#include <math.h>
#include "nrutil.h"

void fit(double x[], double y[], int ndata, double sig[], int mwt, double *a,
	double *b, double *siga, double *sigb, double *chi2, double *q)
{
	int i;
	double wt,t,sxoss,sx=0.0,sy=0.0,st2=0.0,ss,sigdat;

	*b=0.0;
	if (mwt) {
		ss=0.0;
		for (i=0;i<ndata;i++) {
			wt=1.0/SQR(sig[i]);
			ss += wt;
			sx += x[i]*wt;
			sy += y[i]*wt;
		}
	} else {
		for (i=0;i<ndata;i++) {
			sx += x[i];
			sy += y[i];
		}
		ss=ndata;
	}
	sxoss=sx/ss;
	if (mwt) {
		for (i=0;i<ndata;i++) {
			t=(x[i]-sxoss)/sig[i];
			st2 += t*t;
			*b += t*y[i]/sig[i];
		}
	} else {
		for (i=0;i<ndata;i++) {
			t=x[i]-sxoss;
			st2 += t*t;
			*b += t*y[i];
		}
	}
	*b /= st2;
	*a=(sy-sx*(*b))/ss;
	*siga=sqrt((1.0+sx*sx/(ss*st2))/ss);
	*sigb=sqrt(1.0/st2);
	*chi2=0.0;
	if (mwt == 0) {
		for (i=0;i<ndata;i++)
			*chi2 += SQR(y[i]-(*a)-(*b)*x[i]);
		*q=1.0;
		sigdat=sqrt((*chi2)/(ndata-2));
		*siga *= sigdat;
		*sigb *= sigdat;
	} else {
		for (i=0;i<ndata;i++)
			*chi2 += SQR((y[i]-(*a)-(*b)*x[i])/sig[i]);
		*q=gammq(0.5*(ndata-2),0.5*(*chi2));
	}
}