Recursive simulation

using System;
using System.Diagnostics;
using TMPro;
using Unity.VisualScripting;
using UnityEngine;
using UnityEngine.Profiling;
using UnityEngine.Serialization;
using Random = UnityEngine.Random;

public class Simulation : MonoBehaviour
{
   public struct Particle
   {
      public Vector2 position, velocity;
      public float mass;
   }
   public int particleCount;
   public float radius;
   public float mass;
   public float timeScale;

   [Header("Quad Tree")]
   public float approximationThreshold;
   public int capacity;
   public int maxDepth;

   [Header("")]
   public bool centerAttraction;
   public float centerMass;

   public float particleForce;

   public Material material;

   Particle[] particles;
   QuadTree quadTree;
   new ParticleRenderer renderer;
   Particle center;
   TextMeshProUGUI latency;

   const int HalfScreenSize = 5;

   void Start()
   {
      center = new Particle
      {
         position = Vector2.zero,
         velocity = Vector2.zero,
         mass = centerMass
      };
      particles = new Particle[particleCount];
      for (int i = 0; i < particleCount; i++)
      {
         float posX = Random.Range(-HalfScreenSize + radius, HalfScreenSize - radius);
         float posY = Random.Range(-HalfScreenSize + radius, HalfScreenSize - radius);

         float centerDist = new Vector2(posX, posY).magnitude;
         float force = Mathf.Sqrt(centerMass / centerDist);
         Vector2 direction = new Vector2(posY, posX).normalized;
         Vector2 velocity = new Vector2(direction.x, -direction.y) * force * particleForce;

         particles[i] = new()
         {
            position = new Vector2(posX, posY),
            velocity = Vector2.zero,
            mass = mass
         };
      }

      latency = GameObject.Find("Latency").GetComponent<TextMeshProUGUI>();
      renderer = new(particles, material, radius);
      RebuildQuadTree();
   }

   void FixedUpdate()
   {
      Stopwatch watch = new();
      watch.Start();

      Profiler.BeginSample("UpdateVelocities");
      UpdateVelocities(Time.fixedDeltaTime * timeScale);
      Profiler.EndSample();
      Profiler.BeginSample("UpdatePositions");
      UpdatePositions(Time.fixedDeltaTime * timeScale);
      Profiler.EndSample();
      Profiler.BeginSample("RebuildQuadTree");
      RebuildQuadTree();
      Profiler.EndSample();

      watch.Stop();
      latency.text = watch.ElapsedMilliseconds + " ms";
   }

   void Update()
   {
      renderer.Render(particles);
   }
   void OnDestroy()
   {
      renderer.ReleaseBuffers();
   }

   void UpdateVelocities(float timeStep)
   {
      for (int i = 0; i < particleCount; i++)
      {
         particles[i] = quadTree.UpdateVelocity(particles[i], timeStep);
      }
   }

   public static Vector2 CalculateAcceleration(Particle p1, Particle p2)
   {
      Vector2 diff = p2.position - p1.position;
      Vector2 forceDir = diff.normalized;
      float sqrDst = Mathf.Max(diff.sqrMagnitude, 0.05f);
      Vector2 force = forceDir * (p1.mass * p2.mass) / sqrDst;
      Vector2 acceleration = force / p1.mass;

      return acceleration;
   }

   void UpdatePositions(float timeStep)
   {
      float minPosX = -HalfScreenSize + radius;
      float maxPosX = HalfScreenSize - radius;
      float minPosY = -HalfScreenSize + radius;
      float maxPosY = HalfScreenSize - radius;

      for (int i = 0; i < particleCount; i++)
      {
         Particle particle = particles[i];
         particle.position += particle.velocity * timeStep;

         Vector2 absPos = new(Mathf.Abs(particle.position.x), Mathf.Abs(particle.position.y));
         if (absPos.x < minPosX || absPos.x > maxPosX || absPos.y < minPosY || absPos.y > maxPosY)
         {
            float posX = Random.Range(minPosX, maxPosX);
            float posY = Random.Range(minPosY, maxPosY);
            particle.position = new Vector2(posX, posY);
         }

         particles[i] = particle;
      }
   }

   void RebuildQuadTree()
   {
      Quad boundary = new()
      {
         position = new Vector2(-5f, -5f),
         size = 10
      };
      quadTree = new(boundary);
      QuadTree.OverlapTolerance = radius;
      QuadTree.ApproximationThreshold = approximationThreshold;
      QuadTree.Capacity = capacity;
      QuadTree.MaxDepth = maxDepth;
      QuadTree.OriginalSize = HalfScreenSize * 2;

      for (int i = 0; i < particles.Length; i++)
      {
         quadTree.Insert(particles[i]);
      }

      quadTree.CalculateMassAndCenterOfMass();
   }
}