package org.apache.lucene.util;

/**
 * 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.
 */

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>
  <li>store and load, as bit set or d-gaps, depending on sparseness;</li> 
  </ul>

  @author Doug Cutting
  @version $Id: BitVector.java 494136 2007-01-08 18:11:08Z mikemccand $
  */
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) {
    if (bit >= size) {
      throw new ArrayIndexOutOfBoundsException(bit);
    }
    bits[bit >> 3] |= 1 << (bit & 7);
    count = -1;
  }

  /** Sets the value of <code>bit</code> to zero. */
  public final void clear(int bit) {
    if (bit >= size) {
      throw new ArrayIndexOutOfBoundsException(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) {
    if (bit >= size) {
      throw new ArrayIndexOutOfBoundsException(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 {
      if (isSparse()) { 
        writeDgaps(output); // sparse bit-set more efficiently saved as d-gaps.
      } else {
        writeBits(output);
      }
    } finally {
      output.close();
    }
  }
     
  /** Write as a bit set */
  private void writeBits(IndexOutput output) throws IOException {
    output.writeInt(size());        // write size
    output.writeInt(count());       // write count
    output.writeBytes(bits, bits.length);
  }
  
  /** Write as a d-gaps list */
  private void writeDgaps(IndexOutput output) throws IOException {
    output.writeInt(-1);            // mark using d-gaps                         
    output.writeInt(size());        // write size
    output.writeInt(count());       // write count
    int last=0;
    int n = count();
    int m = bits.length;
    for (int i=0; i<m && n>0; i++) {
      if (bits[i]!=0) {
        output.writeVInt(i-last);
        output.writeByte(bits[i]);
        last = i;
        n -= BYTE_COUNTS[bits[i] & 0xFF];
      }
    }
  }

  /** Indicates if the bit vector is sparse and should be saved as a d-gaps list, or dense, and should be saved as a bit set. */
  private boolean isSparse() {
    // note: order of comparisons below set to favor smaller values (no binary range search.)
    // note: adding 4 because we start with ((int) -1) to indicate d-gaps format.
    // note: we write the d-gap for the byte number, and the byte (bits[i]) itself, therefore
    //       multiplying count by (8+8) or (8+16) or (8+24) etc.:
    //       - first 8 for writing bits[i] (1 byte vs. 1 bit), and 
    //       - second part for writing the byte-number d-gap as vint. 
    // note: factor is for read/write of byte-arrays being faster than vints.  
    int factor = 10;  
    if (bits.length < (1<< 7)) return factor * (4 + (8+ 8)*count()) < size();
    if (bits.length < (1<<14)) return factor * (4 + (8+16)*count()) < size();
    if (bits.length < (1<<21)) return factor * (4 + (8+24)*count()) < size();
    if (bits.length < (1<<28)) return factor * (4 + (8+32)*count()) < size();
    return                            factor * (4 + (8+40)*count()) < size();
  }

  /** 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
      if (size == -1) {
        readDgaps(input);
      } else {
        readBits(input);
      }
    } finally {
      input.close();
    }
  }

  /** Read as a bit set */
  private void readBits(IndexInput input) throws IOException {
    count = input.readInt();        // read count
    bits = new byte[(size >> 3) + 1];     // allocate bits
    input.readBytes(bits, 0, bits.length);
  }

  /** read as a d-gaps list */ 
  private void readDgaps(IndexInput input) throws IOException {
    size = input.readInt();       // (re)read size
    count = input.readInt();        // read count
    bits = new byte[(size >> 3) + 1];     // allocate bits
    int last=0;
    int n = count();
    while (n>0) {
      last += input.readVInt();
      bits[last] = input.readByte();
      n -= BYTE_COUNTS[bits[last] & 0xFF];
    }          
  }
  
}