flatten an array of arbitrarily nested arrays of integers into a flat array of integers (can't use Array#flatten built-in method)

flattenizer.rb

class Flattenizer

  def initialize(params)
    if params.empty?
      raise ArgumentError, 'Data set is empty'
    end
    if !params.is_a?(Array)
      raise TypeError, 'Data set is not an array'
    end
    @input_array = params
  end


  def flatten
    @resulted_array = []
    inner_flatten_loop(@input_array)
    return @resulted_array
  end


  private

  def inner_flatten_loop(inner_array)
    inner_array.each do |i|
      if i.is_a?(Fixnum)
        @resulted_array << i
      elsif i.is_a?(Array)
        inner_flatten_loop(i)
      end
    end
  end

end



require 'minitest/autorun'

class TestFlattenizer < Minitest::Unit::TestCase

  def test_that_array1_flattens
    object = Flattenizer.new([[1,2,[3]],4]).flatten
    assert_equal( [1,2,3,4], object )
  end

  def test_that_array2_flattens
    object = Flattenizer.new([[1,2,[3],[[1]]],[1],[[[[3]]]],4]).flatten
    assert_equal( [1,2,3,1,1,3,4], object )
  end

  def test_that_array3_flattens
    object = Flattenizer.new([[1,2,[3]],"","bug",4]).flatten
    assert_equal( [1,2,3,4], object )
  end

  def test_that_string_doesnt_flatten
    assert_raises( TypeError ) { Flattenizer.new("string not an array") }
  end

  def test_that_empty_array_doesnt_flatten
    assert_raises( ArgumentError ) { Flattenizer.new([]) }
  end

end