module ActionDispatch::Http::Cache::Response

def etag=(etag)
  key = ActiveSupport::Cache.expand_cache_key(etag)
  @etag = self[ETAG] = %("#{Digest::MD5.hexdigest(key)}")
end

def handle_conditional_get!
  if etag? || last_modified? || !@cache_control.empty?
    set_conditional_cache_control!
  end
end

def last_modified
  if last = headers[LAST_MODIFIED]
    Time.httpdate(last)
  end
end

def last_modified=(utc_time)
  headers[LAST_MODIFIED] = utc_time.httpdate
end

def last_modified?
  headers.include?(LAST_MODIFIED)
end

def prepare_cache_control!
  @cache_control = {}
  @etag = self[ETAG]
  if cache_control = self[CACHE_CONTROL]
    cache_control.split(/,\s*/).each do |segment|
      first, last = segment.split("=")
      @cache_control[first.to_sym] = last || true
    end
  end
end

def set_conditional_cache_control!
  return if self[CACHE_CONTROL].present?
  control = @cache_control
  if control.empty?
    headers[CACHE_CONTROL] = DEFAULT_CACHE_CONTROL
  elsif control[:no_cache]
    headers[CACHE_CONTROL] = NO_CACHE
  else
    extras  = control[:extras]
    max_age = control[:max_age]
    options = []
    options << "max-age=#{max_age.to_i}" if max_age
    options << (control[:public] ? PUBLIC : PRIVATE)
    options << MUST_REVALIDATE if control[:must_revalidate]
    options.concat(extras) if extras
    headers[CACHE_CONTROL] = options.join(", ")
  end
end