class Rack::ConditionalGet

github.com/wycats/merb/blob/master/merb-core/lib/merb-core/rack/middleware/conditional_get.rb<br>Adapted from Michael Klishin’s Merb implementation:
a conditional GET matches.
message is received will avoid response body generation completely when
Applications that defer response body generation until the body’s each
length and the response status is set to 304 Not Modified.
either of the conditions is met, the response body is set to be zero
last-modified or etag response headers according to RFC 2616. When
if-modified-since. The application should set either or both of the
Middleware that enables conditional GET using if-none-match and

def call(env)

modified since the last request.
Return empty 304 response if the response has not been

def call(env)
  case env[REQUEST_METHOD]
  when "GET", "HEAD"
    status, headers, body = response = @app.call(env)
    if status == 200 && fresh?(env, headers)
      response[0] = 304
      headers.delete(CONTENT_TYPE)
      headers.delete(CONTENT_LENGTH)
      response[2] = Rack::BodyProxy.new([]) do
        body.close if body.respond_to?(:close)
      end
    end
    response
  else
    @app.call(env)
  end
end

def etag_matches?(none_match, headers)

If so, the request has not been modified.
Whether the etag response header matches the if-none-match request header.

def etag_matches?(none_match, headers)
  headers[ETAG] == none_match
end

def fresh?(env, headers)

last request.
Return whether the response has not been modified since the

def fresh?(env, headers)
  # if-none-match has priority over if-modified-since per RFC 7232
  if none_match = env['HTTP_IF_NONE_MATCH']
    etag_matches?(none_match, headers)
  elsif (modified_since = env['HTTP_IF_MODIFIED_SINCE']) && (modified_since = to_rfc2822(modified_since))
    modified_since?(modified_since, headers)
  end
end

def initialize(app)

def initialize(app)
  @app = app
end

def modified_since?(modified_since, headers)

request header. If so, the request has not been modified.
Whether the last-modified response header matches the if-modified-since

def modified_since?(modified_since, headers)
  last_modified = to_rfc2822(headers['last-modified']) and
    modified_since >= last_modified
end

def to_rfc2822(since)

format), or nil if the string cannot be parsed.
Return a Time object for the given string (which should be in RFC2822

def to_rfc2822(since)
  # shortest possible valid date is the obsolete: 1 Nov 97 09:55 A
  # anything shorter is invalid, this avoids exceptions for common cases
  # most common being the empty string
  if since && since.length >= 16
    # NOTE: there is no trivial way to write this in a non exception way
    #   _rfc2822 returns a hash but is not that usable
    Time.rfc2822(since) rescue nil
  end
end

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