From 2acbfffc5f77cf94017875e79e0b47c548fc4c68 Mon Sep 17 00:00:00 2001
From: root <>
Date: Thu, 24 Apr 2025 12:26:57 +0000
Subject: [PATCH] bugfix: velocity unbounded accel not projected

strange behaviour with velocity.mag()
sometimes it is unbounded, sometimes it remains bounded
the issue is `project_onto_plane` mutates its first argument

(1) mutating fn    yyyynnnn
(2) call fn        yynnyynn
(3) use fn retval  ynynynyn

(1) & (2)          yynnnnnn
[(1) & (2)] | (3)  yyynynyn

bounded?           yy-nyn-n

the case that we want is
(1) yes
(2) yes
(3) no

or this one
(1) no
(2) yes
(3) yes
---
 sketch.js | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/sketch.js b/sketch.js
index e08bbe8..362e59f 100644
--- a/sketch.js
+++ b/sketch.js
@@ -38,18 +38,16 @@ function move_charges() {
       const acceleration_mag = 1 / displacement.mag() * 0.005;
       let ai = displacement.copy().normalize().mult(acceleration_mag);
       let aj = p5.Vector.mult(ai, -1);
-      let ai_norm = project_onto_plane(ai, charges[i].position);
-
-      let aj_norm = project_onto_plane(aj, charges[j].position);
-
-      charges[i].acceleration.add(ai_norm);
-      charges[j].acceleration.add(aj_norm);
+      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.velocity = project_onto_plane(charge.velocity, charge.position);
+    //project_onto_plane(charge.velocity, charge.position);
     charge.position = charge.position.add(charge.velocity);
     charge.position.normalize();
   }
@@ -61,9 +59,10 @@ function project_onto_unit_vector(v, unit_vector) {
 }
 
 /// 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);
-  return v.sub(v_proj)
+  v.sub(v_proj)
 }
 
 function make_charges(n) {