Array Flattener in Ruby for Citrusbyte

flattener.rb

require 'test/unit'
extend Test::Unit::Assertions

class Flattener
  # Public interface does input validation and delegates to private method
  def flatten(input)
    raise ArgumentError, 'Argument is not array' unless input.is_a? Array
    recurse_flatten(input)
  end

  private
  # No outside dependencies or side effects for simplicity
  def recurse_flatten(arr)
    accumulator = []

    if arr.kind_of?(Array)
      arr.each do |arr|
        accumulator.concat(recurse_flatten(arr))
      end
    else
      accumulator.concat([arr])
    end

    return accumulator
  end
end

flattener = Flattener.new

single_level_array = [4, 8, 1]
four_level_array = [1, [2,3, [4, [3, 9, 3]]]]
multi_type_array = [[1,2,[3, ['a', 'b', 'c'], false]],4, 'x', {name: "andrew", age: 28}]

# Check that flattener doesn't hurt single level arrays.
assert_equal(flattener.flatten(single_level_array) , [4, 8, 1])

# Check that flattener can handle at least four levels of nesting
assert_equal(flattener.flatten(four_level_array) , [1, 2, 3, 4 ,3, 9, 3])

# Check that flattener can handle arrays of various classes
assert_equal(flattener.flatten(multi_type_array), [1, 2, 3, "a", "b", "c", false, 4, "x", {:name=>"andrew", :age=>28}])

# Check that flattener will not accept arguments for flatten other than arrays
assert_raise ArgumentError do
  flattener.flatten(3)
end