simplify child index calculations

src/barnes_hut.rs

@@ -80,12 +80,11 @@ impl Octree {
 		else {
 			//println!("A{}", self.nodes[cur_node].num_particles);
 			//Divide and conquer
-                        let (base, mask) = self.children[cur_node];
-                        let mut off = 0;
+                        let (mut child, mask) = self.children[cur_node];
 			for c in 0..8 {
 				if mask & (1 << c) != 0 {
-                                    self.barnes_hut_specific_node(particle, base as usize + off, delta);
-                                    off += 1;
+                                    self.barnes_hut_specific_node(particle, child as usize, delta);
+                                    child += 1;
 				}
 			}
 		}
@@ -151,12 +150,11 @@ impl Octree {
                         }
                 }
 
-                let mut off = 0;
-		for (i, particle_oct) in particle_octs.iter().enumerate() {
+                let mut c = idx as usize;
+		for particle_oct in &particle_octs {
 			if !particle_oct.is_empty() {
-				self.add_particles_to_specific_node(&particle_oct,
-					self.children[cur_node].0 as usize + off);
-                                off += 1;
+				self.add_particles_to_specific_node(&particle_oct, c);
+                                c += 1;
 			}
 		}
 	}