Module: Fontisan::SfntBuilder

Defined in:
lib/fontisan/sfnt_builder.rb

Overview

Builds an SFNT (TrueType/OpenType) binary from a flavor + per-tag table data. Used whenever WOFF2-derived table data needs to be reassembled into a parseable SFNT (single-font decoding, WOFF2 collection → font).

Single source of truth for the SFNT byte layout: 12-byte offset table, 16-byte table directory entries, padded table data.

Class Method Summary collapse

Class Method Details

.build(flavor:, tables:) ⇒ String 



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# File 'lib/fontisan/sfnt_builder.rb', line 17

def self.build(flavor:, tables:)
  num_tables = tables.length
  entry_selector = (Math.log(num_tables) / Math.log(2)).floor
  search_range = (2**entry_selector) * 16
  range_shift = (num_tables * 16) - search_range

  out = String.new(encoding: Encoding::BINARY)
  # Offset table (12 bytes)
  out << [flavor].pack("N")
  out << [num_tables].pack("n")
  out << [search_range].pack("n")
  out << [entry_selector].pack("n")
  out << [range_shift].pack("n")

  # Compute layout for table directory + table data with 4-byte padding
  data_start = 12 + (num_tables * 16)
  cursor = data_start
  records = tables.map do |tag, data|
    length = data.bytesize
    checksum = Utilities::ChecksumCalculator.calculate_table_checksum(data)
    padding = Utilities::Padding.boundary(length)
    record = { tag:, checksum:, offset: cursor, length:, data:, padding: }
    cursor += length + padding
    record
  end

  # Write all table directory entries first (16 bytes each, sorted
  # alphabetically per SFNT convention for stable output), then all
  # table data with 4-byte padding. The directory entries' offset
  # fields were precomputed assuming this layout.
  sorted = records.sort_by { |r| r[:tag] }
  # rubocop:disable Style/CombinableLoops
  # SFNT requires all directory entries before any table data
  sorted.each do |r|
    out << r[:tag].ljust(4, "\x00")
    out << [r[:checksum]].pack("N")
    out << [r[:offset]].pack("N")
    out << [r[:length]].pack("N")
  end

  sorted.each do |r|
    out << r[:data]
    out << ("\x00" * r[:padding])
  end
  # rubocop:enable Style/CombinableLoops

  out
end