Handle reference definitions when filling paragraphs

Previously, when fill-paragraph was called (or auto-fill-mode is on)
lines could be broken between the label and URL for a reference
definition if the line is long. Additionally, multiple reference
definitions could become jumpled together into the same paragraph.

Example A before fill-paragraph:
[long-url]: http://a-really-long-url.example.com/...

Example A after fill-paragraph (or autofill):
[long-url]:
http://a-really-long-url.example.com/...

Example B before fill-paragraph:
[label1]: http://short.example.com/
[label2]: http://also-short.example.com

Example B after fill-paragraph:
[label1]: http://short.example.com/ [label2]: http://also-short.example.com

markdown-mode.el

@@ -4523,11 +4523,16 @@ This is an exact copy of `line-number-at-pos' for use in emacs21."
       (forward-line 0)
       (1+ (count-lines start (point))))))
 
-(defun markdown-nobreak-p ()
-  "Return nil if it is acceptable to break the current line at the point."
-  ;; inside in square brackets (e.g., link anchor text)
+(defun markdown-inside-link-text-p ()
+  "Return nil if not currently within link anchor text."
   (looking-back "\\[[^]]*"))
 
+(defun markdown-line-is-reference-definition-p ()
+  "Return whether the current line is a reference defition."
+  (save-excursion
+    (move-beginning-of-line 1)
+    (looking-at markdown-regex-reference-definition)))
+
 (defun markdown-adaptive-fill-function ()
   "Return prefix for filling paragraph or nil if not determined."
   (cond
@@ -4630,10 +4635,29 @@ if ARG is omitted or nil."
   (set (make-local-variable 'end-of-defun-function)
        'markdown-end-of-defun)
   ;; Paragraph filling
-  (set (make-local-variable 'paragraph-start)
-       "\f\\|[ \t]*$\\|[ \t]*[*+-][ \t]+\\|[ \t]*[0-9]+\\.[ \t]+\\|[ \t]*: ")
-  (set (make-local-variable 'paragraph-separate)
-       "\\(?:[ \t\f]*\\|.*  \\)$")
+  (set
+   ; Should match lines that start or seaprate paragraphs
+   (make-local-variable 'paragraph-start)
+       (mapconcat 'identity
+                  '(
+                    "\f" ; starts with a literal line-feed
+                    "[ \t\f]*$" ; space-only line
+                    "[ \t]*[*+-][ \t]+" ; unordered list item
+                    "[ \t]*[0-9]+\\.[ \t]+" ; ordered list item
+                    "[ \t]*\\[\\S-*\\]:[ \t]+" ; link ref def
+                    )
+                  "\\|"))
+  (set
+   ; Should match lines that separate paragraphs without being
+   ; part of any paragraph:
+   (make-local-variable 'paragraph-separate)
+   (mapconcat 'identity
+              '("[ \t\f]*$" ; space-only line
+                ; The following is not ideal, but the Fill customization
+                ; options really only handle paragraph-starting prefixes,
+                ; not paragraph-ending suffixes:
+                ".*  $") ; line ending in two spaces
+              "\\|"))
   (set (make-local-variable 'adaptive-fill-first-line-regexp)
        "\\`[ \t]*>[ \t]*?\\'")
   (set (make-local-variable 'adaptive-fill-function)
@@ -4647,9 +4671,16 @@ if ARG is omitted or nil."
   (setq outline-level 'markdown-outline-level)
   ;; Cause use of ellipses for invisible text.
   (add-to-invisibility-spec '(outline . t))
-  ;; Indentation and filling
-  (make-local-variable 'fill-nobreak-predicate)
-  (add-hook 'fill-nobreak-predicate 'markdown-nobreak-p)
+
+  ;; Inhibiting line-breaking:
+  ;; Separating out each condition into a separate function so that users can
+  ;; override if desired (with remove-hook)
+  (add-hook 'fill-nobreak-predicate
+            'markdown-inside-link-text-p :local t)
+  (add-hook 'fill-nobreak-predicate
+            'markdown-line-is-reference-definition-p :local t)
+
+  ;; Indentation
   (setq indent-line-function markdown-indent-function)
 
   ;; Prepare hooks for XEmacs compatibility

tests/Makefile

@@ -11,7 +11,7 @@ TEST_HTML = $(TEST_SRCS:.text=.html)
 	$(MARKDOWN) < $< > $@
 
 test:
-	$(EMACS) -batch -l ert -l ../markdown-mode.el -l markdown-test.el \
+	$(EMACS) -Q -batch -l ert -l ../markdown-mode.el -l markdown-test.el \
 	    -f ert-run-tests-batch-and-exit
 
 html: $(TEST_HTML)

tests/markdown-test.el

@@ -2198,6 +2198,14 @@ See `paragraph-separate'."
    (fill-paragraph)
    (should (string-equal (buffer-string) "The circumference of a circle divided by it's radius is around 3.14."))))
 
+(ert-deftest test-markdown-filling/no-break-link-reference ()
+  "Shouldn't break line between label and url, or combine two link references."
+  (let ((str "[label1]: http://long-url.example.com\n[label2]: http://another-long-url.example.com/"))
+    (markdown-test-string str
+     (let ((fill-column 15)) ; after end of label, before end of URL
+       (fill-paragraph)
+       (should (string-equal (buffer-string) str))))))
+
 ;;; Export tests:
 
 (ert-deftest test-markdown-hook/xhtml-standalone ()