Add: check that there's exactly one goal

1 files changed, 21 insertions, 2 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 1fc6435..b186ee4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -192,6 +192,11 @@ impl Roadmap {
             }
         }
 
+        // Is there exactly one goal?
+        if self.count_goals() != 1 {
+            return Err(format!("must have exactly one goal for roadmap"));
+        }
+
         // Does every dependency exist?
         for step in self.steps.iter() {
             for depname in step.dependencies() {
@@ -211,6 +216,17 @@ impl Roadmap {
         Ok(())
     }
 
+    // Count number of steps that nothing depends on.
+    fn count_goals(&self) -> usize {
+        self.steps
+            .iter()
+            .map(|step| {
+                self.is_goal(step)
+            })
+            .filter(|b| *b)
+            .count()
+    }
+
     /// Return list of step names.
     pub fn step_names<'a>(&'a self) -> Vec<&'a str> {
         let mut names = vec![];
@@ -536,8 +552,11 @@ first -> second;
 
     #[test]
     fn from_empty_yaml() {
-        let roadmap = Roadmap::from_yaml("{}").unwrap();
-        assert_eq!(roadmap.step_names().len(), 0);
+        let roadmap = Roadmap::from_yaml("{}");
+        match roadmap {
+            Ok(_) => panic!("expected error for empty dict"),
+            _ => (),
+        }
     }
 
     #[test]