Reference checking

Using public domain code by Dmitry Dzhus <mail@sphinx.net.ru>
from markdown-goodies (with permission).

markdown-mode.el

@@ -87,6 +87,11 @@
 ;;   output in another buffer while `C-c C-c p` runs Markdown on the
 ;;   current buffer and previews the output in a browser.
 ;;
+;;   `C-c C-c c` will check for undefined references.  If there are any,
+;;   a small buffer will open with a list.  Selecting a reference from
+;;   this list and pressing `RET` will insert a reference template at
+;;   the end of the buffer.
+;;
 ;; * Images: `C-c C-i`
 ;;
 ;;   `C-c C-i i` inserts an image, using the active region (if any) as
@@ -151,6 +156,7 @@
 ;; * Greg Bognar <greg_bognar@hms.harvard.edu> for menus and a patch.
 ;; * Daniel Burrows <dburrows@debian.org> for filing Debian bug #456592.
 ;; * Peter S. Galbraith <psg@debian.org> for maintaining emacs-goodies-el.
+;; * Dmitry Dzhus <mail@sphinx.net.ru> for reference checking functions.
 
 ;;; Bugs:
 
@@ -642,6 +648,8 @@ as preformatted text."
     ;; Markdown functions
     (define-key markdown-mode-map "\C-c\C-cm" 'markdown)
     (define-key markdown-mode-map "\C-c\C-cp" 'markdown-preview)
+    ;; References
+    (define-key markdown-mode-map "\C-c\C-cc" 'markdown-check-refs)
     markdown-mode-map)
   "Keymap for Markdown major mode")
 
@@ -673,11 +681,145 @@ as preformatted text."
     ["Insert image" markdown-insert-image]
     ["Insert horizontal rule" markdown-insert-hr]
     "---"
+    ["Check references" markdown-check-refs]
+    "---"
     ["Version" markdown-show-version]
     ))
 
 
 
+;;; References ================================================================
+
+;;; Undefined reference checking code by Dmitry Dzhus <mail@sphinx.net.ru>.
+
+(defconst markdown-refcheck-buffer
+  "*Undefined references for %BUFFER%*"
+  "Name of buffer which will contain a list of undefined
+references in `markdown-mode' buffer named %BUFFER%.")
+
+(defun markdown-has-reference-definition (reference)
+    "Find out whether Markdown REFERENCE is defined.
+
+REFERENCE should include the square brackets, like [this]."
+    (let ((reference (downcase reference)))
+      (save-excursion
+        (goto-char (point-min))
+        (catch 'found
+          (while (re-search-forward markdown-regex-reference-definition nil t)
+            (when (string= reference (downcase (match-string-no-properties 1)))
+              (throw 'found t)))))))
+
+(defun markdown-get-undefined-refs ()
+  "Return a list of undefined Markdown references.
+
+Result is an alist of pairs (reference . occurencies), where
+occurencies is itself another alist of pairs (label .
+line-number).
+
+For example, an alist corresponding to [Nice editor][Emacs] at line 12,
+\[GNU Emacs][Emacs] at line 45 and [manual][elisp] at line 127 is
+\((\"[emacs]\" (\"[Nice editor]\" . 12) (\"[GNU Emacs]\" . 45)) (\"[elisp]\" (\"[manual]\" . 127)))."
+  (let ((missing))
+    (save-excursion
+      (goto-char (point-min))
+      (while
+          (re-search-forward markdown-regex-link-reference nil t)
+        (let* ((label (match-string-no-properties 1))
+               (reference (match-string-no-properties 2))
+               (target (downcase (if (string= reference "[]") label reference))))
+          (unless (markdown-has-reference-definition target)
+            (let ((entry (assoc target missing)))
+              (if (not entry)
+                  (add-to-list 'missing (cons target
+                                              (list (cons label (line-number-at-pos)))) t)
+                (setcdr entry
+                        (append (cdr entry) (list (cons label (line-number-at-pos))))))))))
+      missing)))
+
+(defun markdown-add-missing-ref-definition (ref buffer &optional recheck)
+  "Add blank REF definition to the end of BUFFER.
+
+REF is a Markdown reference in square brackets, like \"[lisp-history]\".
+
+When RECHECK is non-nil, BUFFER gets rechecked for undefined
+references so that REF disappears from the list of those links."
+  (with-current-buffer buffer
+      (when (not (eq major-mode 'markdown-mode))
+        (error "Not available in current mdoe"))
+      (goto-char (point-max))
+      (indent-new-comment-line)
+      (insert (concat ref ": ")))
+  (switch-to-buffer-other-window buffer)
+  (goto-char (point-max))
+  (when recheck
+    (markdown-check-refs t)))
+
+;; Button which adds an empty Markdown reference definition to the end
+;; of buffer specified as its 'target-buffer property. Reference name
+;; is button's label
+(define-button-type 'markdown-ref-button
+  'help-echo "Push to create an empty reference definition"
+  'face 'bold
+  'action (lambda (b)
+            (markdown-add-missing-ref-definition
+             (button-label b) (button-get b 'target-buffer) t)))
+
+;; Button jumping to line in buffer specified as its 'target-buffer
+;; property. Line number is button's 'line property.
+(define-button-type 'goto-line-button
+  'help-echo "Push to go to this line"
+  'face 'italic
+  'action (lambda (b)
+            (message (button-get b 'buffer))
+            (switch-to-buffer-other-window (button-get b 'target-buffer))
+            (goto-line (button-get b 'target-line))))
+
+(defun markdown-check-refs (&optional silent)
+  "Show all undefined Markdown references in current `markdown-mode' buffer.
+
+If SILENT is non-nil, do not message anything when no undefined
+references found.
+
+Links which have empty reference definitions are considered to be
+defined."
+  (interactive "P")
+  (when (not (eq major-mode 'markdown-mode))
+    (error "Not available in current mode"))
+  (let ((oldbuf (current-buffer))
+        (refs (markdown-get-undefined-refs))
+        (refbuf (get-buffer-create (replace-regexp-in-string
+                                 "%BUFFER%" (buffer-name)
+                                 markdown-refcheck-buffer t))))
+    (if (null refs)
+        (progn
+          (when (not silent)
+            (message "No undefined references found"))
+          (kill-buffer refbuf))
+      (with-current-buffer refbuf
+        (when view-mode
+          (View-exit-and-edit))
+        (erase-buffer)
+        (insert "Following references lack definitions:")
+        (newline 2)
+        (dolist (ref refs)
+          (let ((button-label (format "%s" (car ref))))
+            (insert-text-button button-label
+                                :type 'markdown-ref-button
+                                'target-buffer oldbuf)
+            (insert " (")
+            (dolist (occurency (cdr ref))
+              (let ((line (cdr occurency)))
+                (insert-button (number-to-string line)
+                               :type 'goto-line-button
+                               'target-buffer oldbuf
+                               'target-line line)
+                (insert " "))) (delete-backward-char 1)
+                (insert ")")
+                (newline))))
+      (view-buffer-other-window refbuf)
+      (goto-line 4))))
+
+
 ;;; Commands ==================================================================
 
 (defun markdown ()