minor fixes

frontend/src/components/KnotLayer.jsx

@@ -2,6 +2,11 @@ import { useEffect, useRef, useState } from 'preact/hooks'
 import { resampleCurve, simplifyCurve } from '../../lib/math/curves.js'
 import { Vec2 } from '../../lib/math/math.js'
 
+function mod(i, modulus) {
+    const r = i % modulus
+    return r < 0 ? r + modulus : r
+}
+
 function createSimulation2d(positions, velocities, accelerations) {
     return {
         positions,
@@ -86,18 +91,14 @@ class KnotSimulation {
             return
         }
 
-        const { positions, velocities } = this.particleSimulation
+        const { positions } = this.particleSimulation
         const N = positions.length
 
         const newAccelerations = Array.from({ length: N }, () => [0, 0])
 
         for (let i = 0; i < N; i++) {
-            const prev = positions.at((i - 1) % N)
-            const curr = positions.at(i)
-            const next = positions.at((i + 1) % N)
-
             if (this.blackHolePosition) {
-                const v = Vec2.sub(this.blackHolePosition, curr)
+                const v = Vec2.sub(this.blackHolePosition, positions[i])
                 const dir = Vec2.normalize(v)
                 const dist = Vec2.norm2(v)
 
@@ -116,11 +117,8 @@ class KnotSimulation {
 
             const factor = (d - 10) / d
 
-            positions[i] = Vec2.add(curr, Vec2.scale(v, 0.5 * factor))
+            positions[mod(i, N)] = Vec2.add(curr, Vec2.scale(v, 0.5 * factor))
-
+            positions[mod(i + 1, N)] = Vec2.add(next, Vec2.scale(v, -0.5 * factor))
-            const nextPos = Vec2.add(next, Vec2.scale(v, -0.5 * factor))
-            next[0] = nextPos[0]
-            next[1] = nextPos[1]
 
             const baseForce = Vec2.scale(Vec2.add(Vec2.sub(prev, curr), Vec2.sub(next, curr)), 0.5)
             const jointFactor = 1