/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.snpeff.collections;

import java.io.Serializable;
import java.util.Arrays;

/**
 * This class is originally from Apache Lucene-3.0.3 project.
 * Only a few minor changes.
 */

/** An "open" BitSet implementation that allows direct access to the array of words
 * storing the bits.
 * <p/>
 * Unlike java.util.bitset, the fact that bits are packed into an array of longs
 * is part of the interface.  This allows efficient implementation of other algorithms
 * by someone other than the author.  It also allows one to efficiently implement
 * alternate serialization or interchange formats.
 * <p/>
 * <code>OpenBitSet</code> is faster than <code>java.util.BitSet</code> in most operations
 * and *much* faster at calculating cardinality of sets and results of set operations.
 * It can also handle sets of larger cardinality (up to 64 * 2**32-1)
 * <p/>
 * The goals of <code>OpenBitSet</code> are the fastest implementation possible, and
 * maximum code reuse.  Extra safety and encapsulation
 * may always be built on top, but if that's built in, the cost can never be removed (and
 * hence people re-implement their own version in order to get better performance).
 * If you want a "safe", totally encapsulated (and slower and limited) BitSet
 * class, use <code>java.util.BitSet</code>.
 * <p/>
 * <h3>Performance Results</h3>
 *
 Test system: Pentium 4, Sun Java 1.5_06 -server -Xbatch -Xmx64M
<br/>BitSet size = 1,000,000
<br/>Results are java.util.BitSet time divided by OpenBitSet time.
<table border="1">
 <tr>
  <th></th> <th>cardinality</th> <th>intersect_count</th> <th>union</th> <th>nextSetBit</th> <th>get</th> <th>iterator</th>
 </tr>
 <tr>
  <th>50% full</th> <td>3.36</td> <td>3.96</td> <td>1.44</td> <td>1.46</td> <td>1.99</td> <td>1.58</td>
 </tr>
 <tr>
   <th>1% full</th> <td>3.31</td> <td>3.90</td> <td>&nbsp;</td> <td>1.04</td> <td>&nbsp;</td> <td>0.99</td>
 </tr>
</table>
<br/>
Test system: AMD Opteron, 64 bit linux, Sun Java 1.5_06 -server -Xbatch -Xmx64M
<br/>BitSet size = 1,000,000
<br/>Results are java.util.BitSet time divided by OpenBitSet time.
<table border="1">
 <tr>
  <th></th> <th>cardinality</th> <th>intersect_count</th> <th>union</th> <th>nextSetBit</th> <th>get</th> <th>iterator</th>
 </tr>
 <tr>
  <th>50% full</th> <td>2.50</td> <td>3.50</td> <td>1.00</td> <td>1.03</td> <td>1.12</td> <td>1.25</td>
 </tr>
 <tr>
   <th>1% full</th> <td>2.51</td> <td>3.49</td> <td>&nbsp;</td> <td>1.00</td> <td>&nbsp;</td> <td>1.02</td>
 </tr>
</table>

 * @version $Id: OpenBitSet.java,v 1.1 2011/03/10 12:18:16 pcingola Exp $
 */

public class OpenBitSet implements Cloneable, Serializable {

	protected long[] bits;
	protected int wlen; // number of words (elements) used in the array

	/** Returns the popcount or cardinality of "a and not b"
	 * or "intersection(a, not(b))".
	 * Neither set is modified.
	 */
	public static long andNotCount(OpenBitSet a, OpenBitSet b) {
		long tot = BitUtil.pop_andnot(a.bits, b.bits, 0, Math.min(a.wlen, b.wlen));
		if( a.wlen > b.wlen ) {
			tot += BitUtil.pop_array(a.bits, b.wlen, a.wlen - b.wlen);
		}
		return tot;
	}

	/** returns the number of 64 bit words it would take to hold numBits */
	public static int bits2words(long numBits) {
		return (int) (((numBits - 1) >>> 6) + 1);
	}

	/** Returns the popcount or cardinality of the intersection of the two sets.
	  * Neither set is modified.
	  */
	public static long intersectionCount(OpenBitSet a, OpenBitSet b) {
		return BitUtil.pop_intersect(a.bits, b.bits, 0, Math.min(a.wlen, b.wlen));
	}

	/** Returns the popcount or cardinality of the union of the two sets.
	  * Neither set is modified.
	  */
	public static long unionCount(OpenBitSet a, OpenBitSet b) {
		long tot = BitUtil.pop_union(a.bits, b.bits, 0, Math.min(a.wlen, b.wlen));
		if( a.wlen < b.wlen ) {
			tot += BitUtil.pop_array(b.bits, a.wlen, b.wlen - a.wlen);
		} else if( a.wlen > b.wlen ) {
			tot += BitUtil.pop_array(a.bits, b.wlen, a.wlen - b.wlen);
		}
		return tot;
	}

	/** Returns the popcount or cardinality of the exclusive-or of the two sets.
	 * Neither set is modified.
	 */
	public static long xorCount(OpenBitSet a, OpenBitSet b) {
		long tot = BitUtil.pop_xor(a.bits, b.bits, 0, Math.min(a.wlen, b.wlen));
		if( a.wlen < b.wlen ) {
			tot += BitUtil.pop_array(b.bits, a.wlen, b.wlen - a.wlen);
		} else if( a.wlen > b.wlen ) {
			tot += BitUtil.pop_array(a.bits, b.wlen, a.wlen - b.wlen);
		}
		return tot;
	}

	public OpenBitSet() {
		this(64);
	}

	/** Constructs an OpenBitSet large enough to hold numBits.
	 *
	 * @param numBits
	 */
	public OpenBitSet(long numBits) {
		bits = new long[bits2words(numBits)];
		wlen = bits.length;
	}

	/** Constructs an OpenBitSet from an existing long[].
	 * <br/>
	 * The first 64 bits are in long[0],
	 * with bit index 0 at the least significant bit, and bit index 63 at the most significant.
	 * Given a bit index,
	 * the word containing it is long[index/64], and it is at bit number index%64 within that word.
	 * <p>
	 * numWords are the number of elements in the array that contain
	 * set bits (non-zero longs).
	 * numWords should be &lt= bits.length, and
	 * any existing words in the array at position &gt= numWords should be zero.
	 *
	 */
	public OpenBitSet(long[] bits, int numWords) {
		this.bits = bits;
		wlen = numWords;
	}

	//** see {@link intersect} */
	public void and(OpenBitSet other) {
		intersect(other);
	}

	//** see {@link andNot} */
	public void andNot(OpenBitSet other) {
		remove(other);
	}

	/** Returns the current capacity in bits (1 greater than the index of the last bit) */
	public long capacity() {
		return bits.length << 6;
	}

	/** @return the number of set bits */
	public long cardinality() {
		return BitUtil.pop_array(bits, 0, wlen);
	}

	/** Clears a range of bits.  Clearing past the end does not change the size of the set.
	 *
	 * @param startIndex lower index
	 * @param endIndex one-past the last bit to clear
	 */
	public void clear(int startIndex, int endIndex) {
		if( endIndex <= startIndex ) return;

		int startWord = (startIndex >> 6);
		if( startWord >= wlen ) return;

		// since endIndex is one past the end, this is index of the last
		// word to be changed.
		int endWord = ((endIndex - 1) >> 6);

		long startmask = -1L << startIndex;
		long endmask = -1L >>> -endIndex; // 64-(endIndex&0x3f) is the same as -endIndex due to wrap

		// invert masks since we are clearing
		startmask = ~startmask;
		endmask = ~endmask;

		if( startWord == endWord ) {
			bits[startWord] &= (startmask | endmask);
			return;
		}

		bits[startWord] &= startmask;

		int middle = Math.min(wlen, endWord);
		Arrays.fill(bits, startWord + 1, middle, 0L);
		if( endWord < wlen ) {
			bits[endWord] &= endmask;
		}
	}

	/** clears a bit, allowing access beyond the current set size without changing the size.*/
	public void clear(long index) {
		int wordNum = (int) (index >> 6); // div 64
		if( wordNum >= wlen ) return;
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] &= ~bitmask;
	}

	/*
	// alternate implementation of get()
	public boolean get1(int index) {
	  int i = index >> 6;                // div 64
	  int bit = index & 0x3f;            // mod 64
	  return ((bits[i]>>>bit) & 0x01) != 0;
	  // this does a long shift and a bittest (on x86) vs
	  // a long shift, and a long AND, (the test for zero is prob a no-op)
	  // testing on a P4 indicates this is slower than (bits[i] & bitmask) != 0;
	}
	*/

	/** Clears a range of bits.  Clearing past the end does not change the size of the set.
	 *
	 * @param startIndex lower index
	 * @param endIndex one-past the last bit to clear
	 */
	public void clear(long startIndex, long endIndex) {
		if( endIndex <= startIndex ) return;

		int startWord = (int) (startIndex >> 6);
		if( startWord >= wlen ) return;

		// since endIndex is one past the end, this is index of the last
		// word to be changed.
		int endWord = (int) ((endIndex - 1) >> 6);

		long startmask = -1L << startIndex;
		long endmask = -1L >>> -endIndex; // 64-(endIndex&0x3f) is the same as -endIndex due to wrap

		// invert masks since we are clearing
		startmask = ~startmask;
		endmask = ~endmask;

		if( startWord == endWord ) {
			bits[startWord] &= (startmask | endmask);
			return;
		}

		bits[startWord] &= startmask;

		int middle = Math.min(wlen, endWord);
		Arrays.fill(bits, startWord + 1, middle, 0L);
		if( endWord < wlen ) {
			bits[endWord] &= endmask;
		}
	}

	/*
	public boolean get2(int index) {
	  int word = index >> 6;            // div 64
	  int bit = index & 0x0000003f;     // mod 64
	  return (bits[word] << bit) < 0;   // hmmm, this would work if bit order were reversed
	  // we could right shift and check for parity bit, if it was available to us.
	}
	*/

	@Override
	public Object clone() {
		try {
			OpenBitSet obs = (OpenBitSet) super.clone();
			obs.bits = obs.bits.clone(); // hopefully an array clone is as fast(er) than arraycopy
			return obs;
		} catch(CloneNotSupportedException e) {
			throw new RuntimeException(e);
		}
	}

	/** Ensure that the long[] is big enough to hold numBits, expanding it if necessary.
	 * getNumWords() is unchanged by this call.
	 */
	public void ensureCapacity(long numBits) {
		ensureCapacityWords(bits2words(numBits));
	}

	/** Expand the long[] with the size given as a number of words (64 bit longs).
	 * getNumWords() is unchanged by this call.
	 */
	public void ensureCapacityWords(int numWords) {
		if( bits.length < numWords ) {
			bits = ArrayUtil.grow(bits, numWords);
		}
	}

	/** returns true if both sets have the same bits set */
	@Override
	public boolean equals(Object o) {
		if( this == o ) return true;
		if( !(o instanceof OpenBitSet) ) return false;
		OpenBitSet a;
		OpenBitSet b = (OpenBitSet) o;
		// make a the larger set.
		if( b.wlen > wlen ) {
			a = b;
			b = this;
		} else {
			a = this;
		}

		// check for any set bits out of the range of b
		for( int i = a.wlen - 1; i >= b.wlen; i-- ) {
			if( a.bits[i] != 0 ) return false;
		}

		for( int i = b.wlen - 1; i >= 0; i-- ) {
			if( a.bits[i] != b.bits[i] ) return false;
		}

		return true;
	}

	protected int expandingWordNum(long index) {
		int wordNum = (int) (index >> 6);
		if( wordNum >= wlen ) {
			ensureCapacity(index + 1);
			wlen = wordNum + 1;
		}
		return wordNum;
	}

	/** clears a bit.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastClear(int index) {
		int wordNum = index >> 6;
		int bit = index & 0x03f;
		long bitmask = 1L << bit;
		bits[wordNum] &= ~bitmask;
		// hmmm, it takes one more instruction to clear than it does to set... any
		// way to work around this?  If there were only 63 bits per word, we could
		// use a right shift of 10111111...111 in binary to position the 0 in the
		// correct place (using sign extension).
		// Could also use Long.rotateRight() or rotateLeft() *if* they were converted
		// by the JVM into a native instruction.
		// bits[word] &= Long.rotateLeft(0xfffffffe,bit);
	}

	/** clears a bit.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastClear(long index) {
		int wordNum = (int) (index >> 6); // div 64
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] &= ~bitmask;
	}

	/** flips a bit.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastFlip(int index) {
		int wordNum = index >> 6; // div 64
		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] ^= bitmask;
	}

	/** flips a bit.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastFlip(long index) {
		int wordNum = (int) (index >> 6); // div 64
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] ^= bitmask;
	}

	/** Returns true or false for the specified bit index.
	  * The index should be less than the OpenBitSet size
	  */
	public boolean fastGet(int index) {
		int i = index >> 6; // div 64
		// signed shift will keep a negative index and force an
		// array-index-out-of-bounds-exception, removing the need for an explicit check.
		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		return (bits[i] & bitmask) != 0;
	}

	/** Returns true or false for the specified bit index.
	 * The index should be less than the OpenBitSet size.
	 */
	public boolean fastGet(long index) {
		int i = (int) (index >> 6); // div 64
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		return (bits[i] & bitmask) != 0;
	}

	/** Sets the bit at the specified index.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastSet(int index) {
		int wordNum = index >> 6; // div 64
		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] |= bitmask;
	}

	/** Sets the bit at the specified index.
	 * The index should be less than the OpenBitSet size.
	 */
	public void fastSet(long index) {
		int wordNum = (int) (index >> 6);
		int bit = (int) index & 0x3f;
		long bitmask = 1L << bit;
		bits[wordNum] |= bitmask;
	}

	/** flips a bit, expanding the set size if necessary */
	public void flip(long index) {
		int wordNum = expandingWordNum(index);
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] ^= bitmask;
	}

	/** Flips a range of bits, expanding the set size if necessary
	 *
	 * @param startIndex lower index
	 * @param endIndex one-past the last bit to flip
	 */
	public void flip(long startIndex, long endIndex) {
		if( endIndex <= startIndex ) return;
		int startWord = (int) (startIndex >> 6);

		// since endIndex is one past the end, this is index of the last
		// word to be changed.
		int endWord = expandingWordNum(endIndex - 1);

		/*** Grrr, java shifting wraps around so -1L>>>64 == -1
		 * for that reason, make sure not to use endmask if the bits to flip will
		 * be zero in the last word (redefine endWord to be the last changed...)
		long startmask = -1L << (startIndex & 0x3f);     // example: 11111...111000
		long endmask = -1L >>> (64-(endIndex & 0x3f));   // example: 00111...111111
		***/

		long startmask = -1L << startIndex;
		long endmask = -1L >>> -endIndex; // 64-(endIndex&0x3f) is the same as -endIndex due to wrap

		if( startWord == endWord ) {
			bits[startWord] ^= (startmask & endmask);
			return;
		}

		bits[startWord] ^= startmask;

		for( int i = startWord + 1; i < endWord; i++ ) {
			bits[i] = ~bits[i];
		}

		bits[endWord] ^= endmask;
	}

	/** flips a bit and returns the resulting bit value.
	 * The index should be less than the OpenBitSet size.
	 */
	public boolean flipAndGet(int index) {
		int wordNum = index >> 6; // div 64
		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] ^= bitmask;
		return (bits[wordNum] & bitmask) != 0;
	}

	/** flips a bit and returns the resulting bit value.
	 * The index should be less than the OpenBitSet size.
	 */
	public boolean flipAndGet(long index) {
		int wordNum = (int) (index >> 6); // div 64
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		bits[wordNum] ^= bitmask;
		return (bits[wordNum] & bitmask) != 0;
	}

	/** Returns true or false for the specified bit index. */
	public boolean get(int index) {
		int i = index >> 6; // div 64
		// signed shift will keep a negative index and force an
		// array-index-out-of-bounds-exception, removing the need for an explicit check.
		if( i >= bits.length ) return false;

		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		return (bits[i] & bitmask) != 0;
	}

	/*
	public static int pop(long v0, long v1, long v2, long v3) {
	  // derived from pop_array by setting last four elems to 0.
	  // exchanges one pop() call for 10 elementary operations
	  // saving about 7 instructions... is there a better way?
	    long twosA=v0 & v1;
	    long ones=v0^v1;

	    long u2=ones^v2;
	    long twosB =(ones&v2)|(u2&v3);
	    ones=u2^v3;

	    long fours=(twosA&twosB);
	    long twos=twosA^twosB;

	    return (pop(fours)<<2)
	           + (pop(twos)<<1)
	           + pop(ones);

	}
	*/

	/** Returns true or false for the specified bit index
	 */
	public boolean get(long index) {
		int i = (int) (index >> 6); // div 64
		if( i >= bits.length ) return false;
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		return (bits[i] & bitmask) != 0;
	}

	/** Sets a bit and returns the previous value.
	 * The index should be less than the OpenBitSet size.
	 */
	public boolean getAndSet(int index) {
		int wordNum = index >> 6; // div 64
		int bit = index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		boolean val = (bits[wordNum] & bitmask) != 0;
		bits[wordNum] |= bitmask;
		return val;
	}

	/** Sets a bit and returns the previous value.
	 * The index should be less than the OpenBitSet size.
	 */
	public boolean getAndSet(long index) {
		int wordNum = (int) (index >> 6); // div 64
		int bit = (int) index & 0x3f; // mod 64
		long bitmask = 1L << bit;
		boolean val = (bits[wordNum] & bitmask) != 0;
		bits[wordNum] |= bitmask;
		return val;
	}

	/** returns 1 if the bit is set, 0 if not.
	 * The index should be less than the OpenBitSet size
	 */
	public int getBit(int index) {
		int i = index >> 6; // div 64
		int bit = index & 0x3f; // mod 64
		return ((int) (bits[i] >>> bit)) & 0x01;
	}

	/** Expert: returns the long[] storing the bits */
	public long[] getBits() {
		return bits;
	}

	/** Expert: gets the number of longs in the array that are in use */
	public int getNumWords() {
		return wlen;
	}

	@Override
	public int hashCode() {
		// Start with a zero hash and use a mix that results in zero if the input is zero.
		// This effectively truncates trailing zeros without an explicit check.
		long h = 0;
		for( int i = bits.length; --i >= 0; ) {
			h ^= bits[i];
			h = (h << 1) | (h >>> 63); // rotate left
		}
		// fold leftmost bits into right and add a constant to prevent
		// empty sets from returning 0, which is too common.
		return (int) ((h >> 32) ^ h) + 0x98761234;
	}

	/** this = this AND other */
	public void intersect(OpenBitSet other) {
		int newLen = Math.min(wlen, other.wlen);
		long[] thisArr = bits;
		long[] otherArr = other.bits;
		// testing against zero can be more efficient
		int pos = newLen;
		while(--pos >= 0) {
			thisArr[pos] &= otherArr[pos];
		}
		if( wlen > newLen ) {
			// fill zeros from the new shorter length to the old length
			Arrays.fill(bits, newLen, wlen, 0);
		}
		wlen = newLen;
	}

	/** returns true if the sets have any elements in common */
	public boolean intersects(OpenBitSet other) {
		int pos = Math.min(wlen, other.wlen);
		long[] thisArr = bits;
		long[] otherArr = other.bits;
		while(--pos >= 0) {
			if( (thisArr[pos] & otherArr[pos]) != 0 ) return true;
		}
		return false;
	}

	/** Returns true if there are no set bits */
	public boolean isEmpty() {
		return cardinality() == 0;
	}

	/** Returns the index of the first set bit starting at the index specified.
	 *  -1 is returned if there are no more set bits.
	 */
	public int nextSetBit(int index) {
		int i = index >> 6;
		if( i >= wlen ) return -1;
		int subIndex = index & 0x3f; // index within the word
		long word = bits[i] >> subIndex; // skip all the bits to the right of index

		if( word != 0 ) { return (i << 6) + subIndex + BitUtil.ntz(word); }

		while(++i < wlen) {
			word = bits[i];
			if( word != 0 ) return (i << 6) + BitUtil.ntz(word);
		}

		return -1;
	}

	/** Returns the index of the first set bit starting at the index specified.
	 *  -1 is returned if there are no more set bits.
	 */
	public long nextSetBit(long index) {
		int i = (int) (index >>> 6);
		if( i >= wlen ) return -1;
		int subIndex = (int) index & 0x3f; // index within the word
		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)); }

		while(++i < wlen) {
			word = bits[i];
			if( word != 0 ) return (((long) i) << 6) + BitUtil.ntz(word);
		}

		return -1;
	}

	// some BitSet compatability methods

	//** see {@link union} */
	public void or(OpenBitSet other) {
		union(other);
	}

	/** Remove all elements set in other. this = this AND_NOT other */
	public void remove(OpenBitSet other) {
		int idx = Math.min(wlen, other.wlen);
		long[] thisArr = bits;
		long[] otherArr = other.bits;
		while(--idx >= 0) {
			thisArr[idx] &= ~otherArr[idx];
		}
	}

	/** sets a bit, expanding the set size if necessary */
	public void set(long index) {
		int wordNum = expandingWordNum(index);
		int bit = (int) index & 0x3f;
		long bitmask = 1L << bit;
		bits[wordNum] |= bitmask;
	}

	/** Sets a range of bits, expanding the set size if necessary
	 *
	 * @param startIndex lower index
	 * @param endIndex one-past the last bit to set
	 */
	public void set(long startIndex, long endIndex) {
		if( endIndex <= startIndex ) return;

		int startWord = (int) (startIndex >> 6);

		// since endIndex is one past the end, this is index of the last
		// word to be changed.
		int endWord = expandingWordNum(endIndex - 1);

		long startmask = -1L << startIndex;
		long endmask = -1L >>> -endIndex; // 64-(endIndex&0x3f) is the same as -endIndex due to wrap

		if( startWord == endWord ) {
			bits[startWord] |= (startmask & endmask);
			return;
		}

		bits[startWord] |= startmask;
		Arrays.fill(bits, startWord + 1, endWord, -1L);
		bits[endWord] |= endmask;
	}

	/** Expert: sets a new long[] to use as the bit storage */
	public void setBits(long[] bits) {
		this.bits = bits;
	}

	/** Expert: sets the number of longs in the array that are in use */
	public void setNumWords(int nWords) {
		wlen = nWords;
	}

	/**
	 * Returns the current capacity of this set.  Included for
	 * compatibility.  This is *not* equal to {@link #cardinality}
	 */
	public long size() {
		return capacity();
	}

	/** Lowers numWords, the number of words in use,
	 * by checking for trailing zero words.
	 */
	public void trimTrailingZeros() {
		int idx = wlen - 1;
		while(idx >= 0 && bits[idx] == 0)
			idx--;
		wlen = idx + 1;
	}

	/** this = this OR other */
	public void union(OpenBitSet other) {
		int newLen = Math.max(wlen, other.wlen);
		ensureCapacityWords(newLen);

		long[] thisArr = bits;
		long[] otherArr = other.bits;
		int pos = Math.min(wlen, other.wlen);
		while(--pos >= 0) {
			thisArr[pos] |= otherArr[pos];
		}
		if( wlen < newLen ) {
			System.arraycopy(otherArr, wlen, thisArr, wlen, newLen - wlen);
		}
		wlen = newLen;
	}

	/** this = this XOR other */
	public void xor(OpenBitSet other) {
		int newLen = Math.max(wlen, other.wlen);
		ensureCapacityWords(newLen);

		long[] thisArr = bits;
		long[] otherArr = other.bits;
		int pos = Math.min(wlen, other.wlen);
		while(--pos >= 0) {
			thisArr[pos] ^= otherArr[pos];
		}
		if( wlen < newLen ) {
			System.arraycopy(otherArr, wlen, thisArr, wlen, newLen - wlen);
		}
		wlen = newLen;
	}

}
