Slightly highlight root elements

In the dot representation of the techtree, highlight root elements with
a slightly different shade so they are easy to pick out.

This tells people at a glance what topics they can immediately start
working on :)

techtree-manager/src/tree.rs

@@ -15,8 +15,20 @@ pub struct Element {
     pub status: ElementStatus,
 }
 
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum ElementRole {
+    Root,
+    Ultimate,
+    Intermediate,
+    Disjoint,
+}
+
 impl Element {
-    fn to_dot_node_attributes(&self, role: Option<SubtreeElementRole>) -> String {
+    fn to_dot_node_attributes(
+        &self,
+        role: Option<ElementRole>,
+        subtree_role: Option<SubtreeElementRole>,
+    ) -> String {
         let Element {
             issue_number,
             description,
@@ -31,10 +43,18 @@ impl Element {
         ));
         attributes.push(r#"shape = "record""#.to_owned());
 
-        let (color, background) = match (role, status) {
+        let (color, background) = match (subtree_role, status) {
             (Some(SubtreeElementRole::ElementOfInterest), _) => ("black", "white"),
             (Some(SubtreeElementRole::Dependant), _) => ("gray", "gray"),
-            (_, ElementStatus::Missing) => ("#800", "#fcc"),
+            (_, ElementStatus::Missing) => (
+                "#800",
+                // Highlight root elements
+                if role == Some(ElementRole::Root) {
+                    "#ffddc1"
+                } else {
+                    "#fcc"
+                },
+            ),
             (_, ElementStatus::Assigned) => ("#a50", "#ffa"),
             (_, ElementStatus::Completed) => ("#080", "#afa"),
         };
@@ -170,7 +190,30 @@ impl Tree {
         })
     }
 
+    pub fn get_element_role(&self, element: ElementIndex) -> ElementRole {
+        let has_dependencies = self
+            .graph
+            .neighbors_directed(element, petgraph::Direction::Outgoing)
+            .next()
+            .is_some();
+        let has_dependants = self
+            .graph
+            .neighbors_directed(element, petgraph::Direction::Incoming)
+            .next()
+            .is_some();
+
+        match (has_dependencies, has_dependants) {
+            (false, true) => ElementRole::Root,
+            (true, false) => ElementRole::Ultimate,
+            (true, true) => ElementRole::Intermediate,
+            (false, false) => ElementRole::Disjoint,
+        }
+    }
+
     pub fn to_dot(&self) -> String {
+        let to_node_attributes = |_g, (id, element): (ElementIndex, &Element)| {
+            element.to_dot_node_attributes(Some(self.get_element_role(id)), None)
+        };
         let dot = petgraph::dot::Dot::with_attr_getters(
             &self.graph,
             &[
@@ -180,7 +223,7 @@ impl Tree {
                 petgraph::dot::Config::GraphContentOnly,
             ],
             &|_g, _edge_id| "".to_string(),
-            &|_g, (_, element)| element.to_dot_node_attributes(None),
+            &to_node_attributes,
         );
 
         let ultimate_elements: Vec<_> = self
@@ -318,7 +361,11 @@ impl<'a> Subtree<'a> {
                 petgraph::dot::Config::RankDir(petgraph::dot::RankDir::BT),
             ],
             &|_g, _edge_id| "".to_string(),
-            &|_g, (_, element)| element.element.to_dot_node_attributes(Some(element.role)),
+            &|_g, (_, element)| {
+                element
+                    .element
+                    .to_dot_node_attributes(None, Some(element.role))
+            },
         );
 
         format!("{:?}", dot)