ox-html: Number source blocks with listings

* lisp/ox-html.el (org-html-src-block): Number source block if it has
  a caption.  Numbering is done among source blocks with a caption,
  only.

Suggested-by: Clément Pit--Claudel <clement.pit@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/108808>

etc/ORG-NEWS

@@ -189,6 +189,11 @@ If the block has a =#+NAME:= attribute assigned, then the HTML element
 will have an ~id~ attribute with that name in the HTML export. This
 enables one to create links to these elements in other places, e.g.,
 ~<a href="#name">text</a>~.
+**** Listings with captions are now numbered in HTML export
+The class associated to the numbering is "listing-number".  If you
+don't want these blocks to be numbered, as it was the case until now,
+You may want to add ~.listing-number { display: none; }~ to the CSS
+used.
 **** Line Numbering in SRC/EXAMPLE blocks support arbitrary start number
 The ~-n~ option to ~SRC~ and ~EXAMPLE~ blocks can now take a numeric
 argument to specify the staring line number for the source or example

lisp/ox-html.el

@@ -3319,18 +3319,28 @@ contextual information."
   (if (org-export-read-attribute :attr_html src-block :textarea)
       (org-html--textarea-block src-block)
     (let ((lang (org-element-property :language src-block))
-	  (caption (org-export-get-caption src-block))
 	  (code (org-html-format-code src-block info))
 	  (label (let ((lbl (and (org-element-property :name src-block)
 				 (org-export-get-reference src-block info))))
 		   (if lbl (format " id=\"%s\"" lbl) ""))))
       (if (not lang) (format "<pre class=\"example\"%s>\n%s</pre>" label code)
-	(format
-	 "<div class=\"org-src-container\">\n%s%s\n</div>"
-	 (if (not caption) ""
-	   (format "<label class=\"org-src-name\">%s</label>"
-		   (org-export-data caption info)))
-	 (format "\n<pre class=\"src src-%s\"%s>%s</pre>" lang label code))))))
+	(format "<div class=\"org-src-container\">\n%s%s\n</div>"
+		;; Build caption.
+		(let ((caption (org-export-get-caption src-block)))
+		  (if (not caption) ""
+		    (let ((listing-number
+			   (format
+			    "<span class=\"listing-number\">%s </span>"
+			    (format
+			     (org-html--translate "Listing %d:" info)
+			     (org-export-get-ordinal
+			      src-block info nil #'org-html--has-caption-p)))))
+		      (format "<label class=\"org-src-name\">%s%s</label>"
+			      listing-number
+			      (org-trim (org-export-data caption info))))))
+		;; Contents.
+		(format "<pre class=\"src src-%s\"%s>%s</pre>"
+			lang label code))))))
 
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