accept-handler.clj

(defn parse-quality
  "Convert the first 'q' parameter to a number (or 1.0)"
  [extension]
  (let [[_ qvalue] (re-find #";q=([0-9\.]+)" (or extension ""))]
    (java.lang.Double. (or qvalue "1.0"))))

(defn +media-range
  ""
  [accept]
  (->> (re-find #"(([\w\*]+)/([^;]+))(;.*)?" accept)
       (zipmap [:original :media-range :type :subtype :parameters])))

(defn +quality
  "Takes first 'q' parameter with a "
  [accept]
  (->> (if-let [[_ q] (re-find #"q=([10]?\.?[0-9]+)" (:original accept))]
         (java.lang.Double. q)
         1.0)
       (assoc accept :quality)))

(defn +extensions
  "Convert a single extensions string into a list of strings."
  [accept]
  (if-let [params (:parameters accept)]
    (->> (clojure.string/replace-first params #"q=([10]?\.?[0-9]+)" "")
         (re-seq #"[\w\.]+")
         vec
         (assoc accept :extensions))
    accept))

(defn +version
  "Non-standard."
  [accept]
  (->> (:subtype accept)
       (re-find #"v([0-9\.]+)")
       last
       (assoc accept :version)))

(defn +format
  "Non-standard."
  [accept]
  (->> (:subtype accept)
       (re-find #"\+([\w\.]+)")
       last
       (assoc accept :format)))

(defn +specificity
  "Used when sorting indidual accept values"
  [accept]
  (assoc accept :specificity
         (cond->> 0
           (accept :extensions) inc
           (not= (accept :subtype) "*") inc
           (not= (accept :type) "*") inc)))

(def accept-xf (comp (map +media-range)
                     (map +quality)
                     (map +extensions)
                     (map +version)
                     (map +format)
                     (map +specificity)))

(defn accept-builder
  ""
  [accept]
  (if accept
    (->> (clojure.string/split accept #"[,\s]+")
         (sequence accept-xf)
         vec
         (sort-by (juxt :quality :specificity))
         reverse)))

(defn accept-handler
  "A Ring handler that parses the Accept header and updates request
   with map ."
  [handler]
  (fn [request]
    (if-let [header (get-in request [:headers "accept"])]
      (handler (assoc request :accept (accept-builder header))
               (handler request)))))

An Accept header like this...

(accept-builder "text/vndr.moo.v0.5+html;level=1;q=0.9, text/vndr.moo.v0.5+json")

...produces a sequence of maps like this...

({:media-range "text/vndr.moo.v0.5+html",
  :format "html",
  :original "text/vndr.moo.v0.5+html;level=1;q=1.0",
  :type "text",
  :extensions ["level" "1"],
  :quality 1.0,
  :version "0.5",
  :specificity 3,
  :parameters ";level=1;q=1.0",
  :subtype "vndr.moo.v0.5+html"}
 {:original "text/vndr.moo.v0.5+json",
  :media-range "text/vndr.moo.v0.5+json",
  :type "text",
  :subtype "vndr.moo.v0.5+json",
  :parameters nil,
  :quality 1.0,
  :version "0.5",
  :format "json",
  :specificity 2})