Add tests for graph

compiler/passes/src/common/graph/mod.rs

@@ -79,26 +79,19 @@ impl<N: Node> DiGraph<N> {
                 // The set of nodes that are on the path to the current node in the search.
                 let mut discovered: IndexSet<N> = IndexSet::new();
                 // Check if there is a cycle in the graph starting from `node`.
-                if self.contains_cycle_from(*node, &mut discovered, &mut finished) {
-                    let path = match discovered.pop() {
-                        // TODO: Should this error more silently?
-                        None => unreachable!("If `contains_cycle_from` returns `true`, `discovered` is not empty."),
-                        Some(node) => {
-                            let mut path = vec![node];
-                            // Backtrack through the discovered nodes to find the cycle.
-                            while let Some(next) = discovered.pop() {
-                                // Add the node to the path.
-                                path.push(next);
-                                // If the node is the same as the first node in the path, we have found the cycle.
-                                if next == node {
-                                    break;
-                                }
-                            }
-                            // Reverse the path to get the cycle in the correct order.
-                            path.reverse();
-                            path
+                if let Some(node) = self.contains_cycle_from(*node, &mut discovered, &mut finished) {
+                    let mut path = vec![node];
+                    // Backtrack through the discovered nodes to find the cycle.
+                    while let Some(next) = discovered.pop() {
+                        // Add the node to the path.
+                        path.push(next);
+                        // If the node is the same as the first node in the path, we have found the cycle.
+                        if next == node {
+                            break;
                         }
-                    };
+                    }
+                    // Reverse the path to get the cycle in the correct order.
+                    path.reverse();
                     // A cycle was detected. Return the path of the cycle.
                     return Err(GraphError::CycleDetected(path));
                 }
@@ -109,8 +102,13 @@ impl<N: Node> DiGraph<N> {
     }
 
     // Detects if there is a cycle in the graph starting from the given node, via a recursive depth-first search.
+    // If there is no cycle, returns `None`.
+    // If there is a cycle, returns the node that was most recently discovered.
     // Nodes are added to to `finished` in topological order.
-    fn contains_cycle_from(&self, node: N, discovered: &mut IndexSet<N>, finished: &mut IndexSet<N>) -> bool {
+    fn contains_cycle_from(&self, node: N, discovered: &mut IndexSet<N>, finished: &mut IndexSet<N>) -> Option<N>{
+        println!("discovered: {:?}", discovered);
+        println!("finished: {:?}", finished);
+        println!("node: {:?}\n", node);
         // Add the node to the set of discovered nodes.
         discovered.insert(node);
 
@@ -121,12 +119,13 @@ impl<N: Node> DiGraph<N> {
                 if discovered.contains(child) {
                     // Insert the child node into the set of discovered nodes; this is used to reconstruct the cycle.
                     // Note that this case is always hit when there is a cycle.
-                    discovered.insert(*child);
-                    return true;
+                    return Some(*child);
                 }
                 // If the node has not been explored, explore it.
-                if !finished.contains(child) && self.contains_cycle_from(*child, discovered, finished) {
-                    return true;
+                if !finished.contains(child) {
+                    if let Some(child) = self.contains_cycle_from(*child, discovered, finished) {
+                        return Some(child);
+                    }
                 }
             }
         }
@@ -136,6 +135,67 @@ impl<N: Node> DiGraph<N> {
         // Add the node to the set of finished nodes.
         finished.insert(node);
 
-        false
+        None
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    impl Node for u32 {}
+
+    #[test]
+    fn test_toposort() {
+        let mut graph = DiGraph::<u32>::new(IndexSet::new());
+
+        graph.add_edge(1, 2);
+        graph.add_edge(1, 3);
+        graph.add_edge(2, 4);
+        graph.add_edge(3, 4);
+        graph.add_edge(4, 5);
+
+        // At this point, the graph looks like:
+        //    1
+        //   / \
+        //  2   3
+        //  \   /
+        //    4
+        //    |
+        //    5
+
+        let result = graph.topological_sort();
+        assert!(result.is_ok());
+
+        let order: Vec<u32> = result.unwrap().into_iter().collect();
+        let expected = Vec::from([5u32, 4, 2, 3, 1]);
+        assert_eq!(order, expected);
+    }
+
+    #[test]
+    fn test_toposort_cycle() {
+        let mut graph = DiGraph::<u32>::new(IndexSet::new());
+
+        graph.add_edge(1, 2);
+        graph.add_edge(2, 3);
+        graph.add_edge(2, 4);
+        graph.add_edge(4, 1);
+
+        // At this point, the graph looks like:
+        //    1
+        //    |
+        //    2
+        //   / \
+        //  3   4
+        //      |
+        //      1
+
+        let result = graph.topological_sort();
+        assert!(result.is_err());
+
+        let GraphError::CycleDetected(cycle) = result.unwrap_err();
+        let expected = Vec::from([1u32, 2, 4, 1]);
+        assert_eq!(cycle, expected);
+    }
+
+}