void selection_sort(int a[], int len) {
    int pos = 0;
    for (int i = 0; i < len - 1; ++i) {
      pos = i;
      for (int j = i + 1; j < len; ++j) {
        if (a[j] < a[pos]) {
          pos = j;
        }
      }
      swap(&a[i], &a[pos]);
    }
  }