class ReeArray::InGroupsOf

def call(array, number, **opts, &block)
  if number.to_i <= 0
    raise ArgumentError,
      "Group size must be a positive integer, was #{number.inspect}"
  end
  collection = if opts.has_key?(:fill_with)
    # size % number gives how many extra we have;
    # subtracting from number gives how many to add;
    # modulo number ensures we don't add group of just fill.
    padding = (number - array.size % number) % number
    array.dup.concat(Array.new(padding, opts[:fill_with]))
  else
    array
  end
  if block_given?
    collection.each_slice(number, &block)
  else
    collection.each_slice(number).to_a
  end
end