(defn- factory
  "Builds a function to compute the HMAC of a payload, given a secret-key.
   `hmac-type` is the algorithm to use for the HMAC."
  [^String hmac-type input-transformer key-transformer output-transformer]
  (fn [secret-string ^String raw-input]
    (let [secret ^"[B" (key-transformer secret-string)
          input  ^"[B" (input-transformer raw-input)
          key    (SecretKeySpec. secret hmac-type)]
      (-> (doto (Mac/getInstance hmac-type) (.init key))
          (.doFinal input)
          (output-transformer)))))

(def sha256 (factory "HmacSHA256"
                     string->bytes
                     string->bytes
                     b->b64))

(sha256 secret-key payload)

(-> (json/generate-string sample)
      (mac/hash {:key "d5044d4220937b57" :alg :hmac})
      (codecs/bytes->b64)

(ns hmac
  (:import (javax.crypto Mac)
           (javax.crypto.spec SecretKeySpec)))

(defn toHexString [bytes]
  (apply str (map #(format "%02x" %) bytes)))

(defn hmac [^String key ^String string]
  (let [mac (Mac/getInstance "HmacSHA256")
        secretKey (SecretKeySpec. (.getBytes key) (.getAlgorithm mac))]
    (-> (doto mac
          (.init secretKey)
          (.update (.getBytes string)))
        .doFinal
        toHexString)))

  (-> (js/write-str body :escape-slash true)
      (mac/hash {:key "d5044d4220937b57" :alg :hmac})
      (codecs/bytes->hex)
      );; => "b5ada318e1e774c11133240fc7ab05174eca91cae81372e0121205cf08a28262"