add ptree caching

ptree.rkt

@@ -35,8 +35,18 @@
   (check-true (ptree? '(foo (hee (uncle "foo"))))))
 
 
-(define/contract (file->ptree path)
+;; implement the caching with two hashes rather than composite key of (cons file mod-date)
+;; so that cached copies don't pile up indefinitely
+(define ptree-cache (make-hash))
+(define ptree-source-mod-dates (make-hash))
+
+(define (not-modified? ptree-source-path)
+  (and (hash-has-key? ptree-source-mod-dates ptree-source-path) 
+       ((file-or-directory-modify-seconds ptree-source-path) . = . (hash-ref ptree-source-mod-dates ptree-source-path))))
+
+(define/contract (file->ptree p)
   (pathish? . -> . ptree?)
+  (define path (->path p))
   (message "Loading ptree file" (->string (file-name-from-path path)))
   (dynamic-require path MAIN_POLLEN_EXPORT))
 
@@ -46,13 +56,16 @@
     (message "Generating ptree from file listing of" dir)
     (ptree-root->ptree (cons PTREE_ROOT_NODE files))))
 
-
 ;; Try loading from ptree file, or failing that, synthesize ptree.
-(define/contract (make-project-ptree [project-dir PROJECT_ROOT])
+(define/contract (make-project-ptree project-dir)
-  (() (directory-pathish?) . ->* . ptree?)
+  (directory-pathish? . -> . ptree?)
   (define ptree-source (build-path project-dir DEFAULT_PTREE))
   (if (file-exists? ptree-source)
-      (file->ptree ptree-source)
+      (if (not-modified? ptree-source)
+          (hash-ref ptree-cache ptree-source)
+          (begin
+            (hash-set! ptree-source-mod-dates ptree-source (file-or-directory-modify-seconds ptree-source))
+            (hash-ref! ptree-cache ptree-source (file->ptree ptree-source))))
       (directory->ptree project-dir)))