test if precs to be inserted are acylic

CupPlugin/src/de/tum/in/www2/cupplugin/views/CupConflictsView.java

@@ -5,7 +5,9 @@ import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map.Entry;
 import java.util.Set;
+import java.util.Stack;
 
 import org.eclipse.jface.text.BadLocationException;
 import org.eclipse.jface.text.IDocument;
@@ -91,6 +93,16 @@ public class CupConflictsView extends FailableView implements ICupEditorPageVisi
 		// precedence than a)
 		private HashMap<terminal,Set<terminal>> edges = new HashMap<>();
 		
+		private int tarjanIndex;
+		private HashMap<terminal,TarjanNodeInfo> tarjanNodeInfos;
+		private Stack<terminal> tarjanStack;
+		
+		private class TarjanNodeInfo{
+			public int index;
+			public boolean onStack;
+			public int lowlink;
+		}
+		
 		public boolean isResolvedInFavorOfShift(ShiftReduceDetails srd){
 			if(!srd.reduce.equals(srd.shift))
 				//TODO: Check if precedence set for terminal at all
@@ -150,6 +162,66 @@ public class CupConflictsView extends FailableView implements ICupEditorPageVisi
 			}
 		}
 		
+		private boolean isAcylic(){
+			tarjanIndex = 0;
+			tarjanNodeInfos = new HashMap<>();
+			tarjanStack = new Stack<>();
+			
+			for(terminal t : precs.keySet()){
+				TarjanNodeInfo tni = new TarjanNodeInfo();
+				tni.onStack = false;
+				tni.index = -1;
+				tni.lowlink = -1;
+				tarjanNodeInfos.put(t, tni);
+			}
+			
+			for(Entry<terminal, TarjanNodeInfo> e : tarjanNodeInfos.entrySet()){
+				if(e.getValue().index == -1){
+					if(strongConnect(e.getKey())){
+						return false;
+					}
+				}
+			}
+			
+			return true;
+		}
+		
+		private boolean strongConnect(terminal t){
+			TarjanNodeInfo node = tarjanNodeInfos.get(t);
+			
+			node.index = tarjanIndex;
+			node.lowlink = tarjanIndex;
+			node.onStack = true;
+			tarjanIndex++;
+			tarjanStack.push(t);
+			
+			for(terminal succ : edges.get(t)){
+				if(tarjanNodeInfos.get(succ).index == -1){
+					if(strongConnect(succ)){
+						return true;
+					}
+					node.lowlink = Math.min(node.lowlink,tarjanNodeInfos.get(succ).lowlink);
+				}
+				else if(tarjanNodeInfos.get(succ).onStack){
+					node.lowlink = Math.min(node.lowlink,tarjanNodeInfos.get(succ).index);
+				}
+			}
+			
+			if(node.lowlink == node.index){
+				terminal w = tarjanStack.pop();
+				tarjanNodeInfos.get(w).onStack = false;
+				
+				if(!w.equals(t)){
+					return true;
+				}
+			}
+			
+			return false;
+		}
+		
+		//TODO: topological sort of all precedences (probably reverse)
+		
+		
 		private boolean hasPrecedence(terminal t,terminal over){
 			// This works because we have guarantee that the graph is a DAG
 			HashSet<terminal> toCheck = new HashSet<>();
@@ -174,8 +246,7 @@ public class CupConflictsView extends FailableView implements ICupEditorPageVisi
 					toCheck = next;
 				}
 			}
-			
-			
+				
 			return false;
 		}