# 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].

# Implementation
module ArrayUtils
  class << self
    def flatten_integers(array)
      raise ArgumentError, "Suplied argument is not an array" unless array.is_a?(Array)
      process_all_items(array)
    end

    private

    def process_all_items(array)
      [].tap do |result|
        array.each { |item| result.push(*convert_single_item(item)) }
      end
    end

    def convert_single_item(item)
      if item.is_a?(Array)
        process_all_items(item)
      elsif item.is_a?(Integer)
        item
      else
        raise ArgumentError, "Unexpected array element: #{item.class}"
      end
    end
  end
end

# Behaviour Tests
require 'minitest/autorun'
require 'minitest/spec'

describe ArrayUtils do
  it "reises ArgumentError when used with invalid arguments" do
    expect { ArrayUtils.flatten_integers() }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers(1) }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers('string') }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers(nil) }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers([1, nil]) }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers([1, 'string']) }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers([1, 1.1]) }.must_raise(ArgumentError)
    expect { ArrayUtils.flatten_integers([1, 2, ['string']]) }.must_raise(ArgumentError)
  end

  it "works with integer arrays" do
    ArrayUtils.flatten_integers([]).must_equal([])
    ArrayUtils.flatten_integers([1]).must_equal([1])
    ArrayUtils.flatten_integers([1, 2]).must_equal([1, 2])
    ArrayUtils.flatten_integers([1, 2, [3]]).must_equal([1, 2, 3])
    ArrayUtils.flatten_integers([[1, 2, [3]], 4]).must_equal([1, 2, 3, 4])
  end
end