content: /**
   * 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.apache.lucene.util;
  
  import org.apache.lucene.search.DocIdSetIterator;
  
  /** An iterator to iterate over set bits in an OpenBitSet.
   * 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 OpenBitSetIterator extends DocIdSetIterator {
  
    // 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
    protected 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)
    ******/
  
    // 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...
  
    final long[] arr;
    final int words;
    private int i=-1;
    private long word;
    private int wordShift;
    private int indexArray;
    private int curDocId = -1;
  
    public OpenBitSetIterator(OpenBitSet obs) {
      this(obs.getBits(), obs.getNumWords());
    }
  
    public OpenBitSetIterator(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];
    }
  
    /***** alternate shift implementations
    // 32 bit shifts, but a long shift needed at the end
    private void shift2() {
      int y = (int)word;
      if (y==0) {wordShift +=32; y = (int)(word >>>32); }
      if ((y & 0x0000FFFF) == 0) { wordShift +=16; y>>>=16; }
      if ((y & 0x000000FF) == 0) { wordShift +=8; y>>>=8; }
      indexArray = bitlist[y & 0xff];
      word >>>= (wordShift +1);
    }
  
    private void shift3() {
      int lower = (int)word;
      int lowByte = lower & 0xff;
      if (lowByte != 0) {
        indexArray=bitlist[lowByte];
        return;
      }
      shift();
    }
    ******/
  
    @Override
    public int nextDoc() {
      if (indexArray == 0) {
        if (word != 0) {
          word >>>= 8;
          wordShift += 8;
        }
  
        while (word == 0) {
          if (++i >= words) {
            return curDocId = NO_MORE_DOCS;
          }
          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 curDocId = (i<<6) + bitIndex;
    }
    
    @Override
    public int advance(int target) {
      indexArray = 0;
      i = target >> 6;
      if (i >= words) {
        word = 0; // setup so next() will also return -1
        return curDocId = NO_MORE_DOCS;
      }
      wordShift = target & 0x3f;
      word = arr[i] >>> wordShift;
      if (word != 0) {
        wordShift--; // compensate for 1 based arrIndex
      } else {
        while (word == 0) {
          if (++i >= words) {
            return curDocId = NO_MORE_DOCS;
          }
          word = arr[i];
        }
        wordShift = -1;
      }
  
      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 curDocId = (i<<6) + bitIndex;
    }
  
    @Override
    public int docID() {
      return curDocId;
    }
    
  }
  