Class: AcroThat::ObjectResolver

Inherits:
Object
  • Object
show all
Defined in:
lib/acro_that/object_resolver.rb

Overview

Parses xref (tables and streams) and exposes object bodies uniformly, including objects embedded in /ObjStm. Also gives you the trailer and /Root.

Defined Under Namespace

Classes: Entry

Instance Method Summary collapse

Constructor Details

#initialize(bytes) ⇒ ObjectResolver

Returns a new instance of ObjectResolver.



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# File 'lib/acro_that/object_resolver.rb', line 9

def initialize(bytes)
  @bytes = bytes
  @entries = {}
  @objstm_cache = {}
  parse_cross_reference
end

Instance Method Details

#apply_png_predictor(data, columns) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 263

def apply_png_predictor(data, columns)
  # PNG predictor: each row starts with a filter byte, followed by 'columns' data bytes
  row_size = columns + 1  # 1 byte for predictor + columns bytes of data
  num_rows = data.bytesize / row_size
  result = []
  prev_row = [0] * columns

  num_rows.times do |i|
    row_start = i * row_size
    filter_type = data.getbyte(row_start)
    row_bytes = (1..columns).map { |j| data.getbyte(row_start + j) }

    decoded_row = case filter_type
                  when 0  # None
                    row_bytes
                  when 1  # Sub
                    out = []
                    columns.times do |j|
                      left = j.positive? ? out[j - 1] : 0
                      out << ((row_bytes[j] + left) & 0xFF)
                    end
                    out
                  when 2  # Up
                    row_bytes.map.with_index { |b, j| (b + prev_row[j]) & 0xFF }
                  when 3  # Average
                    out = []
                    columns.times do |j|
                      left = j.positive? ? out[j - 1] : 0
                      up = prev_row[j]
                      out << ((row_bytes[j] + ((left + up) / 2)) & 0xFF)
                    end
                    out
                  when 4  # Paeth
                    out = []
                    columns.times do |j|
                      left = j.positive? ? out[j - 1] : 0
                      up = prev_row[j]
                      up_left = j.positive? ? prev_row[j - 1] : 0
                      out << ((row_bytes[j] + paeth_predictor(left, up, up_left)) & 0xFF)
                    end
                    out
                  else
                    row_bytes # Unknown filter, pass through
                  end

    result.concat(decoded_row)
    prev_row = decoded_row
  end

  result.pack("C*")
end

#balanced_from(str, start_idx) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 330

def balanced_from(str, start_idx)
  depth = 0
  j = start_idx
  while j < str.length
    if str[j, 2] == "<<"
      depth += 1
      j += 2
    elsif str[j, 2] == ">>"
      depth -= 1
      j += 2
      return j if depth.zero?
    else
      j += 1
    end
  end
  raise "unterminated dict"
end

#clear_cache ⇒ Object

Clear the object stream cache to free memory



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# File 'lib/acro_that/object_resolver.rb', line 53

def clear_cache
  @objstm_cache.clear
end

#decode_stream_data(dict_src, stream_chunk) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 239

def decode_stream_data(dict_src, stream_chunk)
  s_match = /\bstream\r?\n/.match(stream_chunk) or raise "stream keyword missing"
  body = stream_chunk[s_match.end(0)..]
  body = body.sub(/\bendstream\b.*/m, "")

  # Decompress if FlateDecode (handle both "/Filter /FlateDecode" and "/Filter/FlateDecode")
  data = if dict_src =~ %r{/Filter\s*/FlateDecode}
           Zlib::Inflate.inflate(body)
         else
           body
         end

  # Apply PNG predictor if present
  if dict_src =~ %r{/DecodeParms\s*<<[^>]*/Predictor\s+(\d+)}
    predictor = ::Regexp.last_match(1).to_i
    if predictor.between?(10, 15) # PNG predictors
      columns = dict_src =~ %r{/Columns\s+(\d+)} ? ::Regexp.last_match(1).to_i : 1
      data = apply_png_predictor(data, columns)
    end
  end

  data
end

#each_object ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 46

def each_object
  @entries.each_key do |ref|
    yield(ref, object_body(ref))
  end
end

#find_startxref(bytes) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 348

def find_startxref(bytes)
  return nil if bytes.nil? || bytes.empty?

  if bytes =~ /startxref\s+(\d+)\s*%%EOF\s*\z/m
    return Integer(::Regexp.last_match(1))
  end

  m = bytes.rindex("startxref")
  return nil unless m

  tail = bytes[m, bytes.length - m]
  tail[/startxref\s+(\d+)/m, 1]&.to_i
end

#JSON_like_array(tok) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 234

def JSON_like_array(tok)
  inner = tok[1..-2]
  inner.split(/\s+/).map { |t| t =~ /\A\d+\z/ ? t.to_i : t }
end

#load_objstm(container_ref) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 362

def load_objstm(container_ref)
  return if @objstm_cache.key?(container_ref)

  body = object_body(container_ref)
  raise "Object stream #{container_ref.inspect} not found in xref table" unless body

  dict_start = body.index("<<") || 0
  dict_end = balanced_from(body, dict_start)
  dict_src = body[dict_start...dict_end]
  s_pos = body.index(/\bstream\r?\n/m, dict_end) or raise "objstm stream missing"
  e_pos = body.index(/\bendstream\b/m, s_pos) or raise "objstm end missing"
  data = body[s_pos..e_pos]
  raw = decode_stream_data(dict_src, data)
  n = DictScan.value_token_after("/N", dict_src).to_i
  first = DictScan.value_token_after("/First", dict_src).to_i
  parsed = AcroThat::ObjStm.parse(raw, n: n, first: first)
  @objstm_cache[container_ref] = parsed
end

#object_body(ref) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 57

def object_body(ref)
  case (e = @entries[ref])&.type
  when :in_file
    i = e.offset
    # Find "obj" start near offset (handle any preceding whitespace)
    hdr = /\bobj\b/m.match(@bytes, i) or return nil
    after = hdr.end(0)
    # Skip optional whitespace and one line break if present
    after += 1 while (ch = @bytes.getbyte(after)) && ch <= 0x20
    j = @bytes.index(/\bendobj\b/m, after) or return nil
    @bytes[after...j]
  when :in_objstm
    load_objstm([e.objstm_num, 0])
    @objstm_cache[[e.objstm_num, 0]][e.objstm_index][:body]
  end
end

#paeth_predictor(a, b, c) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 315

def paeth_predictor(a, b, c)
  # a = left, b = up, c = up-left
  p = a + b - c
  pa = (p - a).abs
  pb = (p - b).abs
  pc = (p - c).abs
  if pa <= pb && pa <= pc
    a
  elsif pb <= pc
    b
  else
    c
  end
end

#parse_classic_xref(start) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 111

def parse_classic_xref(start)
  pos = @bytes.rindex("xref", start) or raise "xref not found"
  i = pos + 4

  loop do
    m = /\s*(\d+)\s+(\d+)/m.match(@bytes, i) or break
    first = m[1].to_i
    count = m[2].to_i
    i = m.end(0)

    count.times do |k|
      # Skip whitespace/newlines before reading the 20-byte record
      i += 1 while (ch = @bytes.getbyte(i)) && [0x0A, 0x0D, 0x20].include?(ch)

      rec = @bytes[i, 20]
      raise "bad xref record" unless rec && rec.bytesize == 20

      off = rec[0, 10].to_i
      gen = rec[11, 5].to_i
      typ = rec[17, 1]
      i += 20
      # consume line ending(s)
      i += 1 while (ch = @bytes.getbyte(i)) && [0x0A, 0x0D].include?(ch)

      ref = [first + k, gen]
      @entries[ref] ||= Entry.new(type: :in_file, offset: off) if typ == "n"
      # (ignore 'f' free entries)
    end

    break if @bytes[i, 7] == "trailer"
  end

  tpos = @bytes.index("trailer", i)
  if tpos
    dpos = @bytes.index("<<", tpos)
    if dpos
      dend = balanced_from(@bytes, dpos)
      @last_xref_stream_dict = nil
      @trailer_explicit = @bytes[dpos...dend]
      return @trailer_explicit
    end
  end

  # No trailer found (might be at an intermediate xref in the chain)
  nil
end

#parse_cross_reference ⇒ Object

— internals ———————————————————–



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# File 'lib/acro_that/object_resolver.rb', line 76

def parse_cross_reference
  start = find_startxref(@bytes) or raise "startxref not found"
  parse_xref_at_offset(start)
end

#parse_xref_at_offset(offset) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 81

def parse_xref_at_offset(offset)
  # 1) If 'xref' is literally at that offset => classic table
  if @bytes[offset, 4] == "xref"
    tr = parse_classic_xref(offset)

    # 2) Classic trailers may include /XRefStm <offset> to an xref stream with compressed entries
    xrefstm_tok = DictScan.value_token_after("/XRefStm", tr) if tr
    if xrefstm_tok && (ofs = xrefstm_tok.to_i).positive?
      parse_xref_stream_at(ofs) # merge entries from xref stream (type 0/1/2)
    end

    # 3) Follow /Prev pointer if present
    prev_tok = DictScan.value_token_after("/Prev", tr) if tr
    if prev_tok && (prev_ofs = prev_tok.to_i).positive?
      parse_xref_at_offset(prev_ofs)
    end
  else
    # Direct xref stream case (offset points to the xref stream obj header)
    dict_src = parse_xref_stream_at(offset)

    # Follow /Prev in the xref stream's dictionary
    if dict_src
      prev_tok = DictScan.value_token_after("/Prev", dict_src)
      if prev_tok && (prev_ofs = prev_tok.to_i).positive?
        parse_xref_at_offset(prev_ofs)
      end
    end
  end
end

#parse_xref_stream_at(header_ofs) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 158

def parse_xref_stream_at(header_ofs)
  # Expect "<num> <gen> obj" at header_ofs
  m = /\A(\d+)\s+(\d+)\s+obj\b/m.match(@bytes[header_ofs, 50])
  unless m
    # Sometimes header_ofs might land on whitespace; search forward a bit
    win = @bytes[header_ofs, 256]
    m2 = /(\d+)\s+(\d+)\s+obj\b/m.match(win) or raise "xref stream header not found"
    header_ofs += m2.begin(0)
    m = m2
  end
  obj_ref = [m[1].to_i, m[2].to_i]

  dpos = @bytes.index("<<", header_ofs + m[0].length) or raise "xref stream dict missing"
  dend = balanced_from(@bytes, dpos)
  dict_src = @bytes[dpos...dend]
  @last_xref_stream_dict ||= dict_src # Keep first one for trailer_dict

  spos = @bytes.index(/\bstream\r?\n/m, dend) or raise "xref stream body missing"
  epos = @bytes.index(/\bendstream\b/m, spos) or raise "xref stream end missing"
  data = @bytes[spos..epos]
  raw = decode_stream_data(dict_src, data)

  # W is mandatory in xref streams; if missing, bail (don't crash)
  w_tok = DictScan.value_token_after("/W", dict_src)
  return nil unless w_tok

  w = JSON_like_array(w_tok)
  idx_tok = DictScan.value_token_after("/Index", dict_src)
  index = idx_tok ? JSON_like_array(idx_tok) : [0, DictScan.value_token_after("/Size", dict_src).to_i]

  parse_xref_stream_records(raw, w, index)

  # Ensure the xref stream object itself is registered (type 1 entry usually exists,
  # but if not, add it so object_body can find the stream if needed)
  unless @entries.key?(obj_ref)
    # Approximate offset at header_ofs
    @entries[obj_ref] = Entry.new(type: :in_file, offset: header_ofs)
  end

  dict_src # Return dict for /Prev checking
end

#parse_xref_stream_records(raw, w, index) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 200

def parse_xref_stream_records(raw, w, index)
  w0, w1, w2 = w
  s = StringScanner.new(raw)
  (0...(index.length / 2)).each do |i|
    obj = index[2 * i].to_i
    count = index[(2 * i) + 1].to_i
    count.times do |k|
      t  = read_int(s, w0)
      f1 = read_int(s, w1)
      f2 = read_int(s, w2)
      ref = [obj + k, 0]
      case t
      when 0 then next # free
      when 1 then @entries[ref] ||= Entry.new(type: :in_file, offset: f1)
      when 2 then @entries[ref] ||= Entry.new(type: :in_objstm, objstm_num: f1, objstm_index: f2)
      end
    end
  end
end

#read_int(scanner, width) ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 220

def read_int(scanner, width)
  # Ensure width is an integer
  w = width.is_a?(Integer) ? width : width.to_i
  return 0 if w.zero?

  bytes = scanner.peek(w)
  return 0 unless bytes && bytes.bytesize == w

  scanner.pos += w
  val = 0
  bytes.each_byte { |b| val = (val << 8) | b }
  val
end

#root_ref ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 16

def root_ref
  tr = trailer_dict
  return nil unless tr =~ %r{/Root\s+(\d+)\s+(\d+)\s+R}

  [Integer(::Regexp.last_match(1)), Integer(::Regexp.last_match(2))]
end

#trailer_dict ⇒ Object 



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# File 'lib/acro_that/object_resolver.rb', line 23

def trailer_dict
  # Priority order:
  # 1. Explicit trailer from classic xref (incremental updates)
  # 2. Xref stream dictionary (original PDFs)
  # 3. Search for trailer (fallback)
  @trailer_dict ||= if @trailer_explicit
                      @trailer_explicit
                    elsif @last_xref_stream_dict
                      @last_xref_stream_dict
                    else
                      # Find last 'trailer << ... >>' before last startxref
                      start = find_startxref(@bytes) || 0
                      head = @bytes[0...start]
                      idx = head.rindex("trailer")
                      raise "trailer not found" unless idx

                      # naive grab following dict
                      m = head.index("<<", idx)
                      n = balanced_from(head, m)
                      head[m...n]
                    end
end