class Rack::ETag

defaults to nil, while the second defaults to “max-age=0, private, must-revalidate”
used when etag is absent and a directive when it is present. The first
On initialization, you can pass two parameters: a cache-control directive
should be handled by apache/nginx).
a sendfile body (body.responds_to :to_path) is given (since such cases
The etag header is skipped if etag or last-modified headers are sent or if
Automatically sets the etag header on all String bodies.

def call(env) 

def call(env)
  status, headers, body = response = @app.call(env)
  if etag_status?(status) && body.respond_to?(:to_ary) && !skip_caching?(headers)
    body = body.to_ary
    digest = digest_body(body)
    headers[ETAG_STRING] = %(W/"#{digest}") if digest
  end
  unless headers[CACHE_CONTROL]
    if digest
      headers[CACHE_CONTROL] = @cache_control if @cache_control
    else
      headers[CACHE_CONTROL] = @no_cache_control if @no_cache_control
    end
  end
  response
end

def digest_body(body) 

def digest_body(body)
  digest = nil
  body.each do |part|
    (digest ||= Digest::SHA256.new) << part unless part.empty?
  end
  digest && digest.hexdigest.byteslice(0,32)
end

def etag_status?(status) 

def etag_status?(status)
  status == 200 || status == 201
end

def initialize(app, no_cache_control = nil, cache_control = DEFAULT_CACHE_CONTROL) 

def initialize(app, no_cache_control = nil, cache_control = DEFAULT_CACHE_CONTROL)
  @app = app
  @cache_control = cache_control
  @no_cache_control = no_cache_control
end

def skip_caching?(headers) 

def skip_caching?(headers)
  headers.key?(ETAG_STRING) || headers.key?('last-modified')
end