[p19] Implement caching as an advice

p19/p19.org

@@ -14,10 +14,21 @@ First parse with regex to make into a list
 
       (eval-buffer))
 
+#+end_src
+
+Here we define a caching advice; this implementation works under the
+assumption that the original function accepts one string as a
+parameter
+#+begin_src emacs-lisp :results none
+  (setq cache nil)
 
-  (setq possible-cache nil
-        impossible-cache nil)
+  (defun cached (fun r)
+    (if (assoc-string r cache) (cdr (assoc-string r cache))
+      (let ((result (funcall fun r)))
+        (push (cons r result) cache)
+        result)))
 #+end_src
+
 Here we recur down the tree obtained by splitting with all possible
 prefixes.  This is for part 1
 #+begin_src emacs-lisp
@@ -25,13 +36,10 @@ prefixes.  This is for part 1
     (--map (substring word (length it)) (--filter (string-prefix-p it word) avail)))
 
   (defun possible-p (word)
-    (and (not (-contains-p impossible-cache word))
-         (or (string= "" word)
-             (-contains-p possible-cache word)
-             (if (-any #'possible-p (suffixes word))
-                 (push word possible-cache)
-               (push word impossible-cache)
-               nil ))))
+    (or (string= "" word)
+        (-any #'possible-p (suffixes word))))
+
+  (advice-add #'possible-p :around 'cached)
 
   (length (-filter #'possible-p patterns))
 #+end_src
@@ -45,14 +53,10 @@ This is for part 2;
     (--map (substring word (length it)) (--filter (string-prefix-p it word) avail)))
 
   (defun possible-arrangements (word)
-    (if (-contains-p impossible-cache word) 0
-      (cond
-       ((string= "" word) 1)
-       ((assoc-string word possible-cache) (cdr (assoc-string word possible-cache)))
-       (t (let ((arrangements (-sum (-map #'possible-arrangements (suffixes word)))))
-            (if (= arrangements 0) (push word impossible-cache)
-              (push (cons word arrangements) possible-cache))
-            arrangements)))))
+    (if (string= "" word) 1
+      (-sum (-map #'possible-arrangements (suffixes word)))))
+
+  (advice-add #'possible-arrangements :around #'cached)
 
   (-sum (-map #'possible-arrangements patterns))
 #+end_src