fix: adding a self-loop should not add two edges

[dlyongemallo]

Fixes #183.

[Copilot]

Pull Request Overview

This PR fixes a bug where self-loops (edges from a vertex to itself) were being added twice to the graph's internal data structure. The fix adds a check to prevent the duplicate addition of the reverse half-edge when source and target vertices are the same.

• Added a check if s != t to prevent adding the reverse half-edge for self-loops in add_edge_with_type
• Added a test case self_loop_not_duplicated to verify the fix

Comments suppressed due to low confidence (2)

quizx/src/vec_graph.rs:250

• The remove_edge method will incorrectly remove self-loops twice. When s == t, both remove_half_edge(s, t) and remove_half_edge(t, s) will attempt to remove the same edge from the adjacency list, but the second call will fail to find it (since it was already removed). While this may not cause a crash, it's inconsistent with the add_edge_with_type fix. Consider adding a similar if s != t check here to only call remove_half_edge(t, s) when the vertices differ.

    fn remove_edge(&mut self, s: V, t: V) {
        self.nume -= 1;
        self.remove_half_edge(s, t);
        self.remove_half_edge(t, s);
    }

quizx/src/vec_graph.rs:279

• The set_edge_type method has the same issue with self-loops as the original bug in add_edge_with_type. When s == t, the method will update the same edge twice (lines 268 and 275), which is unnecessary and could cause issues if the implementation changes. Consider adding an if s != t check before the second update block (lines 273-278) to match the fix in add_edge_with_type.

    fn set_edge_type(&mut self, s: V, t: V, ety: EType) {
        if let Some(Some(nhd)) = self.edata.get_mut(s) {
            let i = Graph::index(nhd, t).expect("Edge not found");
            nhd[i] = (t, ety);
        } else {
            panic!("Source vertex not found");
        }

        if let Some(Some(nhd)) = self.edata.get_mut(t) {
            let i = Graph::index(nhd, s).expect("Edge not found");
            nhd[i] = (s, ety);
        } else {
            panic!("Target vertex not found");
        }
    }

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