Class: Fontisan::Tables::Cmap

Inherits:
Binary::BaseRecord show all
Defined in:
lib/fontisan/tables/cmap.rb

Overview

Parser for the ‘cmap’ (Character to Glyph Index Mapping) table

The cmap table maps character codes to glyph indices. It supports multiple encoding formats to accommodate different character sets and Unicode planes.

This implementation focuses on:

  • Format 4: Segment mapping for BMP (Basic Multilingual Plane, U+0000-U+FFFF)

  • Format 12: Segmented coverage for full Unicode support

Reference: OpenType specification, cmap table

Constant Summary collapse

PLATFORM_UNICODE =

Platform IDs

0
PLATFORM_MACINTOSH = 
1
PLATFORM_MICROSOFT = 
3
ENC_MS_UNICODE_BMP =

Microsoft Encoding IDs

1
ENC_MS_UNICODE_UCS4 =

Unicode BMP (UCS-2)

10

Instance Method Summary collapse

Methods inherited from Binary::BaseRecord

#raw_data, read, #valid?

Instance Method Details

#extract_subtable_data(record, data) ⇒ Object

Extract subtable data if record exists and offset is valid



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# File 'lib/fontisan/tables/cmap.rb', line 125

def extract_subtable_data(record, data)
  return nil unless record
  return nil unless record[:offset] < data.length

  data[record[:offset]..]
end

#find_best_unicode_subtable(records, data) ⇒ Object

Find the best Unicode subtable from encoding records



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# File 'lib/fontisan/tables/cmap.rb', line 93

def find_best_unicode_subtable(records, data)
  # Try in priority order: UCS-4, BMP, Unicode
  find_ucs4_subtable(records, data) ||
    find_bmp_subtable(records, data) ||
    find_unicode_subtable(records, data)
end

#find_bmp_subtable(records, data) ⇒ Object

Find Microsoft Unicode BMP subtable



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# File 'lib/fontisan/tables/cmap.rb', line 110

def find_bmp_subtable(records, data)
  record = records.find do |r|
    r[:platform_id] == PLATFORM_MICROSOFT &&
      r[:encoding_id] == ENC_MS_UNICODE_BMP
  end
  extract_subtable_data(record, data)
end

#find_ucs4_subtable(records, data) ⇒ Object

Find Microsoft Unicode UCS-4 subtable (full Unicode)



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# File 'lib/fontisan/tables/cmap.rb', line 101

def find_ucs4_subtable(records, data)
  record = records.find do |r|
    r[:platform_id] == PLATFORM_MICROSOFT &&
      r[:encoding_id] == ENC_MS_UNICODE_UCS4
  end
  extract_subtable_data(record, data)
end

#find_unicode_subtable(records, data) ⇒ Object

Find Unicode platform subtable (any encoding)



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# File 'lib/fontisan/tables/cmap.rb', line 119

def find_unicode_subtable(records, data)
  record = records.find { |r| r[:platform_id] == PLATFORM_UNICODE }
  extract_subtable_data(record, data)
end

#has_bmp_coverage? ⇒ Boolean

Validation helper: Check if BMP coverage exists

Checks if the Basic Multilingual Plane (U+0000-U+FFFF) has mappings

Returns:

  • (Boolean) —

    True if BMP characters are mapped



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# File 'lib/fontisan/tables/cmap.rb', line 309

def has_bmp_coverage?
  mappings = unicode_mappings
  return false if mappings.nil? || mappings.empty?

  # Check if any BMP characters (0x0000-0xFFFF) are mapped
  mappings.keys.any? { |code| code.between?(0x0000, 0xFFFF) }
end

#has_format_4_subtable? ⇒ Boolean

Validation helper: Check if format 4 subtable exists

Format 4 is the minimum requirement for Unicode BMP support

Returns:

  • (Boolean) —

    True if format 4 subtable is found



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# File 'lib/fontisan/tables/cmap.rb', line 335

def has_format_4_subtable?
  data = to_binary_s
  records = read_encoding_records(data)

  records.any? do |record|
    subtable_data = extract_subtable_data(record, data)
    next false unless subtable_data && subtable_data.length >= 2

    format = subtable_data[0, 2].unpack1("n")
    format == 4
  end
rescue StandardError
  false
end

#has_required_characters?(*required_chars) ⇒ Boolean

Validation helper: Check if required characters are mapped

Checks for essential characters like space (U+0020)

Parameters:

  • required_chars (Array<Integer>) —

    Character codes that must be present

Returns:

  • (Boolean) —

    True if all required characters are mapped



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# File 'lib/fontisan/tables/cmap.rb', line 323

def has_required_characters?(*required_chars)
  mappings = unicode_mappings
  return false if mappings.nil?

  required_chars.all? { |code| mappings.key?(code) }
end

#has_subtables? ⇒ Boolean

Validation helper: Check if at least one subtable exists

Returns:

  • (Boolean) —

    True if num_tables > 0



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# File 'lib/fontisan/tables/cmap.rb', line 293

def has_subtables?
  num_tables&.positive?
end

#has_unicode_mapping? ⇒ Boolean

Validation helper: Check if Unicode mapping exists

Returns:

  • (Boolean) —

    True if Unicode mappings are present



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# File 'lib/fontisan/tables/cmap.rb', line 300

def has_unicode_mapping?
  !unicode_mappings.nil? && !unicode_mappings.empty?
end

#map_character_range(start_char, end_char, start_glyph, mappings) ⇒ Object

Map a range of characters to glyphs



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# File 'lib/fontisan/tables/cmap.rb', line 274

def map_character_range(start_char, end_char, start_glyph, mappings)
  (start_char..end_char).each do |code|
    glyph_index = start_glyph + (code - start_char)
    mappings[code] = glyph_index if glyph_index != 0
  end
end

#parse_format_12(data, mappings) ⇒ Object

Parse Format 12 subtable (segmented coverage) Format 12 supports full Unicode range



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# File 'lib/fontisan/tables/cmap.rb', line 233

def parse_format_12(data, mappings)
  header = parse_format_12_header(data)
  return unless header

  parse_format_12_groups(data, header[:num_groups], header[:length],
                         mappings)
end

#parse_format_12_groups(data, num_groups, length, mappings) ⇒ Object

Parse Format 12 sequential map groups



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# File 'lib/fontisan/tables/cmap.rb', line 257

def parse_format_12_groups(data, num_groups, length, mappings)
  offset = 16
  num_groups.times do
    break if offset + 12 > length

    start_char_code = data[offset, 4].unpack1("N")
    end_char_code = data[offset + 4, 4].unpack1("N")
    start_glyph_id = data[offset + 8, 4].unpack1("N")

    map_character_range(start_char_code, end_char_code, start_glyph_id,
                        mappings)

    offset += 12
  end
end

#parse_format_12_header(data) ⇒ Object

Parse Format 12 header



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# File 'lib/fontisan/tables/cmap.rb', line 242

def parse_format_12_header(data)
  return nil if data.length < 16

  format = data[0, 2].unpack1("n")
  return nil unless format == 12

  length = data[4, 4].unpack1("N")
  return nil if length > data.length

  num_groups = data[12, 4].unpack1("N")

  { length: length, num_groups: num_groups }
end

#parse_format_4(data, mappings) ⇒ Object

Parse Format 4 subtable (segment mapping to delta values) Format 4 is the most common format for BMP Unicode fonts rubocop:disable Metrics/MethodLength rubocop:disable Metrics/CyclomaticComplexity rubocop:disable Metrics/PerceivedComplexity



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# File 'lib/fontisan/tables/cmap.rb', line 137

def parse_format_4(data, mappings)
  return if data.length < 14

  # Format 4 header
  format = data[0, 2].unpack1("n")
  return unless format == 4

  length = data[2, 2].unpack1("n")
  return if length > data.length

  seg_count_x2 = data[6, 2].unpack1("n")
  seg_count = seg_count_x2 / 2

  # Arrays start at offset 14
  offset = 14

  # Read endCode array
  end_codes = []
  seg_count.times do
    break if offset + 2 > length

    end_codes << data[offset, 2].unpack1("n")
    offset += 2
  end

  # Skip reservedPad (2 bytes)
  offset += 2

  # Read startCode array
  start_codes = []
  seg_count.times do
    break if offset + 2 > length

    start_codes << data[offset, 2].unpack1("n")
    offset += 2
  end

  # Read idDelta array
  id_deltas = []
  seg_count.times do
    break if offset + 2 > length

    id_deltas << data[offset, 2].unpack1("n")
    offset += 2
  end

  # Read idRangeOffset array
  id_range_offsets = []
  id_range_offset_pos = offset
  seg_count.times do
    break if offset + 2 > length

    id_range_offsets << data[offset, 2].unpack1("n")
    offset += 2
  end

  # Process each segment
  seg_count.times do |i|
    start_code = start_codes[i]
    end_code = end_codes[i]
    id_delta = id_deltas[i]
    id_range_offset = id_range_offsets[i]

    # Skip the final segment (0xFFFF)
    next if start_code == 0xFFFF

    if id_range_offset.zero?
      # Use idDelta directly
      (start_code..end_code).each do |code|
        glyph_index = (code + id_delta) & 0xFFFF
        mappings[code] = glyph_index if glyph_index != 0
      end
    else
      # Use glyphIdArray
      (start_code..end_code).each do |code|
        # Calculate position in glyphIdArray
        array_offset = id_range_offset_pos + (i * 2) + id_range_offset
        array_offset += (code - start_code) * 2

        next if array_offset + 2 > length

        glyph_index = data[array_offset, 2].unpack1("n")
        next if glyph_index.zero?

        glyph_index = (glyph_index + id_delta) & 0xFFFF
        mappings[code] = glyph_index if glyph_index != 0
      end
    end
  end
end

#parse_mappings ⇒ Object

Parse all encoding records and extract Unicode mappings



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# File 'lib/fontisan/tables/cmap.rb', line 40

def parse_mappings
  mappings = {}

  # Get the full binary data
  data = to_binary_s

  # Read encoding records
  records = read_encoding_records(data)

  # Try to find the best Unicode subtable
  # Prefer Microsoft Unicode UCS-4 (format 12), then Unicode BMP (format 4)
  subtable_data = find_best_unicode_subtable(records, data)

  return mappings unless subtable_data

  # Parse the subtable based on its format
  format = subtable_data[0, 2].unpack1("n")

  case format
  when 4
    parse_format_4(subtable_data, mappings)
  when 12
    parse_format_12(subtable_data, mappings)
  end

  mappings
end

#read_encoding_records(data) ⇒ Object

Read encoding records from the beginning of the table



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# File 'lib/fontisan/tables/cmap.rb', line 69

def read_encoding_records(data)
  records = []
  offset = 4 # Skip version and numTables

  num_tables.times do
    break if offset + 8 > data.length

    platform_id = data[offset, 2].unpack1("n")
    encoding_id = data[offset + 2, 2].unpack1("n")
    subtable_offset = data[offset + 4, 4].unpack1("N")

    records << {
      platform_id: platform_id,
      encoding_id: encoding_id,
      offset: subtable_offset,
    }

    offset += 8
  end

  records
end

#unicode_mappings ⇒ Object

Parse encoding records and subtables



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# File 'lib/fontisan/tables/cmap.rb', line 35

def unicode_mappings
  @unicode_mappings ||= parse_mappings
end

#valid_glyph_indices?(max_glyph_id) ⇒ Boolean

Validation helper: Check if glyph indices are within bounds

Parameters:

  • max_glyph_id (Integer) —

    Maximum valid glyph ID from maxp table

Returns:

  • (Boolean) —

    True if all mapped glyph IDs are valid



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# File 'lib/fontisan/tables/cmap.rb', line 354

def valid_glyph_indices?(max_glyph_id)
  mappings = unicode_mappings
  return true if mappings.nil? || mappings.empty?

  mappings.values.all? do |glyph_id|
    glyph_id >= 0 && glyph_id < max_glyph_id
  end
end

#valid_version? ⇒ Boolean

Validation helper: Check if version is valid

cmap version should be 0

Returns:

  • (Boolean) —

    True if version is 0



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# File 'lib/fontisan/tables/cmap.rb', line 286

def valid_version?
  version.zero?
end