Add support for headline CUSTOM_ID property & linking

this introduces the PropertyDrawer node to make it easier to access the
properties associated to a headline - normal drawers don't parse their content
into kv pairs

org/drawer.go

@@ -10,8 +10,13 @@ type Drawer struct {
 	Children []Node
 }
 
+type PropertyDrawer struct {
+	Properties [][]string
+}
+
 var beginDrawerRegexp = regexp.MustCompile(`^(\s*):(\S+):\s*$`)
 var endDrawerRegexp = regexp.MustCompile(`^(\s*):END:\s*$`)
+var propertyRegexp = regexp.MustCompile(`^(\s*):(\S+):(\s+(.*)$|\s*$)`)
 
 func lexDrawer(line string) (token, bool) {
 	if m := endDrawerRegexp.FindStringSubmatch(line); m != nil {
@@ -23,7 +28,11 @@ func lexDrawer(line string) (token, bool) {
 }
 
 func (d *Document) parseDrawer(i int, parentStop stopFn) (int, Node) {
-	drawer, start := Drawer{Name: strings.ToUpper(d.tokens[i].content)}, i
+	name := strings.ToUpper(d.tokens[i].content)
+	if name == "PROPERTIES" {
+		return d.parsePropertyDrawer(i, parentStop)
+	}
+	drawer, start := Drawer{Name: name}, i
 	i++
 	stop := func(d *Document, i int) bool {
 		if parentStop(d, i) {
@@ -49,3 +58,34 @@ func (d *Document) parseDrawer(i int, parentStop stopFn) (int, Node) {
 	}
 	return i - start, drawer
 }
+
+func (d *Document) parsePropertyDrawer(i int, parentStop stopFn) (int, Node) {
+	drawer, start := PropertyDrawer{}, i
+	i++
+	stop := func(d *Document, i int) bool {
+		return parentStop(d, i) || (d.tokens[i].kind != "text" && d.tokens[i].kind != "beginDrawer")
+	}
+	for ; !stop(d, i); i++ {
+		m := propertyRegexp.FindStringSubmatch(d.tokens[i].matches[0])
+		k, v := strings.ToUpper(m[2]), strings.TrimSpace(m[4])
+		drawer.Properties = append(drawer.Properties, []string{k, v})
+	}
+	if i < len(d.tokens) && d.tokens[i].kind == "endDrawer" {
+		i++
+	} else {
+		return 0, nil
+	}
+	return i - start, drawer
+}
+
+func (d *PropertyDrawer) Get(key string) (string, bool) {
+	if d == nil {
+		return "", false
+	}
+	for _, kvPair := range d.Properties {
+		if kvPair[0] == key {
+			return kvPair[1], true
+		}
+	}
+	return "", false
+}