Flatten

flatten.exs

defmodule Flat do
  # [ 1 , 2, 3, [ 1, 2]]
  def flatten(list) do
    flatten(list, [])
  end

  def flatten([h|t], acc_list) when is_list(h) do
    # this may be expensive!
    flatten(h ++ t, acc_list)
  end

  def flatten([h|t], acc_list) do
    flatten(t, [h|acc_list])
  end

  def flatten([], acc_list) do
    Enum.reverse(acc_list)
  end
end

ExUnit.start
defmodule FlatTest do
  use ExUnit.Case

  test "should flatten multiple nested list" do
    assert Flat.flatten([1 , 2, 3, [ 11, 12, [:a, :b, [33, 44], :bar], 99], :awesome]) == [1, 2, 3, 11, 12, :a, :b, 33, 44, :bar, 99, :awesome]
  end

  test "should flatten an already flat list" do
    assert Flat.flatten([1 , 2, :awesome]) == [1 , 2, :awesome]
  end

  test "should flatten nested empty arrays" do
    assert Flat.flatten([1 , 2, [[], [], []], :awesome]) == [1 , 2, :awesome]
  end
end

flatten.rb

class Flatten
  def self.flatten(array)
    out = []
    array.each do |el|
      if el.is_a?(Array)
        out = out + flatten(el)
      else
        out << el
      end
    end
    out
  end
end

require 'minitest/autorun'
class FlattenTest < Minitest::Test
  def test_without_nesting
    assert_equal Flatten.flatten([1, 3, 4]), [1, 3, 4]
    assert_equal Flatten.flatten([]), []
  end

  def test_simple_nesting
    assert_equal Flatten.flatten([1, 3, [4]]), [1, 3, 4]
    assert_equal Flatten.flatten([1, [3], [4]]), [1, 3, 4]
  end

  def test_deep_nesting
    assert_equal Flatten.flatten([1, [3, [5, [4, 20, [44, []]]]], [4]]), [1, 3, 5, 4, 20, 44, 4]
  end

end