/*
** Return the index of a rail currently not in use for any row between
** top and bottom, inclusive.
*/
static int findFreeRail(
  GraphContext *p,         /* The graph context */
  int top, int btm,        /* Span of rows for which the rail is needed */
  u64 inUseMask,           /* Mask or rails already in use */
  int iNearto              /* Find rail nearest to this rail */
){
  GraphRow *pRow;
  int i;
  int iBest = 0;
  int iBestDist = 9999;

  for(pRow=p->pFirst; pRow && pRow->idx<top; pRow=pRow->pNext){}
  while( pRow && pRow->idx<=btm ){
    inUseMask |= pRow->railInUse;
    pRow = pRow->pNext;
  }
  for(i=0; i<32; i++){
    if( (inUseMask & BIT(i))==0 ){

      ** on the same rail. */
      pParent->mergeOut = pParent->iRail;
      pParent->mergeUpto = pChild->idx;
    }else{
      /* The thin merge arrow riser is taller than the thick primary
      ** child riser, so use separate rails. */
      int iTarget = pParent->iRail;
      pParent->mergeOut = findFreeRail(p, pChild->idx, pParent->idx-1,
                                       0, iTarget);
      pParent->mergeUpto = pChild->idx;
      mask = BIT(pParent->mergeOut);
      for(pLoop=pChild->pNext; pLoop && pLoop->rid!=pParent->rid;
           pLoop=pLoop->pNext){
        pLoop->railInUse |= mask;
      }
    }

/*
** Compute the complete graph
*/
void graph_finish(GraphContext *p, int omitDescenders){
  GraphRow *pRow, *pDesc, *pDup, *pLoop, *pParent;
  int i;
  u64 mask;
  u64 inUse;
  int hasDup = 0;      /* True if one or more isDup entries */
  const char *zTrunk;

  if( p==0 || p->pFirst==0 || p->nErr ) return;
  p->nErr = 1;   /* Assume an error until proven otherwise */

  /* Initialize all rows */

      if( i==0 ){
        if( pRow->zBranch!=zTrunk ) continue;
      }else {
        if( pRow->iRail>=0 ) continue;
      }
      if( pRow->nParent==0 || hashFind(p,pRow->aParent[0])==0 ){
        if( omitDescenders ){
          pRow->iRail = findFreeRail(p, pRow->idxTop, pRow->idx, 0, 0);
        }else{
          pRow->iRail = ++p->mxRail;
        }
        if( p->mxRail>=GR_MAX_RAIL ) return;
        mask = BIT(pRow->iRail);
        if( !omitDescenders ){
          pRow->bDescender = pRow->nParent>0;
          for(pLoop=pRow; pLoop; pLoop=pLoop->pNext){
            pLoop->railInUse |= mask;
          }
        }
        assignChildrenToRail(pRow);
      }
    }
  }

  /* Assign rails to all rows that are still unassigned.
  */
  inUse = BIT(p->mxRail+1) - 1;
  for(pRow=p->pLast; pRow; pRow=pRow->pPrev){
    int parentRid;

    if( pRow->iRail>=0 ){
      if( pRow->pChild==0 && !pRow->timeWarp ){
        if( omitDescenders || count_nonbranch_children(pRow->rid)==0 ){
          inUse &= ~BIT(pRow->iRail);
        }else{
          riser_to_top(pRow);
          /*pRow->aiRiser[pRow->iRail] = 0;
          mask = BIT(pRow->iRail);
          for(pLoop=pRow; pLoop; pLoop=pLoop->pPrev){
            pLoop->railInUse |= mask;
          }*/
        }
      }
      continue;
    }
    if( pRow->isDup ){
      continue;
    }else{
      assert( pRow->nParent>0 );
      parentRid = pRow->aParent[0];
      pParent = hashFind(p, parentRid);
      if( pParent==0 ){
        pRow->iRail = ++p->mxRail;
        if( p->mxRail>=GR_MAX_RAIL ) return;
        pRow->railInUse = BIT(pRow->iRail);
        continue;
      }
      if( pParent->idx>pRow->idx ){
        /* Common case:  Child occurs after parent and is above the
        ** parent in the timeline */
        pRow->iRail = findFreeRail(p, 0, pParent->idx, inUse, pParent->iRail);
        if( p->mxRail>=GR_MAX_RAIL ) return;
        pParent->aiRiser[pRow->iRail] = pRow->idx;
      }else{
        /* Timewarp case:  Child occurs earlier in time than parent and
        ** appears below the parent in the timeline. */
        int iDownRail = ++p->mxRail;
        if( iDownRail<1 ) iDownRail = ++p->mxRail;
        pRow->iRail = ++p->mxRail;
        if( p->mxRail>=GR_MAX_RAIL ) return;
        pRow->railInUse = BIT(pRow->iRail);
        pParent->aiRiser[iDownRail] = pRow->idx;
        mask = BIT(iDownRail);
        inUse |= mask;
        for(pLoop=p->pFirst; pLoop; pLoop=pLoop->pNext){
          pLoop->railInUse |= mask;
        }
      }
    }
    mask = BIT(pRow->iRail);
    pRow->railInUse |= mask;
    if( pRow->pChild==0 ){
      inUse &= ~mask;
    }else{
      inUse |= mask;
      assignChildrenToRail(pRow);


    }
    if( pParent ){
      for(pLoop=pParent->pPrev; pLoop && pLoop!=pRow; pLoop=pLoop->pPrev){
        pLoop->railInUse |= mask;
      }
    }
  }

  /*
  ** Insert merge rails and merge arrows
  */
  for(pRow=p->pFirst; pRow; pRow=pRow->pNext){
    for(i=1; i<pRow->nParent; i++){
      int parentRid = pRow->aParent[i];
      pDesc = hashFind(p, parentRid);
      if( pDesc==0 ){
        /* Merge from a node that is off-screen */
        int iMrail = findFreeRail(p, pRow->idx, p->nRow, 0, 0);
        if( p->mxRail>=GR_MAX_RAIL ) return;
        mask = BIT(iMrail);
        pRow->mergeIn[iMrail] = 1;
        pRow->mergeDown |= mask;
        for(pLoop=pRow->pNext; pLoop; pLoop=pLoop->pNext){
          pLoop->railInUse |= mask;
        }