ftp.nice.ch/pub/next/developer/languages/r/R.0.12.I.b.tar.gz#/R.0.12.I.b/demos/Dynload.dir/zero.c

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#include <math.h>

static void *func;

static double zfun(double z)
{
	void *args[1], *values[1];
	double zz[1], *result;
	char *mode[1]; long length[1];

	mode[0] = "double"; length[0] = 1;
	args[0] = (void*)(zz); zz[0] = z;

	call_S(func, 1L, args, mode, length, 0L, 1L, values);

	result = (double*)values[0];
	return result[0];
}

static double zero_approx(double(*f)(), double x0, double x1, double tol)
{
	double f0, f1, fc, xc;
	f0 = zfun(x0);
	f1 = zfun(x1);
	if(f0 == 0.0) return x0;
	if(f1 == 0.0) return x1;
	if(f0*f1 > 0.0) error("x[0] and x[1] have the same sign\n");
	if(tol <= 0.0) error("non-positive tol value\n");
	for(;;) {
		xc = 0.5*(x0+x1);
		if(fabs(x0-x1) < tol) return xc;
		fc = zfun(xc);
		if(fc == 0) return xc;
		if(f0*fc > 0.0) {
			x0 = xc;
			f0 = fc;
		}
		else {
			x1 = xc;
			f1 = fc;
		}
	}
}

zero_find(void *f, double *x, double *tol)
{
	func = f;
	x[0] = zero_approx(zfun, x[0], x[1], tol[0]);
}

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.