package de.lmu.ifi.dbs.elki.math.linearalgebra.randomprojections;

/*
 This file is part of ELKI:
 Environment for Developing KDD-Applications Supported by Index-Structures

 Copyright (C) 2013
 Ludwig-Maximilians-Universität München
 Lehr- und Forschungseinheit für Datenbanksysteme
 ELKI Development Team

 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Affero General Public License as published by
 the Free Software Foundation, either version 3 of the License, or
 (at your option) any later version.

 This program 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 Affero General Public License for more details.

 You should have received a copy of the GNU Affero General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
import de.lmu.ifi.dbs.elki.math.linearalgebra.Matrix;
import de.lmu.ifi.dbs.elki.utilities.RandomFactory;
import de.lmu.ifi.dbs.elki.utilities.documentation.Reference;
import de.lmu.ifi.dbs.elki.utilities.optionhandling.OptionID;
import de.lmu.ifi.dbs.elki.utilities.optionhandling.constraints.CommonConstraints;
import de.lmu.ifi.dbs.elki.utilities.optionhandling.parameterization.Parameterization;
import de.lmu.ifi.dbs.elki.utilities.optionhandling.parameters.DoubleParameter;

/**
 * Random projections as suggested by Dimitris Achlioptas.
 * 
 * Reference:
 * <p>
 * Database-friendly random projections: Johnson-Lindenstrauss with binary coins
 * <br />
 * Dimitris Achlioptas<br />
 * In: Proceedings of the twentieth ACM SIGMOD-SIGACT-SIGART symposium on
 * Principles of database systems
 * </p>
 * 
 * @author Erich Schubert
 */
@Reference(title = "Database-friendly random projections: Johnson-Lindenstrauss with binary coins", authors = "Dimitris Achlioptas", booktitle = "Proceedings of the twentieth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems", url = "http://dx.doi.org/10.1145/375551.375608")
public class AchlioptasRandomProjectionFamily extends AbstractRandomProjectionFamily {
  /**
   * Projection sparsity.
   */
  private double sparsity;

  /**
   * Constructor.
   * 
   * @param sparsity Projection sparsity
   * @param random Random number generator.
   */
  public AchlioptasRandomProjectionFamily(double sparsity, RandomFactory random) {
    super(random);
    this.sparsity = sparsity;
  }

  @Override
  public Projection generateProjection(int idim, int odim) {
    final double pPos = .5 / sparsity;
    final double pNeg = pPos + pPos; // Threshold
    double baseValuePart = Math.sqrt(this.sparsity);

    Matrix projectionMatrix = new Matrix(odim, idim);
    for(int i = 0; i < odim; ++i) {
      for(int j = 0; j < idim; ++j) {
        final double r = random.nextDouble();
        final double value;
        if(r < pPos) {
          value = baseValuePart;
        }
        else if(r < pNeg) {
          value = -baseValuePart;
        }
        else {
          value = 0.;
        }

        projectionMatrix.set(i, j, value);
      }
    }
    return new MatrixProjection(projectionMatrix);
  }

  /**
   * Parameterization class.
   * 
   * @author Erich Schubert
   * 
   * @apiviz.exclude
   */
  public static class Parameterizer extends AbstractRandomProjectionFamily.Parameterizer {
    /**
     * Parameter for the projection sparsity.
     */
    public static final OptionID SPARSITY_ID = new OptionID("achlioptas.sparsity", "Frequency of zeros in the projection matrix.");

    /**
     * Projection sparsity
     */
    private double sparsity = 3.;

    @Override
    protected void makeOptions(Parameterization config) {
      super.makeOptions(config);
      DoubleParameter sparsP = new DoubleParameter(SPARSITY_ID);
      sparsP.setDefaultValue(3.);
      sparsP.addConstraint(CommonConstraints.GREATER_EQUAL_ONE_DOUBLE);
      if(config.grab(sparsP)) {
        sparsity = sparsP.doubleValue();
      }
    }

    @Override
    protected AchlioptasRandomProjectionFamily makeInstance() {
      return new AchlioptasRandomProjectionFamily(sparsity, random);
    }
  }
}