[completion] Cache last result of cider-complete

cider-completion.el

@@ -142,6 +142,25 @@ completion functionality."
     (mapcar #'cider-completion--parse-candidate-map (cider-sync-request:completion prefix)))
    (t nil)))
 
+(defvar cider-complete-last-result nil
+  "Stores a cons where car is a list of (PREFIX BUFFER POINT), and the cdr is
+the last completion result.")
+
+(defun cider-complete-with-cache (prefix)
+  "Call `cider-complete', optionally returning the result from cache.
+This function will use cached result if prefix and other completion
+conditions have not changed."
+  (when (cider-connected-p)
+    (let ((cached (with-current-buffer (cider-current-repl)
+                    cider-complete-last-result))
+          (key (list prefix (current-buffer) (point))))
+      (if (equal key (car cached))
+          (cdr cached)
+        (let* ((result (cider-complete prefix)))
+          (with-current-buffer (cider-current-repl)
+            (setq-local cider-complete-last-result (cons key result)))
+          result)))))
+
 (defun cider-completion--get-candidate-type (symbol)
   "Get candidate type for SYMBOL."
   (let ((type (get-text-property 0 'type symbol)))
@@ -189,16 +208,17 @@ performed by `cider-annotate-completion-function'."
               ;; When the 'action is 'metadata, this lambda returns metadata about this
               ;; capf, when action is (boundaries . suffix), it returns nil. With every
               ;; other value of 'action (t, nil, or lambda), 'action is forwarded to
-              ;; (complete-with-action), together with (cider-complete), prefix and pred.
-              ;; And that function performs the completion based on those arguments.
+              ;; (complete-with-action), together with (cider-complete-with-cache),
+              ;; prefix and pred.  And that function performs the completion based
+              ;; on those arguments.
               ;;
               ;; This api is better described in the section
               ;; '21.6.7 Programmed Completion' of the elisp manual.
               (cond ((eq action 'metadata) `(metadata (category . cider))) ;; defines a completion category named 'cider, used later in our `completion-category-overrides` logic.
                     ((eq (car-safe action) 'boundaries) nil)
                     (t (with-current-buffer (current-buffer)
-                         (complete-with-action action
-                                               (cider-complete prefix) prefix pred)))))
+                         (complete-with-action
+                          action (cider-complete-with-cache prefix) prefix pred)))))
             :annotation-function #'cider-annotate-symbol
             :company-kind #'cider-company-symbol-kind
             :company-doc-buffer #'cider-create-compact-doc-buffer