Parsing WOFF / OTF fonts in Ruby

parse_otf.rb

# WOFF spec                                https://www.w3.org/TR/2012/REC-WOFF-20121213/
# OTF spec                                 https://www.microsoft.com/en-us/Typography/OpenTypeSpecification.aspx
# Really nice inspector                    https://opentype.js.org/font-inspector.html
# unpack instructions                      http://www.rubydoc.info/stdlib/core/String#unpack-instance_method
# Font programming instruction definitions https://developer.apple.com/fonts/TrueType-Reference-Manual/RM05/Chap5.html#WS
# Font forge has some useful info, too, get it with homebrew cask
FONT_FILE = '/Users/xjxc322/gamut/bots/pxgamut_regular.woff'

require 'zlib'

SIZES = {
  ''   => 0,
  'a4' => 4, # 4 chars, 8 bits each
  'L>' => 4, # 32 bits unsigned
  'l>' => 4, # 32 bits signed
  'S>' => 2, # 16 bits unsigned
  's>' => 2, # 16 bits signed
}

def read_and_unpack(binary, format_hash)
  struct = Struct.new(*format_hash.keys).new
  format_hash.each do |name, format|
    struct[name] = binary.unpack(format)[0]
    binary = binary[SIZES.fetch(format)..-1]
  end
  return struct, binary
end



rest = font = File.read(FONT_FILE, encoding: 'ASCII-8BIT')

header, rest = read_and_unpack(
  rest,
  signature:        "a4",
  flavor:           "a4",
  length:           "L>",
  num_tables:       "S>",
  reserved:         "S>",
  total_sfnt_size:  "L>",
  major_version:    "S>",
  minor_version:    "S>",
  meta_offset:      "L>",
  meta_length:      "L>",
  meta_orig_length: "L>",
  priv_offset:      "L>",
  priv_length:      "L>",
)
header
# => #<struct 
#     signature="wOFF",
#     flavor="OTTO",
#     length=69456,
#     num_tables=12,
#     reserved=0,
#     total_sfnt_size=133632,
#     major_version=0,
#     minor_version=0,
#     meta_offset=0,
#     meta_length=0,
#     meta_orig_length=0,
#     priv_offset=0,
#     priv_length=0>

entries = header.num_tables.times.map do
  entry, rest = read_and_unpack(
    rest,
    tag:           "a4",
    offset:        "L>",
    comp_length:   "L>",
    orig_length:   "L>",
    orig_checksum: "L>",
  )
  entry
end
entries.sort_by!(&:offset)

[%w[TAG    OFFSET   COMPUTED_LEN   ORIGINAL_LEN   CHECKSUM],
 %w[------ -------- -------------- -------------- ----------],
 *entries.map(&:to_a)
].map { |row| "%-6s%-8s%-14s%-14s%-10s" % row }
# => ["TAG   OFFSET  COMPUTED_LEN  ORIGINAL_LEN  CHECKSUM  ",
#     "----------------------------------------------------",
#     "head  284     54            54            208499658 ",
#     "hhea  340     33            36            247336158 ",
#     "maxp  376     6             6             42094592  ",
#     "OS/2  384     89            96            2363395350",
#     "name  476     499           1190          1917503162",
#     "cmap  976     1103          1712          3922624268",
#     "hmtx  2080    1326          2568          293570700 ",
#     "post  3408    19            32            4290248754",
#     "kern  3428    30983         65538         1282694526",
#     "GPOS  34412   4679          17102         3357198945",
#     "GSUB  39092   3235          8266          1306928084",
#     "CFF   42328   27126         36814         3912817438"]

tables = entries.map do |e|
  table = e.orig_length <= e.comp_length ?
            font[e.offset, e.orig_length] :
            Zlib::Inflate.inflate(font[e.offset, e.comp_length])
  [e.tag, table]
end.to_h

# Hmm, not sure I'm parsing this right, any
glyph_substitution, offsets_binary = read_and_unpack(
  tables['GSUB'],
  major_version:        "S>",
  minor_version:        "S>",
  script_list_offset:   "s>",
  feature_list_offset:  "s>",
  lookup_list_offset:   "s>",
  substitution_format:  "S>", # 2
  coverage_offset:      "s>",
  glyph_count:          "S>",
  substitute_glyph_ids: "",
)
offsets_binary.length # => 8250
glyph_substitution
# => #<struct 
#     major_version=1,
#     minor_version=0,
#     script_list_offset=10,
#     feature_list_offset=296,
#     lookup_list_offset=1780,
#     substitution_format=2,
#     coverage_offset=17478,
#     glyph_count=19540,
#     substitute_glyph_ids=nil>

I have a try but the result is failed.