Add imenu support

Fixes #24.

This is much inspired by the sample code given by @xenodium in
issue #36. Thanks! :)

2 files changed, 24 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4537037..86d0ca1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 # v0.6
 
+Added imenu support! You can now navigate between headings with imenu.
+
 Helpful now correctly handles \= escapes in docstrings.
 
 Added disassemble buttons to byte-code functions in symbol properties.
diff --git a/helpful.el b/helpful.el
index c3f46a9..4eed109 100644
--- a/helpful.el
+++ b/helpful.el
@@ -1389,9 +1389,30 @@ See also `helpful-callable' and `helpful-variable'."
       (helpful-symbol symbol)
     (user-error "There is no symbol at point.")))
 
+(defun helpful--imenu-index ()
+  "Return a list of headings in the current buffer, suitable for
+imenu."
+  (let (headings)
+    (goto-char (point-min))
+    (while (not (eobp))
+      (when (eq (get-text-property (point) 'face)
+                'helpful-heading)
+        (push
+         (cons
+          (buffer-substring-no-properties
+           (line-beginning-position) (line-end-position))
+          (line-beginning-position))
+         headings))
+      (forward-line))
+    (nreverse headings)))
+
 (define-derived-mode helpful-mode special-mode "Helpful"
   "Major mode for *Helpful* buffers."
-  (add-hook 'xref-backend-functions #'elisp--xref-backend nil t))
+  (add-hook 'xref-backend-functions #'elisp--xref-backend nil t)
+
+  (setq imenu-create-index-function #'helpful--imenu-index)
+  ;; Prevent imenu converting "Source Code" to "Source.Code".
+  (setq-local imenu-space-replacement " "))
 
 (defun helpful-visit-reference ()
   "Go to the reference at point."