Flatten arbitrarily nested arrays without using #flatten :-)

bespoke_flatten.rb

# frozen_string_literal: true

require 'test/unit'

class BespokeFlattenTest < Test::Unit::TestCase
  def bespoke_flatten(arr, accum = [])
    return arr unless arr.is_a?(Array)
    until arr.empty?
      elem = arr.shift
      elem.is_a?(Array) ? bespoke_flatten(elem, accum) : (accum << elem)
    end
    accum
  end

  def test_non_array
    a = 5
    assert_equal(5, bespoke_flatten(a))
  end

  def test_single_element_array
    a = [5]
    assert_equal([5], bespoke_flatten(a))
  end

  def test_multi_element_array
    a = [5, 6]
    assert_equal([5, 6], bespoke_flatten(a))
  end

  def test_containing_one_nested_array
    a = [3, [6]]
    assert_equal([3, 6], bespoke_flatten(a))
  end

  def test_containing_empty_arrays
    a = [3, [], 6]
    assert_equal([3, 6], bespoke_flatten(a))
  end

  def test_containing_complex_nested_arrays
    a = [3, [6, 9, [4, 5, [], 3, 2, [2, 4], 0], 3, 5], 23]
    assert_equal([3, 6, 9, 4, 5, 3, 2, 2, 4, 0, 3, 5, 23], bespoke_flatten(a))
  end
end