Switch downstream nodes removal algorithm

[justin0108]

Hi,
I was looking through the ZEN engine (0.26) code and cannot understand the reason for the need to remove the nodes from the source. I can understand the need for removal so that the downstream nodes do not wait for input that will never arrive. However I am puzzle about this portion remove from source

// Remove dead branches from source
let source_outgoing_count = g.edges_directed(source_nid, Outgoing).count();
if source_outgoing_count.is_zero() {
    let edge_ids: Vec<EdgeIndex> = g
        .edges_directed(source_nid, Incoming)
        .map(|edge| edge.id())
        .collect();

    edge_ids.iter().for_each(|edge_id| {
        remove_edge_recursive(g, edge_id.clone());
    });

    if g.edges(source_nid).count().is_zero() {
        g.remove_node(source_nid);
        println!("Removed node from source: {:?}", source_nid);
    }
}

I have debug and step through the codes and it seems that this portion removing from target is sufficient to remove those dead nodes and branches. I have also tried commenting out the removal from source and it still works

// Remove dead branches from target
let target_incoming_count = g.edges_directed(target_nid, Incoming).count();
if target_incoming_count.is_zero() {
    let edge_ids: Vec<EdgeIndex> = g
        .edges_directed(target_nid, Outgoing)
        .map(|edge| edge.id())
        .collect();

    edge_ids.iter().for_each(|edge_id| {
        remove_edge_recursive(g, edge_id.clone());
    });

    if g.edges(target_nid).count().is_zero() {
        g.remove_node(target_nid);
        println!("Removed node from target: {:?}", target_nid);
    }
}

I have added some println! and as seen, the removal is duplicated (though it will not be a concern because remove_node will return None if not found)

Removed node from source: NodeIndex(3)
Removed node from target: NodeIndex(3)
Removed node from source: NodeIndex(1)
Removed node from target: NodeIndex(1)

It is more purely for my understand what kind of switch graph need a removal from source? It could be my graph is too simple. This is a sample of my graph

                 /--- table --- function ---\
Input --- switch                             output
                 \--- table --- function ---/

will become


Input --- switch                             output
                 \--- table --- function ---/

I have also attached the JDM for reference
switch-simple.json