/// <summary>
/// Perform a node contraction (also: node identification) on two nodes.
/// The resulting node will have targetname as name.
/// The attributes of the endpoints are merged and copied to the target node,
/// with head attributes taking precedence over tail attributes.
///
/// Both node1 and node2 will be removed from the graph.
/// Then all the neighbours of both endpoints are attached to the target,
/// preserving direction and attributes.
/// The new edges will be added to the root graph.
/// If the graph is strict, no multiple edges will be added between nodes.
/// </summary>
/// <returns>target</returns>
public Node Contract(Node node1, Node node2, string targetname)
{
Node target = MyRootGraph.GetOrAddNode(targetname);
node1.CopyAttributesTo(target);
node2.CopyAttributesTo(target);
Merge(node1, target);
Merge(node2, target);
return(target);
}
/// <summary>
/// An edge can define a cluster as logical head.
/// This is used to fake edges to and from clusters by clipping the edge on the borders of the logical head.
/// </summary>
public void SetLogicalHead(SubGraph lhead)
{
if (!lhead.IsCluster())
{
throw new InvalidOperationException("ltail must be a cluster");
}
if (!MyRootGraph.IsCompound())
{
throw new InvalidOperationException("rootgraph must be compound for lheads/ltails to be used");
}
string lheadname = lhead.GetName();
SafeSetAttribute("lhead", lheadname, "");
}
/// <summary>
/// An edge can define a cluster as logical head.
/// This is used to fake edges to and from clusters by clipping the edge on the borders of the logical head.
/// </summary>
public SubGraph LogicalHead()
{
if (!MyRootGraph.IsCompound())
{
throw new InvalidOperationException("rootgraph must be compound for lheads/ltails to be used");
}
string lheadname = GetAttribute("lhead");
if (lheadname == null)
{
return(null);
}
return(MyRootGraph.GetSubgraph(lheadname));
}
/// <summary>
/// Merge a node into a target node.
/// Basically, add the neighborhood of the node to the neighborhood of the target.
/// The merge node will be removed from the graph.
/// The new edges will be added to the root graph.
///
/// If the graph is strict, no multiple edges will be added between nodes.
///
/// If add_self_loops is true, edges between the merge node and the target node will be
/// added as self loops to the target node. Self loops that already exist as such are always added.
/// </summary>
public void Merge(Node merge, Node target, bool add_self_loops = false)
{
// .Edges() won't iterate twice over self loops
foreach (var e in merge.Edges())
{
if (!e.IsBetween(merge, target) || add_self_loops) // Only add self loops if we want that
{
Node newtail = e.Tail().Equals(merge) ? target : e.Tail();
Node newhead = e.Head().Equals(merge) ? target : e.Head();
Edge newedge = MyRootGraph.GetOrAddEdge(newtail, newhead, e.GetName());
int returncode = e.CopyAttributesTo(newedge);
// For some reason this may fail, even when the copying seems to have succeeded.
Debug.Assert(returncode == 0);
}
}
// The following will delete all edges connected to merge.
MyRootGraph.Delete(merge);
}
