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Reduce allocation overhead in dag_algo::longest_path #1563

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Reduce allocation overhead in dag_algo::longest_path #1563

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6 changes: 4 additions & 2 deletions rustworkx-core/src/dag_algo.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -302,9 +302,10 @@ where
let mut dist: HashMap<G::NodeId, (T, G::NodeId)> = HashMap::with_capacity(nodes.len()); // Stores the distance and the previous node

// Iterate over nodes in topological order
let mut incoming_path: Vec<(T, G::NodeId)> = Vec::new();
for node in nodes {
let parents = graph.edges_directed(node, petgraph::Direction::Incoming);
let mut incoming_path: Vec<(T, G::NodeId)> = Vec::new(); // Stores the distance and the previous node for each parent
incoming_path.clear();
for p_edge in parents {
let p_node = p_edge.source();
let weight: T = weight_fn(p_edge)?;
Expand All @@ -313,8 +314,9 @@ where
}
// Determine the maximum distance and corresponding parent node
let max_path: (T, G::NodeId) = incoming_path
.into_iter()
.iter()
.max_by(|a, b| a.0.partial_cmp(&b.0).unwrap())
.copied()
.unwrap_or((T::zero(), node)); // If there are no incoming edges, the distance is zero

Comment on lines 305 to 321
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@jakelishman jakelishman Mar 11, 2026

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I don't even think you need a Vec at all, no?

Something like

for node in nodes {
    let max_path = graph
        .edges_directed(node, Incoming)
        .try_fold((T::zero(), node), |((max_length, prev), edge)| -> Result<_, E> {
            let source = edge.source();
            let length = dist[&source].0 + weight_fn(edge)?;
            if length > max_length {
                Ok((length, source))
            else {
                Ok((max_length, prev))
            }
        })?;
    // ...

// Store the maximum distance and the corresponding parent node for the current node
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