Class: Fontisan::Tables::Cff::IndexBuilder

Inherits:
Object
  • Object
show all
Defined in:
lib/fontisan/tables/cff/index_builder.rb

Overview

CFF INDEX structure builder

IndexBuilder constructs binary INDEX structures from arrays of data items. INDEX is a fundamental CFF data structure used for storing arrays of variable-length data.

The builder calculates optimal offset sizes, constructs the offset array, and produces compact binary output.

Structure produced:

  • count (Card16): Number of items
  • offSize (OffSize): Size of offset values (1-4 bytes)
  • offset (Offset): Array of offsets to data
  • data: Concatenated data bytes

Offsets are 1-based (first offset is always 1). The last offset points one byte past the end of the data.

Reference: CFF specification section 5 "INDEX Data" https://adobe-type-tools.github.io/font-tech-notes/pdfs/5176.CFF.pdf

Examples:

Building an INDEX

items = ["data1".b, "data2".b, "data3".b]
index_data = Fontisan::Tables::Cff::IndexBuilder.build(items)

Class Method Summary collapse

Class Method Details

.build(items) ⇒ String

Build INDEX structure from array of binary strings

Parameters:

  • items (Array<String>) —

    Array of binary data items

Returns:

  • (String) —

    Binary INDEX data

Raises:

  • (ArgumentError) —

    If items is not an Array



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# File 'lib/fontisan/tables/cff/index_builder.rb', line 38

def self.build(items)
  validate_items!(items)

  return build_empty_index if items.empty?

  # Calculate total data size
  data_size = items.sum(&:bytesize)

  # Calculate optimal offset size (1-4 bytes)
  # Last offset will be data_size + 1 (1-based)
  off_size = calculate_off_size(data_size + 1)

  # Build offset array (count + 1 offsets)
  offsets = build_offsets(items, off_size)

  # Concatenate all data
  data = items.join

  # Assemble INDEX structure
  output = StringIO.new("".b)

  # Write count (Card16)
  output.write([items.length].pack("n"))

  # Write offSize (OffSize)
  output.putc(off_size)

  # Write offset array
  offsets.each do |offset|
    write_offset(output, offset, off_size)
  end

  # Write data
  output.write(data)

  output.string
end