gistfile1.rb

def generate(params, defaults = {})
  param_or_default = lambda { |p, d, k| (p[k] || d[k]).to_s }
  matches          = lambda { |v| v =~ /^(?-mix:[^\/.,;?]+)$/ }

  v_controller = param_or_default.call(params, defaults, :controller)
  v_action     = param_or_default.call(params, defaults, :action)
  v_id         = param_or_default.call(params, defaults, :id)

  unless matches.call(v_controller)
    raise ArgumentError, "Condition cannot be generated with #{params.inspect}"
  end

  path  = "/#{v_controller}"

  if params[:action] || params[:id] && matches.call(v_action)
    path << "/#{v_action}"
    path << "/#{v_id}" if matches.call(params[:id] && v_id)
  end

  return path
end