//
// Linear least squares fit.
//
// - Given n points in x and y arrays.
// - Slope returned.
//
// This version makes two passes over each data set.
//
// 27-Oct-1999 tvb
//

#include <math.h>

double LineFit (long n, double *x, double *y)
{
    long i;
    double dx, dy;
    double Sdx2, Sdxdy;
    double xmean, ymean;
    double _mean (double *x, long n);

    xmean = _mean(x, n);
    ymean = _mean(y, n);
    Sdx2 = Sdxdy = 0.0;
    for (i = 0; i < n; i += 1) {
        dx = x[i] - xmean;
        dy = y[i] - ymean;
        Sdx2 += dx * dx;
        Sdxdy += dx * dy;
    }

    return (Sdx2 == 0.0) ? 0.0 : (Sdxdy / Sdx2);
}

double _mean (double *x, long n)
{
    long i;
    double sum;

    sum = 0.0;
    for (i = 0; i < n; i += 1) {
        sum += x[i];
    }

    return (sum / n);
}