/****************************************************************
 * This class performs simple linear regression.
 */
public class Regression
{
    /** The y-intercept of the line.
     */
    private double a;

    /** The slope of the line.
     */
    private double b;

    /** The measure of fit for the line.
     */
    private double r2;


    /************************************************************
     * Fit a line y = a + b * x.
     * @param  x    The x coordinates.
     * @param  y    The y coordinates.
     * @return int  Number of data points used in regression.
     */
    public int fit (double [] x, double [] y)
    {
        int n = Math.min (x.length, y.length);
        int i;
        double nn = (double) n;
        double sumX  = 0;
        double sumY  = 0;
        double sumXX = 0;
        double sumYY = 0;
        double sumXY = 0;
  
        for (i = 0; i < n; i++) {
           sumX  += x[i];
           sumY  += y[i];
           sumXX += x[i] * x[i];
           sumYY += y[i] * y[i];
           sumXY += x[i] * y[i];
        } // for

        double sXX = sumXX - sumX * sumX / nn;
        //double sYY = sumYY - sumY * sumY / nn;
        double sXY = sumXY - sumX * sumY / nn;

        b  = sXY / sXX;
        a  = (sumY - b * sumX) / nn;
        r2 = (sumXY * sumXY) / (sumXX * sumYY);

        return n;

    } // fit


    /************************************************************
     * Get the intercept (a).
     * @return double  The intercept (a).
     */
    public double getA () { return a; }


    /************************************************************
     * Get the slope (b).
     * @return double  The slope (b).
     */
    public double getB () { return b; }


    /************************************************************
     * Get the measure of fit (r2).
     * @return double  The slope (b).
     */
    public double getR2 () { return r2; }


    /************************************************************
     * Main method for testing purpose.
     * @param args  Command-line arguments.
     */
    public static void main (String [] args)
    {
       double x [] = { 1.0, 2.0, 3.0, 4.0 };
       double y [] = { 1.9, 4.1, 6.2, 7.9 };

       Regression line = new Regression ();
       line.fit (x, y);

       System.out.println ("Regression results:");
       System.out.println ("a  = " + line.getA ());
       System.out.println ("b  = " + line.getB ());
       System.out.println ("r2 = " + line.getR2 ());

    } // main

} // Regression