diff --git a/Code/DataStructs/SparseIntVect.h b/Code/DataStructs/SparseIntVect.h
index 0e6e20578..c519d503d 100644
--- a/Code/DataStructs/SparseIntVect.h
+++ b/Code/DataStructs/SparseIntVect.h
@@ -337,6 +337,9 @@ namespace RDKit{
   double DiceSimilarity(const SparseIntVect<IndexType> &v1,
                         const SparseIntVect<IndexType> &v2,
                         double bounds=0.0){
+    if(v1.getLength()!=v2.getLength()){
+      throw ValueErrorException("SparseIntVect size mismatch");
+    }
     double v1Sum=v1.getTotalVal();
     double v2Sum=v2.getTotalVal();
     double denom=v1Sum+v2Sum;
@@ -349,7 +352,31 @@ namespace RDKit{
         return 0.0;
       }
     }
-    double numer=(v1&v2).getTotalVal();
+
+    double numer=0.0;
+    // we're doing : (v1&v2).getTotalVal(), but w/o generating
+    // the other vector:
+    typename SparseIntVect<IndexType>::StorageType::const_iterator iter1,iter2;
+    iter1=v1.getNonzeroElements().begin();
+    iter2=v2.getNonzeroElements().begin();
+    while(iter1 != v1.getNonzeroElements().end()){
+      while(iter2!=v2.getNonzeroElements().end() && iter2->first < iter1->first){
+        ++iter2;
+      }
+      if(iter2!=v2.getNonzeroElements().end()){
+        if(iter2->first == iter1->first){
+          if(iter2->second<iter1->second){
+            numer += iter2->second;
+          } else {
+            numer += iter1->second;
+          }
+          ++iter2;
+        }
+        ++iter1;
+      } else {
+        break;
+      }
+    }
     return 2.*numer/denom;
   }
 } 
diff --git a/Code/DataStructs/Wrap/testDiscreteValueVect.py b/Code/DataStructs/Wrap/testDiscreteValueVect.py
index f5b0d9968..e151e072a 100644
--- a/Code/DataStructs/Wrap/testDiscreteValueVect.py
+++ b/Code/DataStructs/Wrap/testDiscreteValueVect.py
@@ -1,3 +1,9 @@
+# $Id$
+#
+# Copyright (C) 2007,2008 Greg Landrum
+#
+#  @@ All Rights Reserved @@
+#
 import RDConfig
 import os,sys,cPickle
 import unittest
diff --git a/Code/DataStructs/Wrap/testSparseIntVect.py b/Code/DataStructs/Wrap/testSparseIntVect.py
index fc8f2537b..bbbf75d9a 100644
--- a/Code/DataStructs/Wrap/testSparseIntVect.py
+++ b/Code/DataStructs/Wrap/testSparseIntVect.py
@@ -1,8 +1,16 @@
+# $Id$
+#
+# Copyright (C) 2007,2008 Greg Landrum
+#
+#  @@ All Rights Reserved @@
+#
 import RDConfig
 import os,sys,cPickle
 import unittest
 import DataStructs as ds
 
+def feq(v1,v2,tol=1e-4):
+  return abs(v1-v2)<tol
 class TestCase(unittest.TestCase):
   def setUp(self) :
     pass
@@ -89,8 +97,48 @@ class TestCase(unittest.TestCase):
     v2.UpdateFromSequence((0,2,3,3,2,3))
     self.failUnless(v1==v2)
     
+  def test5Dice(self):
+    """
 
-    
+    """
+    v1 = ds.IntSparseIntVect(5)
+    v1[4]=4;
+    v1[0]=2;
+    v1[3]=1;
+    self.failUnless(feq(ds.DiceSimilarity(v1,v1),1.0))
+
+    v1 = ds.IntSparseIntVect(5)
+    v1[0]=2;
+    v1[2]=1;
+    v1[3]=4;
+    v1[4]=6;
+    v2 = ds.IntSparseIntVect(5)
+    v2[1]=2;
+    v2[2]=3;
+    v2[3]=4;
+    v2[4]=4;
+    self.failUnless(feq(ds.DiceSimilarity(v1,v2),18.0/26.))
+    self.failUnless(feq(ds.DiceSimilarity(v2,v1),18.0/26.))
+
+  def test6BulkDice(self):
+    """
+
+    """
+    sz=10
+    nToSet=5
+    nVs=6
+    import random
+    vs = []
+    for i in range(nVs):
+      v = ds.IntSparseIntVect(sz)
+      for j in range(nToSet):
+        v[random.randint(0,sz-1)]=random.randint(1,10)
+      vs.append(v)
+
+    baseDs = [ds.DiceSimilarity(vs[0],vs[x]) for x in range(1,nVs)]
+    bulkDs = ds.BulkDiceSimilarity(vs[0],vs[1:])
+    for i in range(len(baseDs)):
+      self.failUnless(feq(baseDs[i],bulkDs[i]))
     
     
 if __name__ == '__main__':
diff --git a/Code/DataStructs/Wrap/wrap_BitOps.cpp b/Code/DataStructs/Wrap/wrap_BitOps.cpp
index 14e9877a9..344508bea 100755
--- a/Code/DataStructs/Wrap/wrap_BitOps.cpp
+++ b/Code/DataStructs/Wrap/wrap_BitOps.cpp
@@ -1,6 +1,6 @@
 // $Id$
 //
-//  Copyright (C) 2003-2006 greg Landrum and Rational Discovery LLC
+//  Copyright (C) 2003-2008 greg Landrum and Rational Discovery LLC
 //
 //  @@ All Rights Reserved @@
 //
@@ -45,7 +45,7 @@ python::list BulkWrapper(const T &bv1,python::list bvs,
   unsigned int nbvs=python::extract<unsigned int>(bvs.attr("__len__")());
   for(unsigned int i=0;i<nbvs;++i){
     double simVal;
-    T bv2=python::extract<T>(bvs[i])();
+    const T &bv2=python::extract<T>(bvs[i])();
     if(bv1.GetNumBits()>bv2.GetNumBits()){
       T *bv1tmp = FoldFingerprint(bv1,bv1.GetNumBits()/bv2.GetNumBits());
       simVal = metric(*bv1tmp,bv2);