Added capability to automatically download the PlantUML jar path (#95)

* Added capability to automatically download the PlantUML jar path

As an effort to ease the first user setup, a new function is added to download the latest
PlantUML JAR release into `plantuml-jar-path` from Maven central.

README.md

@@ -26,6 +26,10 @@ Also, to enable preview you need to tell `plantuml-mode` where to locate the Pla
     M-x customize-variable<RET>
     plantuml-jar-path<RET>
 
+You can also download the latest version of PlantUML straight into `plantuml-jar-path`:
+
+    M-x plantuml-download-jar<RET>
+
 # Features
 
 - Syntax highlight

plantuml-mode.el

@@ -69,6 +69,7 @@
 ;;; Code:
 (require 'thingatpt)
 (require 'dash)
+(require 'xml)
 
 (defgroup plantuml-mode nil
   "Major mode for editing plantuml file."
@@ -163,6 +164,31 @@
             (insert msg)
             (insert "\n"))))))
 
+(defun plantuml-download-jar ()
+  "Download the latest PlantUML JAR file and install it into `plantuml-jar-path'."
+  (interactive)
+  (if (y-or-n-p (format "Download the latest PlantUML JAR file into %s? " plantuml-jar-path))
+      (if (or (not (file-exists-p plantuml-jar-path))
+              (y-or-n-p (format "The PlantUML jar file already exists at %s, overwrite? " plantuml-jar-path)))
+          (with-current-buffer (url-retrieve-synchronously "https://search.maven.org/solrsearch/select?q=g:net.sourceforge.plantuml+AND+a:plantuml&core=gav&start=0&rows=1&wt=xml")
+            (mkdir (file-name-directory plantuml-jar-path) t)
+            (let* ((parse-tree (xml-parse-region))
+                   (doc        (->> parse-tree
+                                    (assq 'response)
+                                    (assq 'result)
+                                    (assq 'doc)))
+                   (strs       (xml-get-children doc 'str))
+                   (version    (->> strs
+                                    (--filter (string-equal "v" (xml-get-attribute it 'name)))
+                                    (first)
+                                    (xml-node-children)
+                                    (first))))
+              (message (concat "Downloading PlantUML v" version " into " plantuml-jar-path))
+              (url-copy-file (format "https://search.maven.org/remotecontent?filepath=net/sourceforge/plantuml/plantuml/%s/plantuml-%s.jar" version version) plantuml-jar-path)
+              (kill-buffer))
+            (message "Aborted.")))
+    (message "Aborted.")))
+
 (defun plantuml-init ()
   "Initialize the keywords or builtins from the cmdline language output."
   (unless (or (eq system-type 'cygwin) (file-exists-p plantuml-jar-path))