class Prawn::SVG::TTF
  SFNT_VERSION_STRINGS = ["\x00\x01\x00\x00", 'true', 'typ1'].freeze
  LANGUAGE_IDS = [0, 0x409].freeze # English, US English
  UTF_16BE_PLATFORM_IDS = [0, 3].freeze # Unicode, Microsoft

  attr_reader :family, :subfamily

  def initialize(filename)
    load_data_from_file(filename)
  end

  private

  def load_data_from_file(filename)
    File.open(filename, 'rb') do |f|
      offset_table = f.read(12)
      return unless offset_table && offset_table.length == 12 && SFNT_VERSION_STRINGS.include?(offset_table[0..3])

      table_count = (offset_table[4].ord * 256) + offset_table[5].ord
      tables = f.read(table_count * 16)
      return unless tables && tables.length == table_count * 16

      offset, length = table_count.times do |index|
        start = index * 16
        break tables[start + 8..start + 15].unpack('NNN') if tables[start..start + 3] == 'name'
      end

      return unless length

      f.seek(offset)
      data = f.read(length)
      return unless data && data.length == length

      _, name_count, string_offset = data[0..5].unpack('nnn')

      names = {}
      name_count.times do |index|
        start = 6 + (index * 12)
        platform_id, _, language_id, name_id, length, offset = data[start..start + 11].unpack('nnnnnn')
        next unless offset
        next unless LANGUAGE_IDS.include?(language_id)
        next unless [1, 2, 16, 17].include?(name_id)

        offset += string_offset
        field = data[offset..offset + length - 1]
        next unless field && field.length == length

        names[name_id] =
          if UTF_16BE_PLATFORM_IDS.include?(platform_id)
            begin
              field.force_encoding(Encoding::UTF_16BE).encode(Encoding::UTF_8)
            rescue StandardError
              field
            end
          else
            field
          end
      end

      @family = names[16] || names[1]
      @subfamily = names[17] || names[2]
    end
  rescue Errno::ENOENT # in case the file disappears between the scan and the load, we don't want to crash
  end
end