diff --git a/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/NodeWritable.java b/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/NodeWritable.java
index 6870ff1..d1fcd6b 100644
--- a/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/NodeWritable.java
+++ b/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/NodeWritable.java
@@ -31,31 +31,32 @@
 
     // merge/update directions
     public static class DirectionFlag {
-        public static final byte DIR_FF = 0b000 << 0;
-        public static final byte DIR_FR = 0b001 << 0;
-        public static final byte DIR_RF = 0b010 << 0;
-        public static final byte DIR_RR = 0b011 << 0;
-        public static final byte DIR_NO = 0b111 << 0;
+        public static final byte DIR_NO = 0b000 << 0;
+        public static final byte DIR_FF = 0b001 << 0;
+        public static final byte DIR_FR = 0b010 << 0;
+        public static final byte DIR_RF = 0b011 << 0;
+        public static final byte DIR_RR = 0b100 << 0;
         public static final byte DIR_MASK = 0b111 << 0;
     }
     
     public static class SpecialVertexFlag extends DirectionFlag {
-        public static final byte IS_RANDOMTAIL = 0b00 << 4;
-        public static final byte IS_STOP = 0b00 << 4;
-        public static final byte IS_HEAD = 0b01 << 4;
-        public static final byte IS_FINAL = 0b10 << 4;
-        public static final byte IS_RANDOMHEAD = 0b11 << 4;
-        public static final byte IS_OLDHEAD = 0b11 << 4;
+        public static final byte IS_NON = 0b00 << 5;
+        public static final byte IS_RANDOMTAIL = 0b00 << 5;
+        public static final byte IS_STOP = 0b00 << 5;
+        public static final byte IS_HEAD = 0b01 << 5;
+        public static final byte IS_FINAL = 0b10 << 5;
+        public static final byte IS_RANDOMHEAD = 0b11 << 5;
+        public static final byte IS_OLDHEAD = 0b11 << 5;
 
-        public static final byte VERTEX_MASK = 0b11 << 4;
+        public static final byte VERTEX_MASK = 0b11 << 5;
         public static final byte VERTEX_CLEAR = (byte) 11001111;
     }
     
     public static class MergeDirFlag extends SpecialVertexFlag{
-        public static final byte NO_MERGE = 0b00 << 2;
-        public static final byte SHOULD_MERGEWITHNEXT = 0b01 << 2;
-        public static final byte SHOULD_MERGEWITHPREV = 0b10 << 2;
-        public static final byte SHOULD_MERGE_MASK = 0b11 << 2;
+        public static final byte NO_MERGE = 0b00 << 3;
+        public static final byte SHOULD_MERGEWITHNEXT = 0b01 << 3;
+        public static final byte SHOULD_MERGEWITHPREV = 0b10 << 3;
+        public static final byte SHOULD_MERGE_MASK = 0b11 << 3;
     }
 
     private PositionWritable nodeID;
diff --git a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/io/MessageWritable.java b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/io/MessageWritable.java
index 979f5f2..9b3a886 100644
--- a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/io/MessageWritable.java
+++ b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/io/MessageWritable.java
@@ -21,6 +21,7 @@
     private KmerBytesWritable kmer;
     private AdjacencyListWritable neighberNode; //incoming or outgoing
     private byte flag;
+    private boolean isFlip;
 
     private byte checkMessage;
 
@@ -29,6 +30,7 @@
         kmer = new KmerBytesWritable(0);
         neighberNode = new AdjacencyListWritable();
         flag = Message.NON;
+        isFlip = false;
         checkMessage = (byte) 0;
     }
     
@@ -118,6 +120,14 @@
     public void setFlag(byte message) {
         this.flag = message;
     }
+    
+    public boolean isFlip() {
+        return isFlip;
+    }
+
+    public void setFlip(boolean isFlip) {
+        this.isFlip = isFlip;
+    }
 
     @Override
     public void write(DataOutput out) throws IOException {
@@ -128,6 +138,7 @@
             kmer.write(out);
         if ((checkMessage & CheckMessage.NEIGHBER) != 0)
             neighberNode.write(out);
+        out.writeBoolean(isFlip);
         out.write(flag);
     }
 
@@ -141,6 +152,7 @@
             kmer.readFields(in);
         if ((checkMessage & CheckMessage.NEIGHBER) != 0)
             neighberNode.readFields(in);
+        isFlip = in.readBoolean();
         flag = in.readByte();
     }
 
diff --git a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/BasicPathMergeVertex.java b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/BasicPathMergeVertex.java
index 05716fe..cd84272 100644
--- a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/BasicPathMergeVertex.java
+++ b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/BasicPathMergeVertex.java
@@ -44,6 +44,7 @@
             maxIteration = getContext().getConfiguration().getInt(ITERATIONS, 1000000);
         outFlag = (byte)0;
         outgoingMsg.reset();
+        headFlag = (byte)(getVertexValue().getState() & MessageFlag.IS_HEAD);
     }
     
     /**
@@ -53,6 +54,10 @@
         headFlag = (byte)(getVertexValue().getState() & MessageFlag.IS_HEAD);
     }
     
+    public byte getHeadFlag(){
+        return (byte)(getVertexValue().getState() & MessageFlag.IS_HEAD);
+    }
+    
     /**
      * get destination vertex
      */
@@ -188,22 +193,22 @@
      */
     public void startSendMsg() {
         if (VertexUtil.isHeadVertexWithIndegree(getVertexValue())) {
-            outgoingMsg.setFlag(MessageFlag.IS_HEAD);
+            outgoingMsg.setFlag((byte)(MessageFlag.IS_HEAD | MessageFlag.SHOULD_MERGEWITHNEXT));
             sendMsgToAllNextNodes(getVertexValue());
             voteToHalt();
         }
         if (VertexUtil.isRearVertexWithOutdegree(getVertexValue())) {
-            outgoingMsg.setFlag(MessageFlag.IS_HEAD);
+            outgoingMsg.setFlag((byte)(MessageFlag.IS_HEAD | MessageFlag.SHOULD_MERGEWITHPREV));
             sendMsgToAllPreviousNodes(getVertexValue());
             voteToHalt();
         }
         if (VertexUtil.isHeadWithoutIndegree(getVertexValue())){
-            outgoingMsg.setFlag(MessageFlag.IS_HEAD);
+            outgoingMsg.setFlag((byte)(MessageFlag.IS_HEAD | MessageFlag.SHOULD_MERGEWITHNEXT));
             sendMsg(getVertexId(), outgoingMsg); //send to itself
             voteToHalt();
         }
         if (VertexUtil.isRearWithoutOutdegree(getVertexValue())){
-            outgoingMsg.setFlag(MessageFlag.IS_HEAD);
+            outgoingMsg.setFlag((byte)(MessageFlag.IS_HEAD | MessageFlag.SHOULD_MERGEWITHPREV));
             sendMsg(getVertexId(), outgoingMsg); //send to itself
             voteToHalt();
         }
@@ -221,7 +226,10 @@
                 voteToHalt();
             } else {
                 incomingMsg = msgIterator.next();
-                getVertexValue().setState(MessageFlag.IS_HEAD);
+                if(getHeadFlag() > 0)
+                    voteToHalt();
+                else 
+                    getVertexValue().setState(incomingMsg.getFlag());
             }
         }
     }
@@ -281,7 +289,7 @@
             case MessageFlag.SHOULD_MERGEWITHPREV:
                 setSuccessorAdjMsg();
                 if(ifFlipWithPredecessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getIncomingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -291,7 +299,7 @@
             case MessageFlag.SHOULD_MERGEWITHNEXT:
                 setPredecessorAdjMsg();
                 if(ifFilpWithSuccessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getOutgoingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -324,7 +332,7 @@
             case MessageFlag.DIR_FR:
                 setSuccessorAdjMsg();
                 if(ifFlipWithPredecessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getIncomingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -335,7 +343,7 @@
             case MessageFlag.DIR_RR:
                 setPredecessorAdjMsg();
                 if(ifFilpWithSuccessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getOutgoingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -356,7 +364,7 @@
             case MessageFlag.SHOULD_MERGEWITHNEXT:
                 setSuccessorAdjMsg();
                 if(ifFlipWithPredecessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getIncomingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -367,7 +375,7 @@
             case MessageFlag.SHOULD_MERGEWITHPREV:
                 setPredecessorAdjMsg();
                 if(ifFilpWithSuccessor())
-                    outFlag |= MessageFlag.FLIP;
+                    outgoingMsg.setFlip(true);
                 outgoingMsg.setFlag(outFlag);
                 outgoingMsg.setNeighberNode(getVertexValue().getOutgoingList());
                 outgoingMsg.setSourceVertexId(getVertexId());
@@ -451,15 +459,11 @@
      * updateAdjList
      */
     public void processUpdate(){
-        byte meToNeighborDir = (byte) (incomingMsg.getFlag() & MessageFlag.DIR_MASK);
+        inFlag = incomingMsg.getFlag();
+        byte meToNeighborDir = (byte) (inFlag & MessageFlag.DIR_MASK);
         byte neighborToMeDir = mirrorDirection(meToNeighborDir);
         
-        boolean flip;
-        if((outFlag & MessageFlag.FLIP) > 0)
-            flip = true;
-        else
-            flip = false;
-        byte neighborToMergeDir = flipDirection(neighborToMeDir, flip);
+        byte neighborToMergeDir = flipDirection(neighborToMeDir, incomingMsg.isFlip());
         
         getVertexValue().processUpdates(neighborToMeDir, incomingMsg.getSourceVertexId(), 
                 neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(incomingMsg.getNeighberNode()));
@@ -479,12 +483,7 @@
             getVertexValue().setState(state);
         }
         
-        boolean flip;
-        if((inFlag & MessageFlag.FLIP) > 0)
-            flip = true;
-        else
-            flip = false;
-        byte neighborToMergeDir = flipDirection(neighborToMeDir, flip);
+        byte neighborToMergeDir = flipDirection(neighborToMeDir, incomingMsg.isFlip());
         
         getVertexValue().processMerges(neighborToMeDir, incomingMsg.getSourceVertexId(), 
                 neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(incomingMsg.getNeighberNode()),
@@ -497,13 +496,8 @@
     public void processMerge(MessageWritable msg){
         byte meToNeighborDir = (byte) (msg.getFlag() & MessageFlag.DIR_MASK);
         byte neighborToMeDir = mirrorDirection(meToNeighborDir);
-        
-        boolean flip;
-        if((outFlag & MessageFlag.FLIP) > 0)
-            flip = true;
-        else
-            flip = false;
-        byte neighborToMergeDir = flipDirection(neighborToMeDir, flip);
+
+        byte neighborToMergeDir = flipDirection(neighborToMeDir, incomingMsg.isFlip());
         
         getVertexValue().processMerges(neighborToMeDir, msg.getSourceVertexId(), 
                 neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(msg.getNeighberNode()),
diff --git a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P4ForPathMergeVertex.java b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P4ForPathMergeVertex.java
index f70a76e..e702bbd 100644
--- a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P4ForPathMergeVertex.java
+++ b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P4ForPathMergeVertex.java
@@ -101,6 +101,9 @@
      * set nextID to the element that's next (in the node's FF or FR list), returning true when there is a next neighbor
      */
     protected boolean setNextInfo(VertexValueWritable value) {
+        if(headFlag > 0 && (getVertexValue().getState() & MessageFlag.SHOULD_MERGEWITHPREV) > 0){
+            return false;
+        }
         if (value.getFFList().getCountOfPosition() > 0) {
             nextID.set(value.getFFList().getPosition(0));
             nextHead = isNodeRandomHead(nextID);
@@ -118,6 +121,9 @@
      * set prevID to the element that's previous (in the node's RR or RF list), returning true when there is a previous neighbor
      */
     protected boolean setPrevInfo(VertexValueWritable value) {
+        if(headFlag > 0 && (getVertexValue().getState() & MessageFlag.SHOULD_MERGEWITHNEXT) > 0){
+            return false;
+        }
         if (value.getRRList().getCountOfPosition() > 0) {
             prevID.set(value.getRRList().getPosition(0));
             prevHead = isNodeRandomHead(prevID);
@@ -163,7 +169,8 @@
                     } else if (hasPrev && !prevHead && (getPreDestVertexId(getVertexValue()) != null)) {
                         // compress this head to the reverse tail
                         sendUpdateMsgToSuccessor();
-                    }
+                    } else
+                        voteToHalt();
                 } else {
                     // I'm a tail
                     if (hasNext && hasPrev) {
@@ -171,20 +178,24 @@
                             // tails on both sides, and I'm the "local minimum"
                             // compress me towards the tail in forward dir
                             sendUpdateMsgToPredecessor();
-                        }
+                        } else
+                            voteToHalt();
                     } else if (!hasPrev) {
                         // no previous node
                         if (!nextHead && curID.compareTo(nextID) < 0) {
                             // merge towards tail in forward dir
                             sendUpdateMsgToPredecessor();
-                        }
+                        } else
+                            voteToHalt();
                     } else if (!hasNext) {
                         // no next node
                         if (!prevHead && curID.compareTo(prevID) < 0) {
                             // merge towards tail in reverse dir
                             sendUpdateMsgToSuccessor();
-                        }
-                    }
+                        } else
+                            voteToHalt();
+                    } else
+                        voteToHalt();
                 }
             }
         }
@@ -193,6 +204,7 @@
             while (msgIterator.hasNext()) {
                 incomingMsg = msgIterator.next();
                 processUpdate();
+                voteToHalt();
             }
         } else if (getSuperstep() % 4 == 1){
             //send message to the merge object and kill self
@@ -205,8 +217,7 @@
                 processMerge();
                 
                 //head meets head, stop
-                headFlag = (byte) (MessageFlag.VERTEX_MASK & incomingMsg.getFlag());
-                if(headFlag == MessageFlag.IS_HEAD && selfFlag == MessageFlag.IS_HEAD)
+                if(getMsgFlag() == MessageFlag.IS_HEAD && selfFlag == MessageFlag.IS_HEAD)
                     voteToHalt();
             }
         }
diff --git a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P5ForPathMergeVertex.java b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P5ForPathMergeVertex.java
index d68c98b..a67d645 100644
--- a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P5ForPathMergeVertex.java
+++ b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/operator/pathmerge/P5ForPathMergeVertex.java
@@ -295,7 +295,7 @@
      * @throws IOException 
      */
     public void broadcastUpdateMsg(){
-        switch(getVertexValue().getState() & 0b0001){
+       /* switch(getVertexValue().getState() & 0b0001){
             case MessageFlag.SHOULD_MERGEWITHPREV:
                 setSuccessorAdjMsg();
                 if(ifFlipWithPredecessor())
@@ -314,7 +314,7 @@
                 outgoingMsg.setSourceVertexId(getVertexId());
                 sendMsg(getPreDestVertexId(getVertexValue()), outgoingMsg);
                 break; 
-        }
+        }*/
     }
     
     /**
@@ -390,7 +390,7 @@
      * updateAdjList
      */
     public void processUpdate(){
-        byte meToNeighborDir = (byte) (incomingMsg.getFlag() & MessageFlag.DIR_MASK);
+        /*byte meToNeighborDir = (byte) (incomingMsg.getFlag() & MessageFlag.DIR_MASK);
         byte neighborToMeDir = mirrorDirection(meToNeighborDir);
         
         boolean flip;
@@ -401,14 +401,14 @@
         byte neighborToMergeDir = flipDirection(neighborToMeDir, flip);
         
         getVertexValue().processUpdates(neighborToMeDir, incomingMsg.getSourceVertexId(), 
-                neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(incomingMsg.getNeighberNode()));
+                neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(incomingMsg.getNeighberNode()));*/
     }
     
     /**
      * merge and updateAdjList
      */
     public void processMerge(){
-        byte meToNeighborDir = (byte) (incomingMsg.getFlag() & MessageFlag.DIR_MASK);
+        /*byte meToNeighborDir = (byte) (incomingMsg.getFlag() & MessageFlag.DIR_MASK);
         byte neighborToMeDir = mirrorDirection(meToNeighborDir);
         
         boolean flip;
@@ -420,7 +420,7 @@
         
         getVertexValue().processMerges(neighborToMeDir, incomingMsg.getSourceVertexId(), 
                 neighborToMergeDir, VertexUtil.getNodeIdFromAdjacencyList(incomingMsg.getNeighberNode()),
-                kmerSize, incomingMsg.getKmer());
+                kmerSize, incomingMsg.getKmer());*/
     }
     
     @Override
diff --git a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/type/MessageFlag.java b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/type/MessageFlag.java
index 855deaa..09ea917 100644
--- a/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/type/MessageFlag.java
+++ b/genomix/genomix-pregelix/src/main/java/edu/uci/ics/genomix/pregelix/type/MessageFlag.java
@@ -4,15 +4,11 @@
 
 public class MessageFlag extends MergeDirFlag {
     
-    public static final byte FLIP = 1 << 6;
+    //public static final byte FLIP = 1 << 6;
 
     
     public static String getFlagAsString(byte code) {
         // TODO: allow multiple flags to be set
-        switch (code) {
-            case FLIP:
-                return "FLIP";
-        }
         return "ERROR_BAD_MESSAGE";
     }
 }