Add speed_controller to maintain system energy

The starting velocities going outwards in a circle means that the sum of
energy in the system is 0. As the boids come back together, they will
slow down to fall into a lattice. This is interesting, but not the
flocking behavior Boids are supposed to have.

I've replaced the `space_brakes` function with this new speed controller
to keep boids moving between two speeds. Anything too slow will speed up
along it's current vector, and anything too fast will slow down.

src/birdoids/mod.rs

@@ -10,11 +10,13 @@ use crate::birdoids::physics::{apply_velocity, Force, Velocity};
 const BACKGROUND_COLOR: Color = Color::srgb(0.4, 0.4, 0.4);
 const PLAYERBOID_COLOR: Color = Color::srgb(1.0, 0.0, 0.0);
 const TURN_FACTOR: f32 = 1.0;
-const BOID_VIEW_RANGE: f32 = 50.0;
+const BOID_VIEW_RANGE: f32 = 15.0;
 const COHESION_FACTOR: f32 = 1.0;
-const SEPARATION_FACTOR: f32 = 1.0;
-const ALIGNMENT_FACTOR: f32 = 10.0;
-const SPACEBRAKES_COEFFICIENT: f32 = 0.01;
+const SEPARATION_FACTOR: f32 = 10.0;
+const ALIGNMENT_FACTOR: f32 = 1.0;
+const SPACEBRAKES_COEFFICIENT: f32 = 0.5;
+const LOW_SPEED_THRESHOLD: f32 = 50.0;
+const HIGH_SPEED_THRESHOLD: f32 = 200.0;
 
 pub struct BoidsPlugin;
 
@@ -37,7 +39,7 @@ impl Plugin for BoidsPlugin {
                 cohesion,
                 separation,
                 alignment,
-                // space_brakes,
+                speed_controller,
             ),
         );
     }
@@ -85,10 +87,16 @@ fn spawn_boids(
     ));
 }
 
-fn space_brakes(mut mobs: Query<&mut Force, With<Boid>>) {
-    for mut accel in &mut mobs {
-        let braking_dir = -accel.0 * SPACEBRAKES_COEFFICIENT;
-        accel.0 += braking_dir;
+/// Controls the boid's minimum and maximum speed according to a low- and
+/// high-threshold. Boids moving too slow are sped up, and boids moving too
+/// fast are slowed down.
+fn speed_controller(mut mobs: Query<(&Velocity, &mut Force), With<Boid>>) {
+    for (vel, mut impulse) in &mut mobs {
+        if vel.0.length() < LOW_SPEED_THRESHOLD {
+            impulse.0 += vel.0 * SPACEBRAKES_COEFFICIENT;
+        } else if vel.0.length() > HIGH_SPEED_THRESHOLD {
+            impulse.0 += -vel.0 * SPACEBRAKES_COEFFICIENT;
+        }
     }
 }