Module: Fontisan::Ufo::Compile::Gsub

Defined in:
lib/fontisan/ufo/compile/gsub.rb

Overview

Builds the OpenType GSUB (Glyph Substitution) table from ligature substitution rules extracted from features.fea via FeatureCompiler.

Emits a minimal but valid GSUB with:

- ScriptList: DFLT script, default language system
- FeatureList: one feature record per ligature feature tag
(e.g. "liga", "dlig", "clig")
- LookupList: one LigatureSubst lookup per feature tag

Each sub A B C by ABC; rule becomes a LigatureSet on the first glyph (A), with a Ligature record pointing at ABC and component count 2 (B, C).

See Also:

Constant Summary collapse

SCRIPT_DFLT = 
"DFLT"
LANGSYS_DEFAULT = 
0

Class Method Summary collapse

Class Method Details

.build(parsed:, name_to_gid:) ⇒ String?

Build GSUB table bytes from parsed feature rules.

Parameters:

  • parsed (FeatureCompiler::ParsedFeatures)
  • name_to_gid (Hash{String=>Integer})

Returns:

  • (String, nil) —

    GSUB bytes, or nil if no ligature rules



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 33

def self.build(parsed:, name_to_gid:)
  features = ligature_features(parsed, name_to_gid)
  return nil if features.empty?

  lookups = features.values
  build_gsub_table(features.keys, lookups)
end

.build_coverage(glyphs) ⇒ Object

Build a Coverage table (format 1 = glyph list).

Parameters:

  • glyphs (Array<Integer>) —

    sorted glyph IDs



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 257

def self.build_coverage(glyphs)
  io = +""
  io << [1].pack("n") # coverageFormat = 1
  io << [glyphs.size].pack("n")
  glyphs.each { |gid| io << [gid].pack("n") }
  io
end

.build_feature_list(tags) ⇒ Object

FeatureList: one FeatureRecord per tag. Each points to a Feature table with one lookup index (the lookup for that tag).



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 136

def self.build_feature_list(tags)
  io = +""
  io << [tags.size].pack("n") # featureCount

  # FeatureRecords: tag(4) + featureOffset(2)
  records_size = tags.size * 6
  header_size = 2 + records_size

  feature_records = +""
  feature_tables = +""
  tags.each_with_index do |tag, idx|
    feature_records << tag.ljust(4, " ")[0, 4]
    feature_records << [header_size + (idx * 4)].pack("n")

    feature_tables << [0].pack("n") # featureParams (null)
    feature_tables << [1].pack("n") # lookupIndexCount
    feature_tables << [idx].pack("n") # lookupIndex (1:1 feature→lookup)
  end

  io << feature_records
  io << feature_tables
  io
end

.build_gsub_table(tags, lookups) ⇒ String

Build the complete GSUB table.

Parameters:

  • tags (Array<String>) —

    feature tags in lookup order

  • lookups (Array<Hash>) —

    one lookup per tag

Returns:

  • (String) 


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# File 'lib/fontisan/ufo/compile/gsub.rb', line 80

def self.build_gsub_table(tags, lookups)
  io = StringIO.new(+"")
  # Header: version (uint32 0x00010000), scriptListOffset,
  # featureListOffset, lookupListOffset (each uint16)
  header_size = 2 + 2 + 2 + 4 # version(4) + 3×uint16

  # Serialize each list independently, then compute offsets.
  script_list = build_script_list(tags.size)
  feature_list = build_feature_list(tags)
  lookup_list = build_lookup_list(lookups)

  script_list_offset = header_size
  feature_list_offset = script_list_offset + script_list.bytesize
  lookup_list_offset = feature_list_offset + feature_list.bytesize

  io << [0x00010000].pack("N")
  io << [script_list_offset].pack("n")
  io << [feature_list_offset].pack("n")
  io << [lookup_list_offset].pack("n")
  io << script_list
  io << feature_list
  io << lookup_list
  io.string
end

.build_ligature(lig) ⇒ Object

Build a single Ligature record.

Parameters:

  • lig (Hash) —

    components: [gids]



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 247

def self.build_ligature(lig)
  io = +""
  io << [lig[:result_gid]].pack("n")
  io << [lig[:components].size + 1].pack("n") # componentCount includes first
  lig[:components].each { |gid| io << [gid].pack("n") }
  io
end

.build_ligature_lookup(sets) ⇒ Object

Build a single LigatureSubst lookup (type 4, format 1).



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 185

def self.build_ligature_lookup(sets)
  io = +""
  # Lookup table: lookupType(2)=4, lookupFlag(2)=0, subTableCount(2)=1
  io << [4].pack("n")
  io << [0].pack("n")
  io << [1].pack("n")

  # Subtable offset (relative to start of Lookup table)
  lookup_header_size = 6
  io << [lookup_header_size].pack("n")

  # LigatureSubstFormat1: format(2)=1, coverageOffset(2),
  # ligatureSetCount(2), ligatureSetOffsets[]
  first_glyphs = sets.keys.sort
  set_count = first_glyphs.size
  subst_header_size = 2 + 2 + 2 + (set_count * 2)

  # Serialize each LigatureSet to compute offsets
  serialized_sets = first_glyphs.map { |gid| sets[gid] }.map { |s| build_ligature_set(s) }
  coverage = build_coverage(first_glyphs)

  pos = subst_header_size
  set_offsets = serialized_sets.map do |s|
    o = pos
    pos += s.bytesize
    o
  end
  coverage_offset = pos

  io << [1].pack("n") # substFormat = 1
  io << [coverage_offset].pack("n")
  io << [set_count].pack("n")
  set_offsets.each { |o| io << [o].pack("n") }
  serialized_sets.each { |s| io << s }
  io << coverage

  io
end

.build_ligature_set(ligatures) ⇒ Object

Build a LigatureSet for one first glyph.

Parameters:

  • ligatures (Array<Hash>) —

    components:



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 226

def self.build_ligature_set(ligatures)
  io = +""
  count = ligatures.size
  io << [count].pack("n")

  # Serialize each Ligature record
  serialized = ligatures.map { |lig| build_ligature(lig) }
  pos = 2 + (count * 2)
  offsets = serialized.map do |s|
    o = pos
    pos += s.bytesize
    o
  end

  offsets.each { |o| io << [o].pack("n") }
  serialized.each { |s| io << s }
  io
end

.build_ligature_sets(rules, name_to_gid) ⇒ Hash{Integer=>Array}

Build ligature sets keyed by the first glyph in each sequence.

Parameters:

  • rules (Array<Hash>) —

    result:

  • name_to_gid (Hash)

Returns:

  • (Hash{Integer=>Array}) —

    gid → [components: [gids]]



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 59

def self.build_ligature_sets(rules, name_to_gid)
  sets = Hash.new { |h, k| h[k] = [] }
  rules.each do |rule|
    seq_gids = rule[:sequence].map { |n| name_to_gid[n] }
    result_gid = name_to_gid[rule[:result]]
    next unless seq_gids.all? && result_gid

    first = seq_gids.first
    components = seq_gids[1..]
    sets[first] << { result_gid: result_gid, components: components }
  end
  sets
end

.build_lookup_list(lookups) ⇒ Object

LookupList: one lookup per feature tag. Each lookup is a LigatureSubst (type 4) lookup.



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 162

def self.build_lookup_list(lookups)
  io = +""
  count = lookups.size
  io << [count].pack("n") # lookupCount

  # Serialize each lookup first, then compute offsets relative
  # to the start of the LookupList.
  serialized = lookups.map { |sets| build_ligature_lookup(sets) }
  offset_table_size = 2 + (count * 2)
  pos = offset_table_size
  offsets = serialized.map do |s|
    o = pos
    pos += s.bytesize
    o
  end

  offsets.each { |o| io << [o].pack("n") }
  serialized.each { |s| io << s }

  io
end

.build_script_list(feature_count) ⇒ Object

ScriptList: one script (DFLT) with a default LangSys that references all features (featureIndex 0..count-1).



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 107

def self.build_script_list(feature_count)
  io = +""
  io << [1].pack("n") # scriptCount = 1

  # ScriptRecord: tag(4) + scriptOffset(2)
  # Script table starts right after the ScriptList header:
  #   scriptCount(2) + ScriptRecord(6) = 8
  script_offset = 8
  io << SCRIPT_DFLT
  io << [script_offset].pack("n")

  # Script table: defaultLangSysOffset(2) + langSysCount(2) = 4
  # LangSys follows immediately
  default_langsys_offset = script_offset + 4
  io << [default_langsys_offset].pack("n")
  io << [0].pack("n") # langSysCount

  # LangSys: lookupOrder(2)=0, requiredFeatureIndex(2)=0xFFFF,
  # featureIndexCount(2), featureIndices[]
  io << [0].pack("n")
  io << [0xFFFF].pack("n")
  io << [feature_count].pack("n")
  feature_count.times { |i| io << [i].pack("n") }

  io
end

.ligature_features(parsed, name_to_gid) ⇒ Hash{String=>Array}

Resolve ligature rules to per-feature lookup data.

Parameters:

  • parsed (FeatureCompiler::ParsedFeatures)
  • name_to_gid (Hash{String=>Integer})

Returns:

  • (Hash{String=>Array}) —

    feature_tag → lookup data



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# File 'lib/fontisan/ufo/compile/gsub.rb', line 46

def self.ligature_features(parsed, name_to_gid)
  parsed.ligatures.keys.each_with_object({}) do |tag, h|
    rules = parsed.ligatures_for(tag)
    sets = build_ligature_sets(rules, name_to_gid)
    h[tag] = sets unless sets.empty?
  end
end