

#xref_section ⇒ Object

Returns an XRefSection that represents the content of this cross-reference stream.

Each invocation returns a new XRefSection object based on the current data in the associated stream and dictionary.



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# File 'lib/hexapdf/type/xref_stream.rb', line 84

def xref_section
  index = self[:Index] || [0, self[:Size]]
  parse_xref_section(index, self[:W])
end