Module: Pdfrb::Source::Recovery

Defined in:
lib/pdfrb/source/recovery.rb

Overview

Last-resort recovery: scan the entire file for \d+ \d+ obj patterns and synthesise an XrefSection from the offsets found. Triggered when the xref table is missing or points to a wrong offset.

Extended recovery capabilities:

* Cross-reference table reconstruction (the original mode).
* Trailer recovery: find /Root and /Encrypt by scanning for
the keys directly when the trailer is corrupt.
* Hybrid xref detection: read both xref table and xref stream
when both are present.
* Object-stream reconstruction: rebuild an XrefSection from a
/Type /ObjStm stream when its header is intact but the
xref stream is missing.

Class Method Summary collapse

Class Method Details

.hybrid_xref?(io) ⇒ Boolean

Detect hybrid xref: PDF 1.7+ allows both a classical xref table AND an xref stream in the same revision, referenced from the table via /XRefStm. Returns true if the file has both. Pure-Ruby: scans for startxref followed by both xref and a Type=XRef stream object.

Returns:

  • (Boolean) 


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# File 'lib/pdfrb/source/recovery.rb', line 72

def hybrid_xref?(io)
  io.seek(0, IO::SEEK_SET)
  data = io.read.to_s
  data.force_encoding(Encoding::BINARY)

  has_table = data.include?("\nxref\n")
  has_stream = data.match?(/\/Type\s*\/XRef\b/)
  has_table && has_stream
end

.rebuild_from_object_stream(objstm, document) ⇒ Object

Reconstruct an XrefSection from a /Type /ObjStm stream by reading its /N + /First + decompressed body. Used when the outer xref stream is corrupt but a known ObjStm is intact.



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# File 'lib/pdfrb/source/recovery.rb', line 85

def rebuild_from_object_stream(objstm, document)
  return nil unless objstm

  pairs = ObjectStreamReader.read(objstm, document)
  section = XrefSection.new
  pairs.each_key do |oid|
    # Mark as compressed in objstm 0 (placeholder; caller
    # fills in the real objstm_oid).
    section.add_compressed(oid, 0, 0, objstm.oid)
  end
  section
end

.rebuild_xref(io, recover: true) ⇒ Object 



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# File 'lib/pdfrb/source/recovery.rb', line 29

def rebuild_xref(io, recover: true)
  return nil unless recover

  io.seek(0, IO::SEEK_SET)
  section = XrefSection.new
  data = io.read
  data.force_encoding(Encoding::BINARY)
  offset = 0
  while (m = data.match(OBJ_PATTERN, offset))
    oid = m[1].to_i
    gen = m[2].to_i
    entry_offset = m.begin(0)
    # Take the last-seen gen per oid (PDF spec: highest gen wins).
    section.add_in_use(oid, gen, entry_offset)
    offset = m.end(0)
  end
  section
end

.recover_trailer_references(io) ⇒ Object

Scan the file for /Root, /Info, /Encrypt references when the trailer dict is unreadable. Returns a Hash with :Root, :Info, :Encrypt keys as References (or nil per key).



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# File 'lib/pdfrb/source/recovery.rb', line 51

def recover_trailer_references(io)
  io.seek(0, IO::SEEK_SET)
  data = io.read.to_s
  data.force_encoding(Encoding::BINARY)

  root_match = data.match(TRAILER_ROOT_PATTERN)
  info_match = data.match(TRAILER_INFO_PATTERN)
  encrypt_match = data.match(TRAILER_ENCRYPT_PATTERN)

  {
    Root: root_match ? Pdfrb::Model::Reference.new(root_match[1].to_i, root_match[2].to_i) : nil,
    Info: info_match ? Pdfrb::Model::Reference.new(info_match[1].to_i, info_match[2].to_i) : nil,
    Encrypt: encrypt_match ? Pdfrb::Model::Reference.new(encrypt_match[1].to_i, encrypt_match[2].to_i) : nil,
  }
end