def in_groups(number, fill_with = nil)

    if number == 0
      raise IndexOutOfBounds.new
    end
    division = size.fdiv number
    modulo = size % number

    groups = [] of (Int32 | String | Char)
    start = 0


    number.times do |index|
      length = division + (modulo > 0 && modulo > index ? 1 : 0)
      last_group = self[start, length]
      groups << last_group
      last_group << fill_with if fill_with != false &&
        modulo > 0 && length == division
      start += length
    end

    groups
end