int splicing_mvrnorm(const splicing_vector_t *mu, double sigma,
splicing_vector_t *resalpha, int len) {
int i;
double sqrtsigma = len == 1 ? sigma : sqrt(sigma);
SPLICING_CHECK(splicing_vector_resize(resalpha, len));
for (i=0; i<len; i++) {
VECTOR(*resalpha)[i] = VECTOR(*mu)[i] + sqrtsigma * RNG_NORMAL(0,1);
}
return 0;
}
int igraph_sample_sphere_surface(igraph_integer_t dim, igraph_integer_t n,
igraph_real_t radius,
igraph_bool_t positive,
igraph_matrix_t *res) {
igraph_integer_t i, j;
if (dim < 2) {
IGRAPH_ERROR("Sphere must be at least two dimensional to sample from "
"surface", IGRAPH_EINVAL);
}
if (n < 0) {
IGRAPH_ERROR("Number of samples must be non-negative", IGRAPH_EINVAL);
}
if (radius <= 0) {
IGRAPH_ERROR("Sphere radius must be positive", IGRAPH_EINVAL);
}
IGRAPH_CHECK(igraph_matrix_resize(res, dim, n));
RNG_BEGIN();
for (i = 0; i < n; i++) {
igraph_real_t *col=&MATRIX(*res, 0, i);
igraph_real_t sum=0.0;
for (j = 0; j < dim; j++) {
col[j] = RNG_NORMAL(0, 1);
sum += col[j] * col[j];
}
sum = sqrt(sum);
for (j = 0; j < dim; j++) {
col[j] = radius * col[j] / sum;
}
if (positive) {
for (j = 0; j < dim; j++) {
col[j] = fabs(col[j]);
}
}
}
RNG_END();
return 0;
}
