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Bobo is a Faceted Search implementation written purely in Java, an extension of Apache Lucene
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/**
 * Bobo Browse Engine - High performance faceted/parametric search implementation
 * that handles various types of semi-structured data.  Written in Java.
 *
 * Copyright (C) 2005-2006  spackle
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 *
 * To contact the project administrators for the bobo-browse project,
 * please go to https://sourceforge.net/projects/bobo-browse/, or
 * contact [email protected].
 */

package com.browseengine.bobo.util;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;

public class MutableSparseFloatArray extends SparseFloatArray {
  private Map _map;
  private boolean _isDirty;

  public MutableSparseFloatArray(float[] floats) {
    super(floats);
    _map = new Hashtable();
    _isDirty = false;
  }

  public boolean isDirty() {
    return _isDirty;
  }

  @Override
  public synchronized float get(int idx) {
    float val = super.get(idx);
    if (val != 0f) {
      return val;
    }
    // else, check here!
    Float stored = _map.get(idx);
    if (stored != null) {
      return stored;
    }
    return 0f;
  }

  public synchronized void set(int idx, float val) {
    _isDirty = true;
    if (null == _bits && null != _floats) {
      _floats[idx] = val;
    } else {
      if (null != _bits && _bits.get(idx)) {
        // count the number of bits that are on BEFORE this idx
        int count;
        int ref = idx / REFERENCE_POINT_EVERY - 1;
        if (ref >= 0) {
          count = _referencePoints[ref];
        } else {
          count = 0;
        }
        int i = idx - idx % REFERENCE_POINT_EVERY;
        while ((i = _bits.nextSetBit(i)) >= 0 && i < idx) {
          count++;
          i++;
        }
        _floats[count] = val;
      } else {
        if (val != 0f) {
          _map.put(idx, val);
        } else {
          Float stored = _map.get(idx);
          if (stored != null) {
            _map.remove(idx);
          }
        }
        int sz = _map.size();
        // keep something on the order of 32KB, or 0.4*compressed size, in _map
        // if _floats is null, then that's the same as it existing but being of length 0
        // if sz > 512, and _floats is null, that's the same as checking if sz > 0.4f*0 and that sz
        // > 2f*0, which is true
        // in other words, if _floats is null, and sz > 512, then our expansion rule says to
        // condense()
        if (sz > 512
            && (null == _floats || (sz > 4096 ? sz > 0.4f * _floats.length
                : sz > 2f * _floats.length))) {
          condense();
        }
      }
    }
  }

  /**
   * See super{@link #expand()}.  Future modifications to this may or may not affect values stored in the
   * returned array.  Modifications to the returned array, cause this object instance to become unusable, as
   * from this point on all operations are UNDEFINED.
   *
   * @return the expanded primitive float array rep. of this.
   */
  @Override
  public synchronized float[] expand() {
    float[] all = super.expand();
    Iterator keysIter = _map.keySet().iterator();
    while (keysIter.hasNext()) {
      int idx = keysIter.next();
      float val = _map.get(idx);
      all[idx] = val;
    }
    return all;
  }

  /**
   * An expensive, but necessary, operation to internally conserve space as things grow.
   * Might be useful to call outside of the automatic maintenance,
   * when you expect very few new non-zero values, just changes to existing values,
   * in the future.
   *
   * Uses an expanded form of the float array as scratch space in memory, so be careful
   * that you have enough memory, and try doing this one at a time.
   */
  public synchronized void condense() {
    super.condense(expand());
    _map = new Hashtable();
  }
}