Class: CorpPdf::IncrementalWriter

Inherits:
Object
  • Object
show all
Defined in:
lib/corp_pdf/incremental_writer.rb

Overview

Appends an incremental update containing the given patches. Each patch is , body:String

Instance Method Summary collapse

Constructor Details

#initialize(original_bytes, patches) ⇒ IncrementalWriter

Returns a new instance of IncrementalWriter.



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# File 'lib/corp_pdf/incremental_writer.rb', line 7

def initialize(original_bytes, patches)
  @orig = original_bytes
  @patches = patches
end

Instance Method Details

#render ⇒ Object 



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# File 'lib/corp_pdf/incremental_writer.rb', line 12

def render
  return @orig if @patches.empty?

  startxref_prev = find_startxref(@orig) or raise "startxref not found"
  max_obj = scan_max_obj_number(@orig)

  # Ensure we end with a newline before appending
  # Avoid dup by concatenating instead of modifying in place
  newline_if_needed = @orig.end_with?("\n") ? "".b : "\n".b
  original_with_newline = @orig + newline_if_needed

  buf = +""
  offsets = []

  # Write patches into an object stream for efficiency
  objstm_data = CorpPdf::ObjStm.create(@patches, compress: true)
  if objstm_data
    # Get the next object number for the object stream itself
    objstm_num = [max_obj + 1, @patches.map { |p| p[:ref][0] }.max.to_i + 1].max

    # Write the object stream object
    objstm_offset = original_with_newline.bytesize + buf.bytesize
    offsets << [objstm_num, 0, objstm_offset]

    buf << "#{objstm_num} 0 obj\n".b
    buf << objstm_data[:dictionary]
    buf << "\nstream\n".b
    buf << objstm_data[:stream_body]
    buf << "\nendstream\n".b
    buf << "endobj\n".b

    # Build xref stream (supports type 2 entries for objects in object streams)
    sorted_patches = objstm_data[:patches]
    xrefstm_num = objstm_num + 1

    # Collect all entries: object stream itself (type 1) + patches (type 2)
    # Format: [obj_num, gen, type, f1, f2]
    # For type 1: f1 is offset, f2 is generation (unused in xref streams)
    # For type 2: f1 is objstm_num, f2 is index in stream
    entries = []
    # Object stream itself - type 1 entry
    entries << [objstm_num, 0, 1, objstm_offset, 0]
    # Patches in object stream - type 2 entries
    sorted_patches.each_with_index do |patch, index|
      num, gen = patch[:ref]
      next if num == objstm_num # Skip the object stream itself

      entries << [num, gen, 2, objstm_num, index]
    end

    # Sort entries by object number
    entries.sort_by! { |num, gen, _type, _f1, _f2| [num, gen] }

    # Build Index array - use single range for simplicity
    # Index format: [first_obj, count]
    obj_nums = entries.map { |num, _gen, _type, _f1, _f2| num }
    min_obj = obj_nums.min
    max_obj = obj_nums.max

    # For Index, we need consecutive entries from min_obj to max_obj
    # So count is (max_obj - min_obj + 1)
    index_count = max_obj - min_obj + 1
    index_array = [min_obj, index_count]

    # Build xref stream data with proper ordering
    # W = [1, 4, 2] means: type (1 byte), offset/f1 (4 bytes), index/f2 (2 bytes)
    w = [1, 4, 2]

    # Create a map for quick lookup by object number
    entry_map = {}
    entries.each { |num, _gen, type, f1, f2| entry_map[num] = [type, f1, f2] }

    # Build xref records in order according to Index range
    # Index says entries start at min_obj and we have index_count entries
    xref_records = []
    index_count.times do |k|
      obj_num = min_obj + k
      if entry_map[obj_num]
        type, f1, f2 = entry_map[obj_num]
        xref_records << [type, f1, f2].pack("C N n")
      else
        # Type 0 (free) entry for missing objects in the range
        xref_records << [0, 0, 0].pack("C N n")
      end
    end

    xref_bytes = xref_records.join("".b)

    # Compress xref stream
    xref_compressed = Zlib::Deflate.deflate(xref_bytes)

    # Size is max object number + 1 (must include xrefstm_num itself)
    size = [max_obj + 1, objstm_num + 1, xrefstm_num + 1].max

    # Write xref stream object
    xrefstm_offset = original_with_newline.bytesize + buf.bytesize
    root_ref = extract_root_from_trailer(@orig)
    xrefstm_dict = "<<\n/Type /XRef\n/W [#{w.join(' ')}]\n/Size #{size}\n/Index [#{index_array.join(' ')}]\n/Prev #{startxref_prev}\n".b
    xrefstm_dict << " /Root #{root_ref}".b if root_ref
    xrefstm_dict << "\n/Filter /FlateDecode\n/Length #{xref_compressed.bytesize}\n>>\n".b

    buf << "#{xrefstm_num} 0 obj\n".b
    buf << xrefstm_dict
    buf << "stream\n".b
    buf << xref_compressed
    buf << "\nendstream\n".b
    buf << "endobj\n".b

    # Build trailer - need to include xref stream itself
    # The xref stream itself must be accessible, so we use a classic trailer pointing to it
    new_size = [max_obj + 1, xrefstm_num, @patches.map { |p| p[:ref][0] }.max.to_i + 1].max
    xref_offset = xrefstm_offset

    trailer = "trailer\n<< /Size #{new_size} /Prev #{startxref_prev}".b
    trailer << " /Root #{root_ref}".b if root_ref
    trailer << " /XRefStm #{xrefstm_offset} >>\n".b
    trailer << "startxref\n#{xref_offset}\n%%EOF\n".b

    result = original_with_newline + buf + trailer

  else
    # Fallback to individual objects if ObjStm.create fails
    @patches.each do |p|
      num, gen = p[:ref]
      offset = original_with_newline.bytesize + buf.bytesize
      offsets << [num, gen, offset]

      # Write object with proper formatting
      buf << "#{num} #{gen} obj\n"
      buf << p[:body]
      buf << "\nendobj\n"
    end

    # Build xref table
    sorted = offsets.sort_by { |n, g, _| [n, g] }
    xref = +"xref\n"

    i = 0
    while i < sorted.length
      first_num = sorted[i][0]
      run = 1
      while (i + run) < sorted.length && sorted[i + run][0] == first_num + run && sorted[i + run][1] == sorted[i][1]
        run += 1
      end
      xref << "#{first_num} #{run}\n"
      run.times do |r|
        abs = sorted[i + r][2]
        gen = sorted[i + r][1]
        xref << format("%010d %05d n \n", abs, gen)
      end
      i += run
    end

    # Debug: verify xref was built
    if xref == "xref\n"
      raise "Xref table is empty! Offsets: #{offsets.inspect}"
    end

    # Build trailer with /Root reference
    new_size = [max_obj + 1, @patches.map { |p| p[:ref][0] }.max.to_i + 1].max
    xref_offset = original_with_newline.bytesize + buf.bytesize

    # Extract /Root from original trailer
    root_ref = extract_root_from_trailer(@orig)
    root_entry = root_ref ? " /Root #{root_ref}" : ""

    trailer = "trailer\n<< /Size #{new_size} /Prev #{startxref_prev}#{root_entry} >>\nstartxref\n#{xref_offset}\n%%EOF\n"

    result = original_with_newline + buf + xref + trailer

    # Verify xref was built correctly
    if xref.length < 10
      warn "Warning: xref table seems too short (#{xref.length} bytes). Expected proper entries."
    end

  end
  result
end