k-means clustering

#include <float.h>
#include <stdio.h>
#include <stdlib.h>
#include <tgmath.h>
#include <time.h>
#define CLUSTER 3
#define ELEMS 1000
typedef struct {
  complex double z;
  int cluster;
} elem;
typedef struct {
  complex double V[2][CLUSTER];
  int flip;
} center;
#define center_at(c, i) (c)->V[(c)->flip][(i)]
static void center_init(center *c) {
  c->flip = 0;
  for (int i = 0; i < CLUSTER; ++i) { c->V[0][i] = c->V[1][i] = 0.; }
}
static void center_update(center *c, elem *elems, int n) {
  c->flip = 1 - c->flip;
  complex double s[CLUSTER] = {0};
  int es[CLUSTER] = {0};
  for (int i = 0; i < n; ++i) {
    s[elems[i].cluster] += elems[i].z, ++es[elems[i].cluster];
  }
  for (int i = 0; i < CLUSTER; ++i) { center_at(c, i) = s[i] / es[i]; }
}
static int center_reassign(center *c, elem *elems, int n) {
  int change = 0;
  for (int i = 0; i < n; ++i) {
    double d[CLUSTER];
    for (int j = 0; j < CLUSTER; ++j) {
      d[j] = fabs(elems[i].z - center_at(c, j));
    }
    double min = DBL_MAX;
    int next_cluster;
    for (int j = 0; j < CLUSTER; ++j) {
      if (d[j] < min) { min = d[j], next_cluster = j; }
    }
    if (next_cluster != elems[i].cluster) {
      elems[i].cluster = next_cluster, ++change;
    }
  }
  return change;
}
static int build_elems(FILE *in, elem *elems, int n) {
  double x, y;
  int used = 0;
  srand(time(NULL));
  while (fscanf(in, "%lf,%lf", &x, &y) == 2) {
    if (n <= used) { exit(1); }
    elems[used].z = CMPLX(x, y), elems[used].cluster = rand() % 3, ++used;
  }
  return used;
}
static void output_elems(FILE *out, elem *elems, int n) {
  for (int i = 0; i < n; ++i) {
    fprintf(out, "%f,%f,%d\n", creal(elems[i].z), cimag(elems[i].z),
            elems[i].cluster);
  }
}
int main(void) {
  FILE *in = fopen("nocluster.csv", "r");
  elem elems[ELEMS];
  int n = build_elems(in, elems, ELEMS);
  fclose(in);
  center c;
  for (center_init(&c), center_update(&c, elems, n);
       center_reassign(&c, elems, n) > 0; center_update(&c, elems, n)) {
    ;
  }
  FILE *out = fopen("out.csv", "w");
  output_elems(out, elems, n), fclose(out);
}