Module: Pdfrb::Font::Sfnt

Defined in:
lib/pdfrb/font/sfnt.rb

Overview

sfnt container (OpenType/TTF) table directory: the shared seam for reading and rebuilding the wrapper around font outlines. Both subsetters (TrueType glyf and CFF) rebuild the directory around their subset tables; this module owns that format so the two implementations cannot drift (they once disagreed even on whether checksums are computed).

Defined Under Namespace

Classes: Entry

Class Method Summary collapse

Class Method Details

.checksum(data) ⇒ Object 



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# File 'lib/pdfrb/font/sfnt.rb', line 65

def checksum(data)
  padded = data + ("\x00".b * ((4 - (data.bytesize % 4)) % 4))
  sum = 0
  padded.unpack("N*").each { |w| sum = (sum + w) & 0xFFFFFFFF }
  sum
end

.parse_directory(sfnt) ⇒ Object

Parse the table directory of an sfnt byte string. Returns [sfnt_version, Array].



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# File 'lib/pdfrb/font/sfnt.rb', line 18

def parse_directory(sfnt)
  num_tables = (sfnt.getbyte(4) * 256) + sfnt.getbyte(5)
  version = sfnt.byteslice(0, 4)
  entries = (0...num_tables).map do |i|
    base = 12 + (i * 16)
    Entry.new(
      sfnt.byteslice(base, 4),
      sfnt.byteslice(base + 8, 4).unpack1("N"),
      sfnt.byteslice(base + 12, 4).unpack1("N")
    )
  end
  [version, entries]
end

.rebuild(sfnt_version, tables) ⇒ Object

Rebuild an sfnt wrapper: sfnt_version is the 4-byte magic (e.g. "\x00\x01\x00\x00" or "OTTO"), tables an Array of [tag, data] pairs. Checksums are computed per the sfnt spec (sum of big-endian u32s over zero-padded data).



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# File 'lib/pdfrb/font/sfnt.rb', line 42

def rebuild(sfnt_version, tables)
  num = tables.length
  search_range = (2**Math.log2(num).floor) * 16
  entry_selector = Math.log2(num).floor
  range_shift = (num * 16) - search_range

  out = +"".b
  out << sfnt_version.b
  out << [num, search_range, entry_selector, range_shift].pack("n4")

  sorted = tables.sort_by { |tag, _| tag }
  offset = 12 + (num * 16)
  sorted.each do |tag, data|
    out << tag.b << [checksum(data), offset, data.bytesize].pack("NNN")
    offset += (data.bytesize + 3) & ~3
  end
  sorted.to_h.each_value do |data|
    out << data.b
    out << ("\x00".b * ((4 - (out.bytesize % 4)) % 4))
  end
  out
end

.table_bytes(sfnt, tag) ⇒ Object

Raw bytes of +tag+'s table, or nil.



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# File 'lib/pdfrb/font/sfnt.rb', line 33

def table_bytes(sfnt, tag)
  entry = parse_directory(sfnt)[1].find { |e| e.tag == tag }
  entry && sfnt.byteslice(entry.offset, entry.byte_length)
end