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The S-Space Package is a collection of algorithms for building Semantic Spaces as well as a highly-scalable library for designing new distributional semantics algorithms. Distributional algorithms process text corpora and represent the semantic for words as high dimensional feature vectors. This package also includes matrices, vectors, and numerous clustering algorithms. These approaches are known by many names, such as word spaces, semantic spaces, or distributed semantics and rest upon the Distributional Hypothesis: words that appear in similar contexts have similar meanings.

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/*
 * Copyright 2011 David Jurgens
 *
 * This file is part of the S-Space package and is covered under the terms and
 * conditions therein.
 *
 * The S-Space package is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as published
 * by the Free Software Foundation and distributed hereunder to you.
 *
 * THIS SOFTWARE IS PROVIDED "AS IS" AND NO REPRESENTATIONS OR WARRANTIES,
 * EXPRESS OR IMPLIED ARE MADE.  BY WAY OF EXAMPLE, BUT NOT LIMITATION, WE MAKE
 * NO REPRESENTATIONS OR WARRANTIES OF MERCHANT- ABILITY OR FITNESS FOR ANY
 * PARTICULAR PURPOSE OR THAT THE USE OF THE LICENSED SOFTWARE OR DOCUMENTATION
 * WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER
 * RIGHTS.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see .
 */

package edu.ucla.sspace.matrix;

import edu.ucla.sspace.vector.SparseHashDoubleVector;
import edu.ucla.sspace.vector.DoubleVector;
import edu.ucla.sspace.vector.SparseDoubleVector;
import edu.ucla.sspace.vector.Vector;
import edu.ucla.sspace.vector.Vectors;


/**
 * A decorator around a {@code SparseMatrix} that keeps only the upper
 * triangular values while providing a symmetric view of the data.  This class
 * only records changes values where row > col.  For all other values, the
 * row and column values are swapped and then the backing matrix is updated.
 * Note, that if the provided backing matrix has existing values for indices row
 * < col, these values will be ignored and never returned from any method.
 * Note the original perfomance characteristics of the backing matrix are
 * retained by this class.
 *
 * The primary benfit of this class is for storing large symmetric sparse
 * matrices in half of the memory.
 *
 * @author David Jurgens
 */
public class SparseSymmetricMatrix extends AbstractMatrix 
        implements SparseMatrix, java.io.Serializable {

    private static final long serialVersionUID = 1L;

    private final SparseMatrix backing;

    /**
     * Constructs a sparse matrix with the specified dimensions.
     */
    public SparseSymmetricMatrix(SparseMatrix backing) {
        this.backing = backing;
    }

    /**
     * {@inheritDoc}
     */
    public int columns() {
        return backing.columns();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public double get(int row, int column) {
        // Swap the ordering so only the upper triangular is accessed.
        if (row > column) {
            int tmp = column;
            column = row;
            row = tmp;
        }
        return backing.get(row, column);
    }

    /**
     * {@inheritDoc}
     */
    @Override public SparseDoubleVector getColumnVector(int column) {
        int rows = rows();
        SparseHashDoubleVector col = new SparseHashDoubleVector(rows);
        for (int r = 0; r < rows; ++r)
            col.set(r, get(r, column));
        return col;
    }
    
    /**
     * {@inheritDoc}
     */
    @Override public SparseDoubleVector getRowVector(int row) {
        int cols = columns();
        SparseHashDoubleVector rowVec = new SparseHashDoubleVector(cols);
        for (int c = 0; c < cols; ++c)
            rowVec.set(c, get(row, c));
        return rowVec;
    }

    /**
     * {@inheritDoc}
     */
    public int rows() {
        return backing.rows();
    }

    /**
     * {@inheritDoc}
     */
    @Override public void set(int row, int column, double val) {
        // Swap the ordering so only the upper triangular is written to
        if (row > column) {
            int tmp = column;
            column = row;
            row = tmp;
        }
        backing.set(row, column, val);
    }
}