自动生成in期间clojure URL缩短程序中的错误

Question

我在Clojure编程中偶然发现了一段代码，它有很多buggy。我是Clojure的新手，不知道代码中的bug在哪里。

(ns ring-tutorial.core
  (:require [compojure.handler :as ch]
        [compojure.core :refer [GET PUT POST defroutes]]
        [compojure.route :as cr]
        [ring.util.response :as response]))

(def ^:private counter (atom 0))
(def ^:private mappings (ref {}))

(defn url-for
[id]
(@mappings id))

(defn shorten!
 "Stores the given URL under a new unique identifier and returns it as a string."
 ([url]
 (let [id (swap! counter inc)
       id (Long/toString id 36)] ;;convert to base 36 string
   (or (shorten! url id)
       (recur url))))
([url id] ;;url is provided with an id.
 (dosync
  (when-not (@mappings id)
    (alter mappings assoc id url)
    id))))

(defn retain
 [& [url id :as args]]
(if-let [id (apply shorten! args)]
{:status 201
  :headers {"Location" id}
 :body (list "URL " url " assigned the short identifier " id " \n")}
{:status 409 :body (format "Short URL %s is already taken \n " id)}))

(defn redirect
 [id]
(if-let [url (url-for id)]
(ring.util.response/redirect url) ;;Appropriately redirect the URL based on ethe ID.
{:status 404 :body (str "No such short URL: \n" id)})) ;;Return a 404 if no URL for the particular ID found.

(defroutes app*
 (GET "/" request "Hello World!")
 (PUT "/:id" [id url] (retain url id))
 (POST "/" [url ] (retain url))
 (GET "/:id" [id] (redirect id))
 (GET "/list/" [] (interpose "\n" (keys @mappings)))
 (cr/not-found "Its a 404!\n"))

(def app (ch/api #'app*))

当我运行$ CURL POST 'localhost:8080/? URL =http://www.yahoo.com/‘时，当我重新运行命令I get’时，我的get -X已经被赋予了标识符1，但是它应该会标记一个错误，说明yahoo.com已经具有标识符1。

这是一个错误，我可以看到它是在缩短！函数，但一直没有能够纠正它。

谢谢!

Answer 1

在shorten!函数中，您需要获取一个URL，并查看是否已经为其分配了ID。相反，您每次都会生成一个新的ID，并在地图中查找它，以查看是否为其分配了URL，但这种情况永远不会发生。

为了解决这个问题，您需要维护第二个映射，即从长URL到短ID的映射。在dosync块中，您将首先在这个新映射中查找URL，如果没有找到URL，则生成一个ID，并将URL对添加到两个映射中。