move charge repelling stuff to its own file

thomson-problem.js

@@ -0,0 +1,44 @@
+function move_charges(charges) {
+  for (charge of charges) {
+    charge.acceleration.setMag(0);
+  }
+  for (let i = 0; i < charges.length; i += 1) {
+    for (let j = 0; j < i; j += 1) {
+      const displacement = p5.Vector.sub(
+        charges[i].position,
+        charges[j].position,
+      );
+      let acceleration_mag = 1 / displacement.mag() * 0.001;
+      let ai;
+      if (acceleration_mag === Infinity) {
+        ai = createVector(random(-1, 1), random(-1, 1), random(-1, 1));
+      } else {
+        ai = displacement.copy();
+      }
+      ai = ai.normalize().mult(acceleration_mag);
+      let aj = p5.Vector.mult(ai, -1);
+      project_onto_plane(ai, charges[i].position);
+      project_onto_plane(aj, charges[j].position);
+      charges[i].acceleration.add(ai);
+      charges[j].acceleration.add(aj);
+    }
+  }
+  for (let i = 0; i < charges.length; i += 1) {
+    let charge = charges[i];
+    charge.velocity = charge.velocity.add(charge.acceleration);
+    charge.position = charge.position.add(charge.velocity);
+    charge.position.normalize();
+  }
+}
+
+function project_onto_unit_vector(v, unit_vector) {
+  let size = p5.Vector.dot(v, unit_vector);
+  return p5.Vector.mult(unit_vector, size);
+}
+
+/// Project `v` onto the plane normal to `unit_vector`
+/// Mutates `v`
+function project_onto_plane(v, unit_vector) {
+  let v_proj = project_onto_unit_vector(v, unit_vector);
+  v.sub(v_proj)
+}