package org.apache.lucene.util;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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.
 */

import java.io.IOException;

import org.apache.lucene.store.Directory;
import org.apache.lucene.store.IndexInput;
import org.apache.lucene.store.IndexOutput;

/** Optimized implementation of a vector of bits.  This is more-or-less like
  java.util.BitSet, but also includes the following:
  <ul>
  <li>a count() method, which efficiently computes the number of one bits;</li>
  <li>optimized read from and write to disk;</li>
  <li>inlinable get() method;</li>
  </ul>

  @author Doug Cutting
  @version $Id: BitVector.java 150536 2004-09-28 18:15:52Z cutting $
  */
public final class BitVector {

  private byte[] bits;
  private int size;
  private int count = -1;

  /** Constructs a vector capable of holding <code>n</code> bits. */
  public BitVector(int n) {
    size = n;
    bits = new byte[(size >> 3) + 1];
  }

  /** Sets the value of <code>bit</code> to one. */
  public final void set(int bit) {
    bits[bit >> 3] |= 1 << (bit & 7);
    count = -1;
  }

  /** Sets the value of <code>bit</code> to zero. */
  public final void clear(int bit) {
    bits[bit >> 3] &= ~(1 << (bit & 7));
    count = -1;
  }

  /** Returns <code>true</code> if <code>bit</code> is one and
    <code>false</code> if it is zero. */
  public final boolean get(int bit) {
    return (bits[bit >> 3] & (1 << (bit & 7))) != 0;
  }

  /** Returns the number of bits in this vector.  This is also one greater than
    the number of the largest valid bit number. */
  public final int size() {
    return size;
  }

  /** Returns the total number of one bits in this vector.  This is efficiently
    computed and cached, so that, if the vector is not changed, no
    recomputation is done for repeated calls. */
  public final int count() {
    // if the vector has been modified
    if (count == -1) {
      int c = 0;
      int end = bits.length;
      for (int i = 0; i < end; i++)
        c += BYTE_COUNTS[bits[i] & 0xFF];	  // sum bits per byte
      count = c;
    }
    return count;
  }

  private static final byte[] BYTE_COUNTS = {	  // table of bits/byte
    0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
    1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
    1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
    1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
    2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
    3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
    3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
    4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
  };


  /** Writes this vector to the file <code>name</code> in Directory
    <code>d</code>, in a format that can be read by the constructor {@link
    #BitVector(Directory, String)}.  */
  public final void write(Directory d, String name) throws IOException {
    IndexOutput output = d.createOutput(name);
    try {
      output.writeInt(size());			  // write size
      output.writeInt(count());			  // write count
      output.writeBytes(bits, bits.length);	  // write bits
    } finally {
      output.close();
    }
  }

  /** Constructs a bit vector from the file <code>name</code> in Directory
    <code>d</code>, as written by the {@link #write} method.
    */
  public BitVector(Directory d, String name) throws IOException {
    IndexInput input = d.openInput(name);
    try {
      size = input.readInt();			  // read size
      count = input.readInt();			  // read count
      bits = new byte[(size >> 3) + 1];		  // allocate bits
      input.readBytes(bits, 0, bits.length);	  // read bits
    } finally {
      input.close();
    }
  }

}