cmd/present: display slide numbers

Fixes golang/go#5331

Change-Id: Iddd45452959823b2e8e3f9b4236a9a9a3968d35f
Reviewed-on: https://go-review.googlesource.com/97058
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

cmd/present/static/styles.css

@@ -415,6 +415,14 @@ p.link {
   margin-left: 20px;
 }
 
+.pagenumber {
+  color: #8c8c8c;
+  font-size: 75%;
+  position: absolute;
+  bottom: 0px;
+  right: 10px;
+}
+
 /* Code */
 div.code {
   outline: 0px solid transparent;

cmd/present/templates/slides.tmpl

@@ -62,8 +62,9 @@
       {{else}}
         <h2>{{$s.Title}}</h2>
       {{end}}
+      <span class="pagenumber">{{pagenum $s 1}}</span>
       </article>
-  <!-- end of slide {{$i}} -->
+  <!-- end of slide {{$s.Number}} -->
   {{end}}{{/* of Slide block */}}
 
       <article>

present/parse.go

@@ -168,8 +168,17 @@ func renderElem(t *template.Template, e Elem) (template.HTML, error) {
 	return execTemplate(t, e.TemplateName(), data)
 }
 
+// pageNum derives a page number from a section.
+func pageNum(s Section, offset int) int {
+	if len(s.Number) == 0 {
+		return offset
+	}
+	return s.Number[0] + offset
+}
+
 func init() {
 	funcs["elem"] = renderElem
+	funcs["pagenum"] = pageNum
 }
 
 // execTemplate is a helper to execute a template and return the output as a