using UnityEngine;
using Random = UnityEngine.Random;

public class StarField : MonoBehaviour
{
    private Transform thisTransform;
    private ParticleSystem.Particle[] points;

    public int starsMax = 100;
    public float starSize = 1f;
    public float starDistance = 10f;

    public float starClipDistance = 1f;

    // do not touch, internal math stuff
    private float starDistanceSqr;
    private float starClipDistanceSqr;

    private ParticleSystem ps;

    // Use this for initialization
    private void Start()
    {
        thisTransform = transform; // caching the transform
        starClipDistanceSqr = starClipDistance * starClipDistance;
        starDistanceSqr = starDistance * starDistance;
        ps = GetComponent<ParticleSystem>();
        var main = ps.main;
        main.simulationSpace = ParticleSystemSimulationSpace.World;
    }

    private void CreateStars()
    {
        points = new ParticleSystem.Particle[starsMax];

        for (int i = 0; i < starsMax; i++)
        {
            points[i].position = Random.insideUnitSphere.normalized * starDistance + thisTransform.position;
            points[i].color = new Color(1, 1, 1, 1);
            points[i].size = starSize;
        }
    }

    // Update is called once per frame
    private void Update()
    {
        if (points == null)
        {
            CreateStars();
        }

        for (int i = 0; i < starsMax; i++)
        {
            if ((points[i].position - thisTransform.position).sqrMagnitude > starDistanceSqr)
            {
                points[i].position = Random.insideUnitSphere.normalized * starDistance + thisTransform.position;
            }

            if ((points[i].position - thisTransform.position).sqrMagnitude <= starDistanceSqr)
            {
                float percent = (points[i].position - thisTransform.position).sqrMagnitude / starClipDistanceSqr;
                points[i].color = new Color(1, 1, 1, percent);
                points[i].size = percent * starSize;
            }
        }

        ps.SetParticles(points, points.Length);
    }
}