flattener.rb

def flatten integers
  [].tap do |memory|
    integers.each do |element|
      memory << element if element.is_a? Fixnum

      next unless element.is_a? Array

      flatten(element).each do |element|
        memory << element if element.is_a? Fixnum
      end
    end
  end
end

require 'test/unit'

class Tests < Test::Unit::TestCase
  def test_flatten
    assert_equal [1], flatten([1])
    assert_equal [1, 2], flatten([1, 2])
    assert_equal [1], flatten([[1]])
    assert_equal [1, 2], flatten([[1], 2])
    assert_equal [1], flatten([[[1]]])
    assert_equal [1, 2], flatten([[[1], 2]])
    assert_equal [1, 2, 3, 4], flatten([[1, 2, [3]], 4])
    assert_equal [1, 2, 3, 4], flatten([[[1, 2, [3]]], 4])
    assert_equal [1, 2, 3, 4], flatten([nil, 1, [2, 3], [nil], 4])
    assert_equal [1, 2, 3, 4], flatten(['foo', 1, 2, 3, 3.5, 4])
  end
end

mildly annoyed by repetition of 4th and 9th lines