Index: lucene/src/java/org/apache/lucene/util/OpenBitSet.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/OpenBitSet.java	(revision 1139572)
+++ lucene/src/java/org/apache/lucene/util/OpenBitSet.java	(working copy)
@@ -626,12 +626,12 @@
     long word = bits[i] >> subIndex;  // skip all the bits to the right of index
 
     if (word!=0) {
-      return (i<<6) + subIndex + BitUtil.ntz(word);
+      return (i<<6) + subIndex + Long.numberOfTrailingZeros(word);
     }
 
     while(++i < wlen) {
       word = bits[i];
-      if (word!=0) return (i<<6) + BitUtil.ntz(word);
+      if (word!=0) return (i<<6) + Long.numberOfTrailingZeros(word);
     }
 
     return -1;
@@ -647,12 +647,12 @@
     long word = bits[i] >>> subIndex;  // skip all the bits to the right of index
 
     if (word!=0) {
-      return (((long)i)<<6) + (subIndex + BitUtil.ntz(word));
+      return (((long)i)<<6) + (subIndex + Long.numberOfTrailingZeros(word));
     }
 
     while(++i < wlen) {
       word = bits[i];
-      if (word!=0) return (((long)i)<<6) + BitUtil.ntz(word);
+      if (word!=0) return (((long)i)<<6) + Long.numberOfTrailingZeros(word);
     }
 
     return -1;
@@ -692,6 +692,39 @@
     return -1;
   }
 
+  /** Returns the index of the first set bit starting downwards at
+   *  the index specified.
+   *  -1 is returned if there are no more set bits.
+   */
+  public long prevSetBit(long index) {
+    int i = (int) (index >> 6);
+    final int subIndex;
+    long word;
+    if (i >= wlen) {
+      i = wlen - 1;
+      if (i < 0) return -1;
+      subIndex = 63;  // last possible bit
+      word = bits[i];
+    } else {
+      if (i < 0) return -1;
+      subIndex = (int)index & 0x3f;  // index within the word
+      word = (bits[i] << (63-subIndex));  // skip all the bits to the left of index
+    }
+
+    if (word != 0) {
+      return (((long)i)<<6) + subIndex - Long.numberOfLeadingZeros(word); // See LUCENE-3197
+    }
+
+    while (--i >= 0) {
+      word = bits[i];
+      if (word !=0 ) {
+        return (((long)i)<<6) + 63 - Long.numberOfLeadingZeros(word);
+      }
+    }
+
+    return -1;
+  }
+
   @Override
   public Object clone() {
     try {