module Sinatra::Helpers

def etag(value, options = {})

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, options = {})
  # Before touching this code, please double check RFC 2616 14.24 and 14.26.
  options      = { kind: options } unless Hash === options
  kind         = options[:kind] || :strong
  new_resource = options.fetch(:new_resource) { request.post? }
  unless ETAG_KINDS.include?(kind)
    raise ArgumentError, ':strong or :weak expected'
  end
  value = format('"%s"', value)
  value = "W/#{value}" if kind == :weak
  response['ETag'] = value
  return unless success? || status == 304
  if etag_matches?(env['HTTP_IF_NONE_MATCH'], new_resource)
    halt(request.safe? ? 304 : 412)
  end
  if env['HTTP_IF_MATCH']
    return if etag_matches?(env['HTTP_IF_MATCH'], new_resource)
    halt 412
  end
  nil
end