Find the root of pi in the outer loop and update it on the fly in the inner...

Find the root of pi in the outer loop and update it on the fly in the inner loop for the brute force version of the FOF.

src/fof.c

@@ -175,10 +175,12 @@ void fof_search_serial(struct space *s) {
     const double piy = pi->x[1];
     const double piz = pi->x[2];
 
+    /* Find the root of pi. */
+    int root_i = fof_find(i, pid);
+    
     for (size_t j = i + 1; j < nr_gparts; j++) {
 
       /* Find the roots of pi and pj. */
-      const int root_i = fof_find(i, pid);
       const int root_j = fof_find(j, pid);
 
       /* Skip particles in the same group. */
@@ -204,8 +206,13 @@ void fof_search_serial(struct space *s) {
       /* Hit or miss? */
       if (r2 < l_x2) {
 
-        if (root_j < root_i)
+        /* If the root ID of pj is lower than pi's root ID set pi's root to point to pj's. 
+         * Otherwise set pj's to root to point to pi's.*/
+        if (root_j < root_i) {
           pid[root_i] = root_j;
+          /* Update root_i on the fly. */
+          root_i = root_j;
+        }
         else
           pid[root_j] = root_i;