double unitrand (double randfn(void))
{ /* --- compute N(0,1) distrib. number */
static double b; /* buffer for random number */
double x, y, r; /* coordinates and radius */
if (b != 0.0) { /* if the buffer is full, */
x = b; b = 0; return x; } /* return the buffered number */
do { /* pick a random point */
x = 2.0*randfn()-1.0; /* in the unit square [-1,1]^2 */
y = 2.0*randfn()-1.0; /* and check whether it lies */
r = x*x +y*y; /* inside the unit circle */
} while ((r > 1) || (r == 0));
r = sqrt(-2*log(r)/r); /* factor for Box-Muller transform */
b = x *r; /* save one of the random numbers */
return y *r; /* and return the other */
} /* unitrand() */
void v_shuffle (void *vec, int n, double randfn (void))
{ /* --- shuffle vector entries */
int i; /* vector index */
void **v = vec, *t; /* vector and exchange buffer */
while (--n > 0) { /* shuffle loop (n random selections) */
i = (int)((n+1) *randfn()); /* compute a random index */
if (i > n) i = n; /* in the remaining section and */
if (i < 0) i = 0; /* exchange the vector elements */
t = v[i]; v[i] = v[n]; v[n] = t;
}
} /* v_shuffle() */
