Don't highlight very large code snippets

This stops big code excerpts hanging Emacs when using helpful.
Fixes #107.

1 files changed, 28 insertions, 19 deletions

diff --git a/helpful.el b/helpful.el
index c82c3b1..3b55f4f 100644
--- a/helpful.el
+++ b/helpful.el
@@ -82,6 +82,14 @@ To disable cleanup entirely, set this variable to nil. See also
   :type 'function
   :group 'helpful)
 
+;; TODO: explore whether more basic highlighting is fast enough to
+;; handle larger functions. See `c-font-lock-init' and its use of
+;; font-lock-keywords-1.
+(defconst helpful-max-highlight 5000
+  "Don't highlight code with more than this many characters.
+This is particularly important for large pieces of C code, such
+as describing `this-command'.")
+
 (defun helpful--kind-name (symbol callable-p)
   "Describe what kind of symbol this is."
   (cond
@@ -905,25 +913,26 @@ hooks.")
   (with-temp-buffer
     (insert source)
 
-    ;; Switch to major-mode MODE, but don't run any hooks.
-    (delay-mode-hooks (funcall mode))
-
-    ;; `delayed-mode-hooks' contains mode hooks like
-    ;; `emacs-lisp-mode-hook'. Build a list of functions that are run
-    ;; when the mode hooks run.
-    (let (hook-funcs)
-      (dolist (hook delayed-mode-hooks)
-        (let ((funcs (symbol-value hook)))
-          (setq hook-funcs (append hook-funcs funcs))))
-
-      ;; Filter hooks to those that relate to highlighting, and run them.
-      (setq hook-funcs (-intersection hook-funcs helpful--highlighting-funcs))
-      (-map #'funcall hook-funcs))
-
-    (if (fboundp 'font-lock-ensure)
-        (font-lock-ensure)
-      (with-no-warnings
-        (font-lock-fontify-buffer)))
+    (when (< (length source) helpful-max-highlight)
+      ;; Switch to major-mode MODE, but don't run any hooks.
+      (delay-mode-hooks (funcall mode))
+
+      ;; `delayed-mode-hooks' contains mode hooks like
+      ;; `emacs-lisp-mode-hook'. Build a list of functions that are run
+      ;; when the mode hooks run.
+      (let (hook-funcs)
+        (dolist (hook delayed-mode-hooks)
+          (let ((funcs (symbol-value hook)))
+            (setq hook-funcs (append hook-funcs funcs))))
+
+        ;; Filter hooks to those that relate to highlighting, and run them.
+        (setq hook-funcs (-intersection hook-funcs helpful--highlighting-funcs))
+        (-map #'funcall hook-funcs))
+
+      (if (fboundp 'font-lock-ensure)
+          (font-lock-ensure)
+        (with-no-warnings
+          (font-lock-fontify-buffer))))
     (buffer-string)))
 
 (defun helpful--source (sym callable-p)