module Sinatra::Helpers

def etag(value, kind=:strong)

GET or HEAD, a '304 Not Modified' response is sent.
matching etag, execution is immediately halted. If the request method is
When the current request includes an 'If-None-Match' header with a

cache validator.
indicates whether the etag should be used as a :strong (default) or :weak
identifies the current version of the resource. The +kind+ argument
GET matches. The +value+ argument is an identifier that uniquely
Set the response entity tag (HTTP 'ETag' header) and halt if conditional 
def etag(value, kind=:strong)
  raise TypeError, ":strong or :weak expected" if ![:strong,:weak].include?(kind)
  value = '"%s"' % value
  value = 'W/' + value if kind == :weak
  response['ETag'] = value
  # Conditional GET check
  if etags = env['HTTP_IF_NONE_MATCH']
    etags = etags.split(/\s*,\s*/)
    halt 304 if etags.include?(value) || etags.include?('*')
  end
end