src/ord/ordMain.c

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#include "ordInt.h"

static char rcsid[] UNUSED = "$Id: ordMain.c,v 1.17 2005/04/16 06:15:25 fabio Exp $";

/*---------------------------------------------------------------------------*/
/* Constant declarations                                                     */
/*---------------------------------------------------------------------------*/

/*---------------------------------------------------------------------------*/
/* Structure declarations                                                     */
/*---------------------------------------------------------------------------*/

/*---------------------------------------------------------------------------*/
/* Static function prototypes                                                */
/*---------------------------------------------------------------------------*/

static int NodesCompareDepth(Ntk_Node_t *node1, Ntk_Node_t *node2);
static long NodeComputeDepth(Ntk_Node_t * node);
static void NetworkAddDanglingNodesToOrderList(Ntk_Network_t * network, lsList nodeOrderList, st_table *nodeToHandle);
static void NetworkAddNSVarsToOrderList(Ntk_Network_t * network, lsList nodeOrderList, st_table *nodeToHandle, boolean nsAfterSupport);
static lsList LatchNSListConvertToLatchDataInputList(lsList latchNSList);
static void NodeSetDepth(Ntk_Node_t * node, long depth);
static void MddGroupVariables(mdd_manager *mddMgr, int initMddId, int blockSize);

/*---------------------------------------------------------------------------*/
/* Definition of exported functions                                          */
/*---------------------------------------------------------------------------*/

void
Ord_NetworkOrderVariables(
  Ntk_Network_t *network,
  Ord_RootMethod rootOrderMethod,
  Ord_NodeMethod nodeOrderMethod,
  boolean        nsAfterSupport,
  Ord_OrderType  generatedOrderType /* Ord_All_c or Ord_InputAndLatch_c */,
  Ord_OrderType  suppliedOrderType /* no restrictions */,
  lsList         suppliedNodeList /* list of nodes Ntk_Node_t* from ordering file */,
  int            verbose)
{
  lsList totalOrderList;      /* list of nodes (Ntk_Node_t *) */
  long   initialTime = util_cpu_time();
  
  /*
   * The condition where suppliedOrderType==InputAndLatch and
   * generatedOrderType==All is not currently allowed because we're not sure
   * if it's useful.
   */
  assert(!((suppliedOrderType == Ord_InputAndLatch_c)
           && (generatedOrderType == Ord_All_c))); 

  /*
   * Either get a total ordering of the network nodes from suppliedNodeList,
   * or compute it working from the network.
   */
  if ((suppliedOrderType == Ord_All_c)
      || (suppliedOrderType == Ord_InputAndLatch_c)) {
    totalOrderList = lsCopy(suppliedNodeList,
                            (lsGeneric (*) (lsGeneric)) NULL);
  }
  else {
    totalOrderList = Ord_NetworkOrderNodes(network, rootOrderMethod,
                                           nodeOrderMethod, nsAfterSupport,
                                           generatedOrderType,
                                           suppliedOrderType,
                                           suppliedNodeList, 
                                           verbose);
  }
    

  OrdNetworkAssignMddIds(network, generatedOrderType, totalOrderList, nsAfterSupport);
  (void) lsDestroy(totalOrderList, (void (*) (lsGeneric)) NULL);

  /*
   * If nsAfterSupport is FALSE, this implies that NS comes right after PS.
   *
   */
  if ( nsAfterSupport == FALSE ) {
    lsGen tmpGen;
    Ntk_Node_t *tmpLatch;
 
   /*
    * Iterate over each latch, and group the PS variable with the next
    * variable in the ordering, which is guaranteed to be the corresponding NS
    * variable.
    */
    Ntk_NetworkForEachLatch(network, tmpGen, tmpLatch) {
      mdd_manager *mddMgr = Ntk_NetworkReadMddManager( network );
          Ntk_Node_t *nsNode = Ntk_NodeReadShadow( tmpLatch );
      int psMddId = Ntk_NodeReadMddId( tmpLatch );
      int nsMddId = Ntk_NodeReadMddId( nsNode );

      /* 
       * Group size = 2 ( ps and ns )
       */
      if (nsMddId == (psMddId+1)) {
        MddGroupVariables(mddMgr, psMddId, 2);
      }
      /* Adnan's fix: See his mail to vis@ic on Oct 24, 1997 */
      if (nsMddId == (psMddId-1)) {
        MddGroupVariables(mddMgr, nsMddId, 2);
      }
    }
  }

  if (verbose > 0) {
    long finalTime = util_cpu_time();
    (void) fprintf(vis_stdout, "\nTotal ordering time = %2.1f\n",
                   (finalTime-initialTime)/1000.0);
  }
}


lsList
Ord_NetworkOrderNodes(
  Ntk_Network_t *network,
  Ord_RootMethod rootOrderMethod,
  Ord_NodeMethod nodeOrderMethod,
  boolean        nsAfterSupport,
  Ord_OrderType  generatedOrderType /* Ord_All_c or Ord_InputAndLatch_c */,
  Ord_OrderType  suppliedOrderType  /* Ord_NextStateNode_c, Ord_Partial_c, or
                                        Ord_Unassigned_c */, 
  lsList         suppliedNodeList   /* list of nodes Ntk_Node_t* from ordering file */,
  int            verbose)
{
  lsList      partialRootOrder;   /* list of nodes (Ntk_Node_t *) */
  lsList      roots;              /* list of nodes (Ntk_Node_t *) */
  lsList      nodeOrderList;      /* list of nodes (Ntk_Node_t *) */
  st_table   *nodeToHandle;       /* Ntk_Node_t* to lsHandle */

  /*
   * Check the input arguments.
   */
  assert(   (generatedOrderType == Ord_All_c)
         || (generatedOrderType == Ord_InputAndLatch_c));

  assert(   (suppliedOrderType == Ord_NextStateNode_c)
         || (suppliedOrderType == Ord_Partial_c)
         || (suppliedOrderType == Ord_Unassigned_c));
  

  /*
   * Compute an ordering on the roots. The nodes of the network are explored
   * in DFS order from the roots, in the root order computed.  If
   * suppliedOrderType is Ord_NextStateNode_c, then suppliedNodeList contains
   * the next state nodes; this list serves as a seed to compute the root
   * ordering.
   */
  partialRootOrder = (suppliedOrderType == Ord_NextStateNode_c)
      ? LatchNSListConvertToLatchDataInputList(suppliedNodeList)
      : (lsList) NULL;
  roots = Ord_NetworkOrderRoots(network, rootOrderMethod,
                                partialRootOrder, verbose);
  if (suppliedOrderType == Ord_NextStateNode_c) {
    (void) lsDestroy(partialRootOrder, (void (*) (lsGeneric)) NULL);
  }
  if (verbose > 0) {
    (void) fprintf(vis_stdout, "\nOrder in which roots are visited:\n");
    OrdNodeListWrite(vis_stdout, roots);
  }
  
  /*
   * Compute an order on all nodes in the transitive fanin of roots, including
   * the roots themselves. Pass in an empty nodeToHandle table.
   */
  nodeToHandle  = st_init_table(st_ptrcmp, st_ptrhash);
  nodeOrderList = OrdNetworkOrderTFIOfRoots(network, roots, nodeOrderMethod,
                                            nodeToHandle, verbose);
  (void) lsDestroy(roots, (void (*) (lsGeneric)) NULL);

  if (verbose > 0) {
    (void) fprintf(vis_stdout, "\nOrder of network nodes without NS:\n");
    OrdNodeListWrite(vis_stdout, nodeOrderList);
  }
  
  /*
   * Add in the next state variables (represented by the shadow nodes of
   * latches).
   */
  NetworkAddNSVarsToOrderList(network, nodeOrderList, nodeToHandle, nsAfterSupport);
  if (verbose > 2) {
    (void) fprintf(vis_stdout, "\nOrder of network nodes with NS:\n");
    OrdNodeListWrite(vis_stdout, nodeOrderList);
  }
  
  
  /*
   * There may be some nodes that are not in the TFI of any root. Put such
   * nodes at the end of nodeOrderList (in no particular order).
   */
  NetworkAddDanglingNodesToOrderList(network, nodeOrderList, nodeToHandle);
  st_free_table(nodeToHandle);
  
  /*
   * If the suppliedOrderType is Partial, then merge (left, arbitrarily) the
   * computed node order into the supplied node order and return the result.
   * Otherwise, just return the nodeOrderList computed.
   */
  if (suppliedOrderType == Ord_Partial_c) {
    lsList suppliedNodeListCopy = lsCopy(suppliedNodeList,
                                         (lsGeneric (*) (lsGeneric)) NULL);

    Ord_ListMergeList(suppliedNodeListCopy, nodeOrderList, Ord_ListMergeLeft_c);
    (void) lsDestroy(nodeOrderList, (void (*) (lsGeneric)) NULL);
    return suppliedNodeListCopy;
  }
  else {
    return nodeOrderList;
  }
}


void
Ord_NetworkAssignMddIdForNode(
  Ntk_Network_t *network,
  Ntk_Node_t    *node)
{
  if (Ntk_NodeReadMddId(node) != NTK_UNASSIGNED_MDD_ID) {
    return;
  }
  else {
    int          mddId;
    mdd_manager *mddManager = Ntk_NetworkReadMddManager(network);
    array_t     *mvarValues = array_alloc(int, 0);
    array_t     *mvarNames  = array_alloc(char *, 0);
  
    /*
     * If the network doesn't already have an MDD manager, then create one.  In
     * any case, set mddId to be the number of variables currently existing in
     * the manager.
     */
    if (mddManager == NIL(mdd_manager)) {
      mddManager = Ntk_NetworkInitializeMddManager(network);
      mddId = 0;
    }
    else {
      array_t *mvarArray  = mdd_ret_mvar_list(mddManager);
      mddId = array_n(mvarArray);
    }
  
    Ntk_NodeSetMddId(node, mddId);

    array_insert_last(int, mvarValues,
                      Var_VariableReadNumValues(Ntk_NodeReadVariable(node)));
    array_insert_last(char *, mvarNames, Ntk_NodeReadName(node));

    /*
     * Create the variable in the MDD manager.  The last argument being NIL
     * means that the variable will not be interleaved.
     */
    mdd_create_variables(mddManager, mvarValues, mvarNames, NIL(array_t));

    array_free(mvarValues);
    array_free(mvarNames);
  }
}


void
Ord_ListMergeLeftListUsingTable(
  lsList    list1,
  lsList    list2,
  st_table *dataToHandle1)
{
  lsGen      gen1;   /* generator for list1 */
  lsGen      gen2;   /* generator for list2 */
  lsHandle   handle; /* handle of an item in list1 */
  lsGeneric  data;   
  lsGeneric  prevData;

  /*
   * Walk through list2 forwards, keeping track of the previous item just visited.
   */
  gen2 = lsStart(list2);
  prevData = NULL;
  while (lsNext(gen2, &data, LS_NH) == LS_OK) {

    if (!st_is_member(dataToHandle1, (char *) data)) {
      /*
       * Data is not present in list1, so we must insert it at the proper
       * location.
       */
      if (prevData == NULL) {
        /*
         * Data is the head of list2, so insert it at the head of list1.
         */
        (void) lsNewBegin(list1, data, &handle);
      }
      else {
        lsGeneric dummyData;
        lsHandle  prevHandle;

        /*
         * Data is not the head of list1.  Hence, insert it just to the right
         * of the location of prevData in list1 (by induction, prevData must
         * currently exist in list1). Do this by getting the handle of
         * prevData in list1, creating a generator initialized to just after
         * prevData, and then inserting data at this point. Note that
         * lsInAfter and lsInBefore are equivalent in this case, since we
         * immediately kill gen1. 
         */
        st_lookup(dataToHandle1, prevData, &prevHandle);
        gen1 = lsGenHandle(prevHandle, &dummyData, LS_AFTER);
        (void) lsInAfter(gen1, data, &handle);
        (void) lsFinish(gen1);
      }
      st_insert(dataToHandle1, data, handle);

    } /* else, data already present in list1, so do nothing */

    prevData = data;

  } /* end while */
  (void) lsFinish(gen2);
}


void
Ord_ListMergeRightListUsingTable(
  lsList    list1,
  lsList    list2,
  st_table *dataToHandle1)
{
  lsGen      gen1;   /* generator for list1 */
  lsGen      gen2;   /* generator for list2 */
  lsHandle   handle; /* handle of an item in list1 */
  lsGeneric  data;   
  lsGeneric  succData;

  /*
   * Walk through list2 backwards, keeping track of the successor item just visited.
   */
  gen2 = lsEnd(list2);
  succData = NULL;
  while (lsPrev(gen2, &data, LS_NH) == LS_OK) {

    if (!st_is_member(dataToHandle1, (char *) data)) {
      /*
       * Data is not present in list1, so we must insert it at the proper
       * location.
       */
      if (succData == NULL) {
        /*
         * Data is the tail of list2, so insert it at the tail of list1.
         */
        (void) lsNewEnd(list1, data, &handle);
      }
      else {
        lsGeneric dummyData;
        lsHandle  succHandle;

        /*
         * Data is not the tail of list1.  Hence, insert it just to the left
         * of the location of succData in list1 (by induction, succData must
         * currently exist in list1). Do this by getting the handle of
         * succData in list1, creating a generator initialized to just before
         * succData, and then inserting data at this point. Note that
         * lsInAfter and lsInBefore are equivalent in this case, since we are
         * immediately kill gen1. 
         */
        st_lookup(dataToHandle1, succData, &succHandle);
        gen1 = lsGenHandle(succHandle, &dummyData, LS_BEFORE);
        (void) lsInAfter(gen1, data, &handle);
        (void) lsFinish(gen1);
      }
      st_insert(dataToHandle1, data, handle);

    } /* else, data already present in list1, so do nothing */

    succData = data;

  } /* end while */
  (void) lsFinish(gen2);
}


void
Ord_ListMergeListUsingTable(
  lsList               list1,
  lsList               list2,
  st_table            *dataToHandle1,
  Ord_ListMergeMethod  method)
{
  if (method == Ord_ListMergeLeft_c) {
    Ord_ListMergeLeftListUsingTable(list1, list2, dataToHandle1);
  }
  else if (method == Ord_ListMergeRight_c) {
    Ord_ListMergeRightListUsingTable(list1, list2, dataToHandle1);
  }
  else {
    fail("unrecognized method");
  }
}
    

void
Ord_ListMergeList(
  lsList              list1,
  lsList              list2,
  Ord_ListMergeMethod method)
{
  lsGen      gen1;   /* generator for list1 */
  lsHandle   handle; /* handle of an item in list1 */
  lsGeneric  data;   
  st_table  *dataToHandle1 = st_init_table(st_ptrcmp, st_ptrhash);

  /*
   * Load a hash table mapping each item in list1 to its handle in list1.
   */
  gen1 = lsStart(list1);
  while (lsNext(gen1, &data, &handle) == LS_OK) {
    st_insert(dataToHandle1, (char *) data, (char *) handle);
  }
  (void) lsFinish(gen1);

  Ord_ListMergeListUsingTable(list1, list2, dataToHandle1, method);

  st_free_table(dataToHandle1);
}

  
void
Ord_ListAppendList(
  lsList list1,
  lsList list2)
{
  lsGen gen;
  lsHandle handle; /* unused */
  lsGeneric data;   

  /*
   * Walk through list2 forwards, inserting each element to the end of list1.
   */
  gen = lsStart(list2);
  while (lsNext(gen, &data, LS_NH) == LS_OK) {
    (void) lsNewEnd(list1, data, &handle);
  } 
  (void) lsFinish(gen);
}

  
/*---------------------------------------------------------------------------*/
/* Definition of internal functions                                          */
/*---------------------------------------------------------------------------*/

int
OrdNodesFromListCompareDepth(
  lsGeneric node1,
  lsGeneric node2)
{
  return NodesCompareDepth((Ntk_Node_t *) node1, (Ntk_Node_t *) node2);
}


int
OrdNodesFromArrayCompareDepth(
  const void * node1,
  const void * node2)
{
  return NodesCompareDepth(*(Ntk_Node_t **) node1, *(Ntk_Node_t **) node2);
}


void
OrdNetworkComputeNodeDepths(
  Ntk_Network_t * network,
  lsList  roots /* list of Ntk_Node_t  */)
{
  lsGen       gen;
  Ntk_Node_t *node;
  Ntk_Node_t *root;

  /*
   * Initialize the depth of each node to unassigned.
   */
  Ntk_NetworkForEachNode(network, gen, node) {
    NodeSetDepth(node, ORDUNASSIGNED_DEPTH);
  }

  /*
   * Start the recursive computation from each root.
   */
  lsForEachItem(roots, gen, root) {
    (void) NodeComputeDepth(root);
  }
}


void
OrdNodeListWrite(
  FILE *fp,
  lsList nodeList)
{
  lsGen gen;
  Ntk_Node_t *node;

  lsForEachItem(nodeList, gen, node) {
    (void) fprintf(fp, "%s\n", Ntk_NodeReadName(node));
  }
}


long
OrdNodeReadDepth(
  Ntk_Node_t * node)
{
  return ((long) Ntk_NodeReadUndef(node));
}


void
OrdNetworkAssignMddIds(
  Ntk_Network_t * network,
  Ord_OrderType  orderType,
  lsList  orderList,
  boolean nsAfterSupport)
{
  lsGen        gen;
  Ntk_Node_t  *node;
  int          mddId;
  mdd_manager *mddManager = Ntk_NetworkReadMddManager(network);
  array_t     *mvarValues = array_alloc(int, lsLength(orderList));
  array_t     *mvarNames  = array_alloc(char *, lsLength(orderList));

  assert((orderType == Ord_All_c) || (orderType == Ord_InputAndLatch_c));

  /*
   * If the network doesn't already have an MDD manager, then create one.  In
   * any case, set mddId to be the number of variables currently existing in
   * the manager.
   */
  if (mddManager == NIL(mdd_manager)) {
    mddManager = Ntk_NetworkInitializeMddManager(network);
    mddId = 0;
  }
  else {
    array_t *mvarArray  = mdd_ret_mvar_list(mddManager);
    mddId = array_n(mvarArray);
  }
  
  lsForEachItem(orderList, gen, node) {
    /*
     * A node gets an MDD id if node is a combinational input, next state
     * node, or orderType is Ord_All_c.
     */
    if (Ntk_NodeTestIsCombInput(node) || Ntk_NodeTestIsNextStateNode(node)
        || (orderType == Ord_All_c) ) {

      Ntk_NodeSetMddId(node, mddId);
      mddId++;

      array_insert_last(int, mvarValues,
                        Var_VariableReadNumValues(Ntk_NodeReadVariable(node)));
      array_insert_last(char *, mvarNames, Ntk_NodeReadName(node));
    }
  }

  /*
   * Create the variables in the MDD manager.  The last argument being NIL
   * means that the variables will not be interleaved.
   */
  mdd_create_variables(mddManager, mvarValues, mvarNames, NIL(array_t));

  array_free(mvarValues);
  array_free(mvarNames);
}
    

/*---------------------------------------------------------------------------*/
/* Definition of static functions                                            */
/*---------------------------------------------------------------------------*/

static int
NodesCompareDepth(
  Ntk_Node_t *node1,
  Ntk_Node_t *node2)
{
  long depth1 = OrdNodeReadDepth(node1);
  long depth2 = OrdNodeReadDepth(node2);

  if (depth1 > depth2) {
    return -1;
  }
  else if (depth1 < depth2) {
    return 1;
  }
  else {
    char *name1 = Ntk_NodeReadName(node1);
    char *name2 = Ntk_NodeReadName(node2);

    /*
     * As a tiebreaker, sort based on name, so that the sorted result is not
     * sensitive to the order in which the nodes are passed to the compare
     * function. When sorting based on name, the order doesn't really matter,
     * but right now it's done so that "foo<i>" will come before "foo<j>" in
     * the ordering, where i < j.  This is just a little heuristic to put lower
     * order bits first. */
    if (strcmp(name1, name2) < 0) {
      return -1;
    }
    else if (strcmp(name1, name2) > 0) {
      return 1;
    }
    else {
      return 0;
    }
  }
}


static long
NodeComputeDepth(
  Ntk_Node_t * node)
{
  long depth = OrdNodeReadDepth(node);

  /*
   * If the node's depth has already been computed (i.e. it's not unassigned),
   * then just return it below.  If it's unassigned, then recursively compute
   * it.
   */
  if (depth == ORDUNASSIGNED_DEPTH) {

    if (Ntk_NodeTestIsCombInput(node) || Ntk_NodeTestIsConstant(node)) {
      /*
       * Latches, and nodes with no fanins, get depth 0. This is the terminal
       * case of recursion. 
       */
      depth = 0;
    }
    else {
      int         i;
      Ntk_Node_t *fanin;
      /*
       * Compute the depth of each fanin node in the support of node, and
       * maintain the maximum.  We start depth at 0 for max calculation.
       */
      depth = 0;
      Ntk_NodeForEachFanin(node, i, fanin) {  
        long faninDepth = NodeComputeDepth(fanin);

        depth = (depth > faninDepth) ? depth : faninDepth;
      }
      
      /*
       * The depth of node is one more than the max depths of its fanins.
       */
      depth++;
    }

    /*
     * Store the depth.
     */
    NodeSetDepth(node, depth);
  }
  
  return depth;
}

  
static void
NetworkAddDanglingNodesToOrderList(
  Ntk_Network_t * network,
  lsList  nodeOrderList /* of Ntk_Node_t*  */,
  st_table *nodeToHandle /* Ntk_Node_t* to lsHandle */)
{
  lsGen       gen;
  Ntk_Node_t *node;

  /*
   * For each network node, if it's not in nodeOrderList, then add it to the
   * end of the list. 
   * (NOTE: may be sensitive to ordering in memory.)
   */
  Ntk_NetworkForEachNode(network, gen, node) {
    if (!st_is_member(nodeToHandle, (char *) node)) {
      lsHandle handle;
      
      (void) lsNewEnd(nodeOrderList, (lsGeneric) node, &handle);
      st_insert(nodeToHandle, (char *) node, (char *) handle);
    }
  }
}


static void
NetworkAddNSVarsToOrderList(
  Ntk_Network_t * network,
  lsList  nodeOrderList /* of Ntk_Node_t  */,
  st_table *nodeToHandle /* Ntk_Node_t* to lsHandle */,
  boolean nsAfterSupport)
{
  lsGen       gen1;
  Ntk_Node_t *latch;

  /*
   * For each latch, add the latch's corresponding NS node to the ordering.
   * (NOTE: may be sensitive to ordering in memory.)
   */
  Ntk_NetworkForEachLatch(network, gen1, latch) {
    lsHandle    handle;
    lsGen       gen2;
    lsGeneric   dummyData;
    Ntk_Node_t *latchNS   = Ntk_NodeReadShadow(latch);

    if (nsAfterSupport) {
      /* Add just after the latch's dataInput. */
      lsHandle    dataInputHandle;
      Ntk_Node_t *dataInput = Ntk_LatchReadDataInput(latch);
      int         status UNUSED = st_lookup(nodeToHandle, dataInput,
                                            &dataInputHandle);

      assert(status);
      gen2 = lsGenHandle(dataInputHandle, &dummyData, LS_AFTER);
    }
    else {
      /* Add just after the latch. */
      lsHandle latchHandle;
      int      status = st_lookup(nodeToHandle, latch, &latchHandle);

      if (!status) {
        /*
         * Latch itself is not yet in the ordering.  This can happen if the
         * latch is not in the TFI of any other latch.  So add the latch first
         * (at the end of the ordering). 
         */
        (void) lsNewEnd(nodeOrderList, (lsGeneric) latch, &latchHandle);
        st_insert(nodeToHandle, (char *) latch, (char *) latchHandle);
      }
    
      gen2 = lsGenHandle(latchHandle, &dummyData, LS_AFTER);
    }

    /* Actually add to list here. */
    (void) lsInAfter(gen2, (lsGeneric) latchNS, &handle);  
    st_insert(nodeToHandle, (char *) latchNS, (char *) handle);
    (void) lsFinish(gen2);
      
  }
}


static lsList
LatchNSListConvertToLatchDataInputList(
  lsList latchNSList)
{
  lsGen       gen;
  Ntk_Node_t *node;
  Ntk_Node_t *latch;
  lsList      latchDataInputList = lsCreate();

  lsForEachItem(latchNSList, gen, node) {
    assert(Ntk_NodeTestIsNextStateNode(node));
    latch = Ntk_ShadowReadOrigin(node);
    (void) lsNewEnd(latchDataInputList, (lsGeneric)
                    Ntk_LatchReadDataInput(latch), LS_NH); 
  }

  return latchDataInputList;
}


static void
NodeSetDepth(
  Ntk_Node_t * node,
  long depth)
{
  Ntk_NodeSetUndef(node, (void *) depth);
}

static void
MddGroupVariables(
  mdd_manager *mddMgr,
  int initMddId,
  int blockSize )
{
  int i, j;
  int length;
  int aIndex;
  int startingBddIndex;
  int sanityCheck;
  mvar_type initMv, anMv;
  bvar_type initBvar, aBvar;
  array_t *mvar_list, *bvar_list;
  bdd_t *bdd;
  
  mvar_list = mdd_ret_mvar_list(mddMgr);
  bvar_list = mdd_ret_bvar_list(mddMgr);

  /*
   * mvar for initMddId 
   */
  initMv = array_fetch(mvar_type, mvar_list, initMddId);

  /*
   * bvar for first bdd for initMddId 
   */
  initBvar = mdd_ret_bvar(&initMv, 0, bvar_list);

  /*
   * number of bdd variables to group 
   */
  length = 0;

  /*
   * startingBddIndex is the level of the first bdd variable 
   */
  startingBddIndex = bdd_top_var_level( mddMgr, initBvar.node );
  sanityCheck = startingBddIndex;

  /*
   * in this loop we are simply ensuring that the bdd variables
   * corresponding to the mdd vars are infact contiguous. If this
   * is not the case we fail. length is the total number of bdd variables
   * to be grouped.
   */
  for( i = 0; i < blockSize; i++) {
    anMv = array_fetch(mvar_type, mvar_list, ( initMddId + i ) );
    for ( j = 0 ; j < anMv.encode_length; j++ ) {

      aBvar = mdd_ret_bvar( & anMv, j, bvar_list );
      aIndex = bdd_top_var_level( mddMgr,  aBvar.node );

      if ( sanityCheck != aIndex ) {
        /* bdd vars are not contiguous!! */
        fprintf(vis_stderr, "Badly formed block - bdd vars for %s (mvar_id = %d) are not contiguous!!\n",
                             anMv.name, anMv.mvar_id );
        fail("Wont go on\n");
      }
      else {
        /* number of variables to group increased by one */
        sanityCheck++;
        length++;
      }
    }
  }

  bdd = bdd_var_with_index(mddMgr, startingBddIndex);
  (void) bdd_new_var_block(bdd, length);
  bdd_free(bdd);
}