Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 812850)
+++ CHANGES.txt	(working copy)
@@ -549,6 +549,10 @@
     Because of this IndexReader.isLocked() and IndexWriter.isLocked() did
     not work correctly.  (Uwe Schindler)
 
+ * LUCENE-1899: Fix O(N^2) CPU cost when setting docIDs in order in an
+   OpenBitSet, due to an inefficiency in how the underlying storage is
+   reallocated.  (Nadav Har'El via Mike McCandless)
+
 New features
 
  * LUCENE-1411: Added expert API to open an IndexWriter on a prior
Index: src/java/org/apache/lucene/util/OpenBitSet.java
===================================================================
--- src/java/org/apache/lucene/util/OpenBitSet.java	(revision 812850)
+++ src/java/org/apache/lucene/util/OpenBitSet.java	(working copy)
@@ -483,7 +483,6 @@
    */
   public void flip(long startIndex, long endIndex) {
     if (endIndex <= startIndex) return;
-    int oldlen = wlen;
     int startWord = (int)(startIndex>>6);
 
     // since endIndex is one past the end, this is index of the last
@@ -742,9 +741,7 @@
    */
   public void ensureCapacityWords(int numWords) {
     if (bits.length < numWords) {
-      long[] newBits = new long[numWords];
-      System.arraycopy(bits,0,newBits,0,wlen);
-      bits = newBits;
+      bits = ArrayUtil.grow(bits, numWords);
     }
   }