page: Add Prev and Next fields

dir.go

@@ -32,6 +32,8 @@ type Page struct {
 	Path    string `yaml:"-"`
 	Content string `yaml:"-"`
 	Params  map[string]string
+	Prev    *Page `yaml:"-"`
+	Next    *Page `yaml:"-"`
 }
 
 // NewDir returns a new Dir with the given path.
@@ -263,6 +265,16 @@ func (d *Dir) sort() {
 	sort.Slice(d.Pages, func(i, j int) bool {
 		return d.Pages[i].Date.After(d.Pages[j].Date)
 	})
+
+	for i := range d.Pages {
+		if i-1 > 0 {
+			d.Pages[i].Prev = d.Pages[i-1]
+		}
+		if i+1 < len(d.Pages) {
+			d.Pages[i].Next = d.Pages[i+1]
+		}
+	}
+
 	// Sort subdirectories
 	for _, d := range d.Dirs {
 		d.sort()