An internal PDF::Reader class that represents the XRef table in a PDF file as a hash-like object.
An Xref table is a map of object identifiers and byte offsets. Any time a particular object needs to be found, the Xref table is used to find where it is stored in the file.
Hash keys are object ids, values are either:
a byte offset where the object starts (regular PDF objects)
a PDF::Reader::Reference instance that points to a stream that contains the desired object (PDF objects embedded in an object stream)
The class behaves much like a standard Ruby hash, including the use of the Enumerable mixin. The key difference is no []= method - the hash is read only.
create a new Xref table based on the contents of the supplied io object
io - must be an IO object, generally either a file or a StringIO
# File lib/pdf/reader/xref.rb, line 54 def initialize (io) @io = io @junk_offset = calc_junk_offset(io) || 0 @xref = {} @trailer = load_offsets end
returns the byte offset for the specified PDF object.
ref - a PDF::Reader::Reference object containing an object ID and revision number
# File lib/pdf/reader/xref.rb, line 71 def [](ref) @xref[ref.id][ref.gen] rescue raise InvalidObjectError, "Object #{ref.id}, Generation #{ref.gen} is invalid" end
iterate over each object in the xref table
# File lib/pdf/reader/xref.rb, line 78 def each(&block) ids = @xref.keys.sort ids.each do |id| gen = @xref[id].keys.sort[-1] yield PDF::Reader::Reference.new(id, gen) end end
return the number of objects in this file. Objects with multiple generations are only counter once.
# File lib/pdf/reader/xref.rb, line 64 def size @xref.size end
Returns the offset of the PDF document in the
stream
. In theory this should always be 0, but all sort of
crazy junk is prefixed to PDF files in the
real world.
Checks up to 50 chars into the file, returns nil if no PDF data detected.
# File lib/pdf/reader/xref.rb, line 227 def calc_junk_offset(io) io.rewind offset = io.pos until (c = io.readchar) == '%' || c == 37 || offset > 50 offset += 1 end io.rewind offset < 50 ? offset : nil end
Read a xref table from the underlying buffer.
If offset is specified the table will be loaded from there, otherwise the default offset will be located and used.
After seeking to the offset, processing is handed of to either #load_xref_table() or #load_xref_stream() based on what we find there.
# File lib/pdf/reader/xref.rb, line 96 def load_offsets(offset = nil) offset ||= new_buffer.find_first_xref_offset offset += @junk_offset buf = new_buffer(offset) tok_one = buf.token return load_xref_table(buf) if tok_one == "xref" || tok_one == "ref" tok_two = buf.token tok_three = buf.token if tok_one.to_i >= 0 && tok_two.to_i >= 0 && tok_three == "obj" buf = new_buffer(offset) stream = PDF::Reader::Parser.new(buf).object(tok_one.to_i, tok_two.to_i) return load_xref_stream(stream) end raise PDF::Reader::MalformedPDFError, "xref table not found at offset #{offset} (#{tok_one} != xref)" end
Read an XRef stream from the underlying buffer instead of a traditional xref table.
# File lib/pdf/reader/xref.rb, line 151 def load_xref_stream(stream) unless stream.is_a?(PDF::Reader::Stream) && stream.hash[:Type] == :XRef raise PDF::Reader::MalformedPDFError, "xref stream not found when expected" end trailer = Hash[stream.hash.select { |key, value| [:Size, :Prev, :Root, :Encrypt, :Info, :ID].include?(key) }] widths = stream.hash[:W] entry_length = widths.inject(0) { |s, w| s + w } raw_data = StringIO.new(stream.unfiltered_data) if stream.hash[:Index] index = stream.hash[:Index] else index = [0, stream.hash[:Size]] end index.each_slice(2) do |start_id, size| obj_ids = (start_id..(start_id+(size-1))) obj_ids.each do |objid| entry = raw_data.read(entry_length) || "" f1 = unpack_bytes(entry[0,widths[0]]) f2 = unpack_bytes(entry[widths[0],widths[1]]) f3 = unpack_bytes(entry[widths[0]+widths[1],widths[2]]) if f1 == 1 && f2 > 0 store(objid, f3, f2 + @junk_offset) elsif f1 == 2 && f2 > 0 store(objid, 0, PDF::Reader::Reference.new(f2, 0)) end end end load_offsets(trailer[:Prev].to_i) if trailer.has_key?(:Prev) trailer end
Assumes the underlying buffer is positioned at the start of a traditional Xref table and processes it into memory.
# File lib/pdf/reader/xref.rb, line 119 def load_xref_table(buf) params = [] while !params.include?("trailer") && !params.include?(nil) if params.size == 2 objid, count = params[0].to_i, params[1].to_i count.times do offset = buf.token.to_i generation = buf.token.to_i state = buf.token store(objid, generation, offset + @junk_offset) if state == "n" && offset > 0 objid += 1 params.clear end end params << buf.token end trailer = Parser.new(buf, self).parse_token raise MalformedPDFError, "PDF malformed, trailer should be a dictionary" unless trailer.kind_of?(Hash) load_offsets(trailer[:XRefStm]) if trailer.has_key?(:XRefStm) load_offsets(trailer[:Prev].to_i) if trailer.has_key?(:Prev) trailer end
Wrap the io stream we're working with in a buffer that can tokenise it for us.
We create multiple buffers so we can be tokenising multiple sections of the file at the same time without worrying about clearing the buffers contents.
# File lib/pdf/reader/xref.rb, line 211 def new_buffer(offset = 0) PDF::Reader::Buffer.new(@io, :seek => offset) end
Stores an offset value for a particular PDF object ID and revision number
# File lib/pdf/reader/xref.rb, line 217 def store (id, gen, offset) (@xref[id] ||= {})[gen] ||= offset end
XRef streams pack info into integers 1-N bytes wide. Depending on the number of bytes they need to be converted to an int in different ways.
# File lib/pdf/reader/xref.rb, line 190 def unpack_bytes(bytes) if bytes.to_s.size == 0 0 elsif bytes.size == 1 bytes.unpack("C")[0] elsif bytes.size == 2 bytes.unpack("n")[0] elsif bytes.size == 3 ("\x00" + bytes).unpack("N")[0] elsif bytes.size == 4 bytes.unpack("N")[0] else raise UnsupportedFeatureError, "Unable to unpack xref stream entries with more than 4 bytes" end end