Verify the unpack string

gistfile1.rb

require 'terminal-table'

def unpack_str_table(unpack_str, data = nil)
  len      = unpack_str.scan(/\d+/)
  sum      = 0; idx = 1
  fields   = len.map { |x| sum += x.to_i }
  headings = %w( Index Length Start End )
  headings << "Data" unless data.nil?
  table = Terminal::Table.new :headings => headings
  fields.each do |stop|
    length = len[idx - 1].to_i
    start  = stop + 1 - length
    row    = [idx, length, start, stop]
    row << data[idx - 1] unless data.nil?
    table << row
    idx += 1
  end
  table
end

line = '123abc456def'
unpack_str = 'A3A3A3A3'
fields_data         = line.unpack(unpack_str)
puts unpack_str_table(unpack_str, fields_data)