/* 
 * Repackaged from org.apache.lucene.util.OpenBitSet (Lucene).
 * svn rev. 893130, http://svn.apache.org/repos/asf/lucene/java/trunk/
 * 
 * Minor changes in class hierarchy, removed serialization.
 */

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.
/**
 * 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 com.carrotsearch.hppc;

An iterator to iterate over set bits in an BitSet. This is faster than nextSetBit() for
iterating over the complete set of bits, especially when the density of the bits set is
high.
/**
 * An iterator to iterate over set bits in an BitSet. This is faster than nextSetBit() for
 * iterating over the complete set of bits, especially when the density of the bits set is
 * high.
 */
public class BitSetIterator {
  // The General Idea: instead of having an array per byte that has
  // the offsets of the next set bit, that array could be
  // packed inside a 32 bit integer (8 4 bit numbers).  That
  // should be faster than accessing an array for each index, and
  // the total array size is kept smaller (256*sizeof(int))=1K
  final static int[] bitlist={
    0x0, 0x1, 0x2, 0x21, 0x3, 0x31, 0x32, 0x321, 0x4, 0x41, 0x42, 0x421, 0x43, 
    0x431, 0x432, 0x4321, 0x5, 0x51, 0x52, 0x521, 0x53, 0x531, 0x532, 0x5321, 
    0x54, 0x541, 0x542, 0x5421, 0x543, 0x5431, 0x5432, 0x54321, 0x6, 0x61, 0x62, 
    0x621, 0x63, 0x631, 0x632, 0x6321, 0x64, 0x641, 0x642, 0x6421, 0x643, 
    0x6431, 0x6432, 0x64321, 0x65, 0x651, 0x652, 0x6521, 0x653, 0x6531, 0x6532, 
    0x65321, 0x654, 0x6541, 0x6542, 0x65421, 0x6543, 0x65431, 0x65432, 0x654321, 
    0x7, 0x71, 0x72, 0x721, 0x73, 0x731, 0x732, 0x7321, 0x74, 0x741, 0x742,
    0x7421, 0x743, 0x7431, 0x7432, 0x74321, 0x75, 0x751, 0x752, 0x7521, 0x753, 
    0x7531, 0x7532, 0x75321, 0x754, 0x7541, 0x7542, 0x75421, 0x7543, 0x75431, 
    0x75432, 0x754321, 0x76, 0x761, 0x762, 0x7621, 0x763, 0x7631, 0x7632, 
    0x76321, 0x764, 0x7641, 0x7642, 0x76421, 0x7643, 0x76431, 0x76432, 0x764321, 
    0x765, 0x7651, 0x7652, 0x76521, 0x7653, 0x76531, 0x76532, 0x765321, 0x7654, 
    0x76541, 0x76542, 0x765421, 0x76543, 0x765431, 0x765432, 0x7654321, 0x8, 
    0x81, 0x82, 0x821, 0x83, 0x831, 0x832, 0x8321, 0x84, 0x841, 0x842, 0x8421, 
    0x843, 0x8431, 0x8432, 0x84321, 0x85, 0x851, 0x852, 0x8521, 0x853, 0x8531, 
    0x8532, 0x85321, 0x854, 0x8541, 0x8542, 0x85421, 0x8543, 0x85431, 0x85432, 
    0x854321, 0x86, 0x861, 0x862, 0x8621, 0x863, 0x8631, 0x8632, 0x86321, 0x864, 
    0x8641, 0x8642, 0x86421, 0x8643, 0x86431, 0x86432, 0x864321, 0x865, 0x8651, 
    0x8652, 0x86521, 0x8653, 0x86531, 0x86532, 0x865321, 0x8654, 0x86541, 
    0x86542, 0x865421, 0x86543, 0x865431, 0x865432, 0x8654321, 0x87, 0x871, 
    0x872, 0x8721, 0x873, 0x8731, 0x8732, 0x87321, 0x874, 0x8741, 0x8742, 
    0x87421, 0x8743, 0x87431, 0x87432, 0x874321, 0x875, 0x8751, 0x8752, 0x87521, 
    0x8753, 0x87531, 0x87532, 0x875321, 0x8754, 0x87541, 0x87542, 0x875421, 
    0x87543, 0x875431, 0x875432, 0x8754321, 0x876, 0x8761, 0x8762, 0x87621, 
    0x8763, 0x87631, 0x87632, 0x876321, 0x8764, 0x87641, 0x87642, 0x876421, 
    0x87643, 0x876431, 0x876432, 0x8764321, 0x8765, 0x87651, 0x87652, 0x876521, 
    0x87653, 0x876531, 0x876532, 0x8765321, 0x87654, 0x876541, 0x876542, 
    0x8765421, 0x876543, 0x8765431, 0x8765432, 0x87654321
  };

  
the python code that generated bitlist
def bits2int(val):
arr=0
for shift in range(8,0,-1):
if val & 0x80:
arr = (arr << 4) | shift
val = val << 1
return arr
def int_table():
tbl = [ hex(bits2int(val)).strip('L') for val in range(256) ]
return ','.join(tbl)
/***** the python code that generated bitlist
  def bits2int(val):
  arr=0
  for shift in range(8,0,-1):
    if val & 0x80:
      arr = (arr << 4) | shift
    val = val << 1
  return arr

  def int_table():
    tbl = [ hex(bits2int(val)).strip('L') for val in range(256) ]
    return ','.join(tbl)
  ******/

  // hmmm, what about an iterator that finds zeros though,
  // or a reverse iterator... should they be separate classes
  // for efficiency, or have a common root interface?  (or
  // maybe both?  could ask for a SetBitsIterator, etc...

  private final long[] arr;
  private final int words;
  private int i=-1;
  private long word;
  private int wordShift;
  private int indexArray;

  public BitSetIterator(BitSet obs) {
    this(obs.bits, obs.wlen);
  }

  public BitSetIterator(long[] bits, int numWords) {
    arr = bits;
    words = numWords;
  }

  // 64 bit shifts
  private void shift() {
    if ((int)word ==0) {wordShift +=32; word = word >>>32; }
    if ((word & 0x0000FFFF) == 0) { wordShift +=16; word >>>=16; }
    if ((word & 0x000000FF) == 0) { wordShift +=8; word >>>=8; }
    indexArray = bitlist[(int)word & 0xff];
  }

  public final static int NO_MORE = -1;

  public int nextSetBit() {
    if (indexArray == 0) {
      if (word != 0) {
        word >>>= 8;
        wordShift += 8;
      }

      while (word == 0) {
        if (++i >= words) {
          return NO_MORE;
        }
        word = arr[i];
        wordShift = -1; // loop invariant code motion should move this
      }

      // after the first time, should I go with a linear search, or
      // stick with the binary search in shift?
      shift();
    }

    int bitIndex = (indexArray & 0x0f) + wordShift;
    indexArray >>>= 4;
    // should i<<6 be cached as a separate variable?
    // it would only save one cycle in the best circumstances.
    return (i<<6) + bitIndex;
  }
}