/// <returns>A boolean value indicating whether the node was found to be zero-equivalent.
/// That is, the returned value is <see langword="true"/> <em>only</em> if the node is
/// zero-equivalent but may be <see langword="false"/> even if the node is zero-equivalent.</returns>
/// <remarks>A node is found to be zero-equivalent either if its path can be killed or if
/// the path is equivalent to (up to replacement) to a path that has previously been
/// determined to be zero-equivalent.</remarks>
private static bool EquivClassExploreNode <TVertex>(
TransformationRuleTreeNode <TVertex> transformationRuleTree,
SearchTreeNodeOld <TVertex> node,
AnalysisStateForSingleStartingVertexOld <TVertex> state,
Stack <SearchTreeNodeOld <TVertex> > equivClassStack,
SearchTreeNodeOld <TVertex> origin)
where TVertex : IEquatable <TVertex>, IComparable <TVertex>
{
node.Explored = true;
var trailingVertexPath = new List <TVertex>(); // In reversed order
foreach (var endingNode in node.ReversePathOfNodes) // The last vertex of endingNode is the last vertex in the subpath
{
var transformationNode = transformationRuleTree;
foreach (var startingNode in endingNode.ReversePathOfNodes) // The last vertex of startingNode is the first vertex in the subpath
{
var pathVertex = startingNode.Vertex;
if (!transformationNode.Children.TryGetValue(pathVertex, out transformationNode))
{
break;
}
if (transformationNode.CanBeKilled)
{
state.EquivalenceClasses.Union(node, state.ZeroDummyNode);
return(true);
}
// If replacement is possible, do the replacement on the subpath
// and add a search tree node for the entire resulting path
if (transformationNode.CanBeReplaced)
{
// Add search tree nodes for replacement path
// This doesn't work when pathNode is the root ...
// var lastUnreplacedNode = pathNode.Parent;
// var curNode = lastUnreplacedNode;
// ... so instead do the following (with Skip), which assumes that the replacement
// path has the same first vertex (which it does for the QPs under consideration)
var firstReplacementNode = startingNode;
var curNode = firstReplacementNode;
foreach (var vertex in transformationNode.ReplacementPath.Vertices.Skip(1))
{
curNode = state.GetInsertChildNode(curNode, vertex, origin);
}
// Add search tree nodes for the trailing path
foreach (var vertex in trailingVertexPath.Reversed())
{
curNode = state.GetInsertChildNode(curNode, vertex, origin);
}
var transformedNode = curNode;
if (state.NodeIsZeroEquivalent(transformedNode))
{
state.EquivalenceClasses.Union(node, transformedNode);
return(true);
}
// transformedNode.Explored may be true and is then a node that we have
// already encountered during the equivalence class search.
// (It cannot be an explored node of some *other* equivalence class,
// because then this current equivalence class search would not have
// started in the first place.)
//if (!transformedNode.Explored)
if (!state.EquivalenceClasses.FindSet(node).Equals(state.EquivalenceClasses.FindSet(transformedNode)))
{
state.EquivalenceClasses.Union(node, transformedNode);
equivClassStack.Push(transformedNode);
}
}
}
trailingVertexPath.Add(endingNode.Vertex);
}
return(false);
}
/// <returns>A boolean value indicating whether the path correspondings to the explored
/// child node is nonzero (up to equivalence).</returns>
private ExploreChildNodeResult ExploreChildNode <TVertex>(
TransformationRuleTreeNode <TVertex> transformationRuleTree,
AnalysisStateForSingleStartingVertexOld <TVertex> state,
SearchTreeNodeOld <TVertex> parentNode,
TVertex nextVertex,
QPAnalysisSettings settings)
where TVertex : IEquatable <TVertex>, IComparable <TVertex>
{
// Either the child node has not already been discovered, or the child node was
// discovered during an equivalence class search.
if (parentNode.Children.TryGetValue(nextVertex, out var childNode))
{
// The child node has already been discovered.
// Do a non-cancellativity check (if we are to detect non-cancellativity):
// If the child node is zero, everything is fine.
// If a different arrow was added to the origin than to the parent to get child node (i.e., origin and parent have different last vertices), things are fine.
// Else, make sure that the origin is equivalent to the parent
// (otherwise, the QP is not cancellative).
if (
settings.DetectCancellativityFailure &&
!state.NodeIsZeroEquivalent(childNode) &&
parentNode.Vertex.Equals(childNode.Origin.Vertex) &&
state.EquivalenceClasses.FindSet(parentNode) != state.EquivalenceClasses.FindSet(childNode.Origin))
{
return(ExploreChildNodeResult.NonCancellativityDetected);
}
}
else
{
// The child node has not been discovered, so let's discover it by inserting it
// into the search tree.
childNode = state.InsertChildNode(parentNode, nextVertex, parentNode);
}
if (childNode.Explored)
{
return(state.NodeIsZeroEquivalent(childNode) ? ExploreChildNodeResult.ZeroEquivalent : ExploreChildNodeResult.NotZeroEquivalent);
}
// Code for equivalence class searching begins here
// Stack containing all the nodes to explore (by path transformation)
var equivClassStack = new Stack <SearchTreeNodeOld <TVertex> >(state.EquivalenceClasses.GetSet(childNode));
// This could happen with the current code, namely when the current childNode was encountered
// in a previous call to ExploreChildNodes (in EquivClassExploreNode, as transformationNode)
// and 'node' was found to be zero equivalent (in which case the search is cancelled
// before transformationNode is explored in EquivClassExploreNode)
// Thought: Would probably be good to have several Explored properties (or an
// enum-valued Explored property) containing information about the extent to which the
// node is explored (explored-as-in-encountered-in-EquivClassExploreNode would be
// useful here; more easily understood and maintained than this comment methinks)
if (equivClassStack.Count != 1)
{
return(ExploreChildNodeResult.ZeroEquivalent);
}
while (equivClassStack.Count > 0)
{
var equivalentNode = equivClassStack.Pop();
if (equivalentNode.Explored)
{
continue;
}
if (settings.UseMaxLength && equivalentNode.PathLengthInVertices > settings.MaxPathLength + 1)
{
return(ExploreChildNodeResult.TooLongPath);
}
var exploredNodeWasFoundZeroEquivalent = EquivClassExploreNode(transformationRuleTree, equivalentNode, state, equivClassStack, parentNode);
if (exploredNodeWasFoundZeroEquivalent)
{
return(ExploreChildNodeResult.ZeroEquivalent);
}
}
// Child node was not zero-equivalent
state.Stack.Push(childNode);
return(ExploreChildNodeResult.NotZeroEquivalent);
}