# Flatten an array.
#
# @param input [Array] An arbitrarily nested array structure of integers to be nested
# @return [Array] Flattened one-dimensional array of integers
def flatten(input)
  return [input] unless input.respond_to?('each')

  output = []
  input.each do |elem|
    if elem.respond_to?('each')
      output.concat(flatten(elem))
    else
      output << elem
    end
  end
  output
end