created two passes

Pass1: compute all forces
Pass2: integrate the forces and do the updates

If we dont separate it, we will have some inter pedestrian forces calculated on a mix of new and updated   values

main.cpp

@@ -58,7 +58,7 @@ int main(int argc, const char *argv[]) {
 
   // # (8 + 55N - 55 + 144 + 4 + 8 + 4 + 10)*n_p*n_iter
   for (int t = 0; t < n_iter; t++) {
-
+    // Pass1: conmpute all forces from the current state
     for (int i = 0; i < n_p; i++) {
 
       Pedestrian &p = pedestrians[i];
@@ -73,7 +73,10 @@ int main(int argc, const char *argv[]) {
 
       // # 144 flops
       compute_repulsive_effects_from_borders(board, &p);
-
+    }
+    // Pass2: integrate the forces and update the current state
+    for (int i = 0; i < n_p; i++) {
+      Pedestrian &p = pedestrians[i];
       // # 4 flops = 2 ADDs, 2 MULTs
       Vector2d w_alpha = p.curr_vel + p.f_i * dt;