Write some code, that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]

array_flattener.rb

class ArrayFlattener

	# ArrayFlattener provides a method to flat nested array.
	# For example, it can turn an array like [1, 2 [3, 4]] to [1, 2, 3, 4]

	# Class method that can be called on ArrayFlattener that will convert nested array into flat array
	def self.flatten(nested_array)
		return [] if nested_array.length == 0
		self._flatten(nested_array, [])
	end

	private

	# Private method that is doing main work by recursively iterating over any nested array found
	def self._flatten(nested_array, flat_array)
		nested_array.each do |elem|
			if (elem.is_a?(Array))
				_flatten(elem, flat_array)
			else
				flat_array << elem
			end
		end
		flat_array
	end
	
end

array_flattener_spec.rb

require "array_flattener"

RSpec.describe ArrayFlattener do

	context "when array is empty" do
		it "returns empty array without performing any operations" do
			flat_array = ArrayFlattener.flatten([])
			expect(flat_array).to eq([])
		end
	end

	context "when array have items" do

		it "flats single 1 level nested array" do
			flat_array = ArrayFlattener.flatten([1,2,[3, 4]])
			expect(flat_array).to eq([1, 2, 3, 4])
		end

		it "flats multiple 1 level nested array" do
			flat_array = ArrayFlattener.flatten([[1, 2, 3], 4, [5, 6]])
			expect(flat_array).to eq([1, 2, 3, 4, 5, 6])
		end

		it "flats single 2 level nested array" do
			flat_array = ArrayFlattener.flatten([[1,2,[3]],4])
			expect(flat_array).to eq([1, 2, 3, 4])
		end

		it "flats multiple 2 level nested array" do 
			flat_array = ArrayFlattener.flatten([[1, 2, [3], 4, 5], [6, 7, [8,9]]])
			expect(flat_array).to eq([1, 2, 3, 4, 5, 6, 7, 8, 9])
		end

		it "flats nested array having elements like Hash" do
			flat_array = ArrayFlattener.flatten([1, 2, [3, 4], {key1: 1, key2: 2}])
			expect(flat_array).to eq([1, 2, 3, 4, {key1: 1, key2: 2}])
		end

	end

end