Adding both forms of flip() to BitSet.

classpath/java/util/BitSet.java

@@ -80,6 +80,30 @@ public class BitSet implements Serializable, Cloneable {
       bits[i] ^= otherBits.bits[i];
     }
   }
+  
+  public void flip(int index) {
+    flip(index, index+1);
+  }
+  
+  public void flip(int fromIndex, int toIndex) {
+    if (fromIndex > toIndex || fromIndex < 0 || toIndex < 0) {
+      throw new IndexOutOfBoundsException();
+    } else if (fromIndex != toIndex) {
+      int basePartition = longPosition(fromIndex);
+      int lastPartition = longPosition(toIndex - 1); //range is [fromIndex, toIndex)
+      int numPartitionsToTraverse = lastPartition - basePartition + 1;
+      enlarge(lastPartition);
+      
+      int currentFirstIndex = fromIndex;
+      for (int i = 0; i < numPartitionsToTraverse; ++i) {
+        int currentToIndex = Math.min(toIndex, (basePartition + i + 1) * BITS_PER_LONG);
+        int currentRange = currentToIndex - currentFirstIndex;
+        long mask = (((1L << currentRange) - 1L) << (currentFirstIndex % BITS_PER_LONG));
+        bits[i + basePartition] ^= mask;
+        currentFirstIndex = currentToIndex;
+      }
+    }
+  }
 
   private void enlarge(int newSize) {
     if (bits == null || bits.length < newSize) {

test/BitsetTest.java

@@ -47,6 +47,33 @@ public class BitsetTest {
     assertEquals("after 5, 6 is empty", 6, bits.nextClearBit(5));
     assertEquals("after 100, 102 is empty", 102, bits.nextClearBit(100));
     
+    testFlip();
+  }
+  
+  private static void testFlip() {
+    /* simple case */
+    BitSet bitset = new BitSet();
+    bitset.set(0);
+    bitset.flip(0, 0);
+    assertTrue("Should not be flipped with 0 length range", bitset.get(0));
+    bitset.flip(0, 1);
+    assertTrue("Should be false with range of one", !bitset.get(0));
+    bitset.flip(0);
+    assertTrue("Should be true again", bitset.get(0));
+    
+    /* need to grow */
+    bitset.flip(1000);
+    assertTrue("1000 should be true", bitset.get(1000));
+    assertTrue("1001 should be false", !bitset.get(1001));
+    assertTrue("999 should be false", !bitset.get(999));
+    
+    /* Range over 2 segments */
+    bitset.flip(60, 70);
+    assertTrue("59 should be false", !bitset.get(59));
+    for (int i=60; i < 70; ++i) {
+      assertTrue(i + " should be true", bitset.get(i));
+    }
+    assertTrue("70 should be false", !bitset.get(70));
   }
   
   static void assertTrue(String msg, boolean flag) {